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Direct Solar Power: Off-Grid Without Batteries

Fri, 25 Aug 2023 00:00:00 +0000

Image: a laptop running on direct solar power. Photo: Marie Verdeil.

Conventional solar installations do not question our dependence on fossil fuels and the energy-guzzling lifestyle that results. Both rooftop solar panels and large-scale solar farms provide us with all the power we want, even when the sun is not shining. That is because these systems use the central power grid, which largely runs on fossil fuels, as a kind of battery to cope with power shortages.

Although grid-connected solar panels can reduce the fossil fuel consumption of thermal power plants, these savings are at least partly offset by the additional fossil fuels required to build and maintain what is essentially a dual energy infrastructure. Combining solar and wind power can further increase the share of renewable energy in the power grid, but this requires further infrastructure development. Apart from energy, this also demands a lot of money and time.

Replacing fossil-fuel-fired power plants with energy storage, so that surplus electricity generated on sunny days can be stored for when there is no or insufficient sun, encounters the same problem. Energy storage, whether integrated into a power grid or located at individual households (off-grid systems), is very expensive and carbon-intensive to build and maintain.

Autonomous solar installation

The production of solar panels obviously costs money and energy. However, the financial and energy costs of the associated back-up infrastructure are many times higher. For grid-connected solar installations, these costs are very difficult to calculate precisely, but for autonomous solar installations (without grid connection and with their own energy storage) it is a lot easier. As an example, I will therefore take the small autonomous solar installation that powers my living room in Barcelona.

This system consists of two 50W solar panels on the balcony, a 100 Ah lead-acid battery and a 10A charge controller. The energy generated is used for lighting, the music system, and charging laptops and other electronic devices, among other things. The initial financial investment was 340 euros: 120 euros for the solar panels, 170 euros for the battery and 50 euros for the charge controller.

But while the solar panels should last 30 years and the charge controller about 10 years, I have to replace the lead battery on average every three to five years. ¹ Over a 30-year lifespan, the costs then amount to €120 for the solar panels, €150 for the charge controllers and – in the best case scenario – €1,020 for the batteries. The batteries (and associated charge controllers) therefore account for about 90% of the total lifetime costs.

Energy storage also dominates the plant’s “embedded” energy (and resulting carbon emissions). Producing my lead-acid battery took 1,200 megajoules (MJ) of energy. ² Over a 30-year lifetime (six batteries at best), that equates to 7,200 MJ. The three charge controllers add another 360 MJ over a 30-year lifetime, bringing the total energy consumption for the battery system to 7,560 MJ. ³ In contrast, the production of the solar panels costs only 2,275 MJ out of a total of 9,835 MJ. ⁴ Conclusion: more than 75% of total fossil energy consumption is due to energy storage.

Image: To the right on the balcony are the two 50W solar panels that power my flat's living room. Next to it is the 30W solar panel that makes this website work. Photo: Marie Verdeil.

Image: The structure for the solar panels, built from waste wood. Photo: Kris De Decker.

Image: The 100 Ah lead-acid battery powering the living room after sunset. Photo: Kris De Decker.

Other types of batteries would not significantly change this conclusion. For a comparable off-grid system with lithium-ion batteries, energy storage would account for about 95% of the total lifetime cost (which is almost double that of a system with lead-acid batteries). Assuming an optimistic lifetime (10 years) and including charge controllers, lithium energy storage accounts for some 70% of the energy invested in a solar grid system. ⁵ ⁶ For nickel-iron batteries, energy storage would account for 85% of the total lifetime cost (there are no energy cost data). ⁷

The scale and location of the solar installation also make no difference. A larger system needs more solar panels, but also larger batteries and more expensive and powerful charge controllers. The ratios remain the same. ⁸ The only factor that may give the solar panels a slightly larger share of the total cost is the structures on which they are mounted. I don’t take this into account because I built them myself from waste wood. However, if the solar panels are mounted on a roof, a DIY solution is less obvious. But even in that case, the cost of energy storage remains by far the biggest consideration.

Direct solar energy: much cheaper and more sustainable

Unlike fossil fuels, the sun and wind are not available on demand. The problem with our approach to renewable energy is that we insist that power should always be infinitely available, regardless of the weather, seasons or time of day. Matching energy demand to supply – as was done in the past – would lead to dramatic reductions in the cost and use of fossil fuels.

For example, if I omitted the battery storage of my solar installation, my system would become about 10 times cheaper: 120 euros instead of 1,290 euros over a 30-year lifetime. Alternatively, I could spend 1,290 euros on solar panels alone, which would give me a solar system of 1,075 watts. That’s ten times the capacity of the setup with batteries, more than what would fit on the balcony.

Without the battery and charge controller, the energy cost of the installation also drops from 9,835 MJ to 2,275 MJ. In other words, I could generate at least four times as much solar energy with the same investment in fossil fuels.

How can direct solar power be practical?

All well and good, but the sun does not shine after sunset and the amount of solar energy varies throughout the day and year. So how then can using solar panels without batteries (or other back-up infrastructure in the case of grid-connected installations) be practical?

To answer that question, we look at a pioneer of “direct solar power”: the Living Energy Farm. This environmental education community in the US state of Virginia is completely “off-the-grid” thanks to solar power, but only 10% of the solar power generated passes through a (nickel-iron) battery. However, the solar panels provide power for several homes, a communal kitchen, a metal workshop, and a farm. ⁹ ¹⁰

Image: direct solar power at the Living Energy Farm.

The solar installation has been in operation since 2011 and consists of separate systems with a total peak power of 1,400 watts. ¹¹ In comparison, the average peak power of a residential solar installation in the UK and the US – for one household – is 4,000 watts and 6,500 watts, respectively. As in my flat, the Living Energy Farm uses energy sparingly, but the fact that hardly any batteries are used has other reasons.

Some appliances are only used during the day

A first reason is obvious: some electrical appliances and machines are only used during the day. This is true, for example, of all machines in the metal workshop, including a band saw, compressor, grinder, circular saw, lathe, milling machine and drilling machine. It also applies to agricultural machinery such as a grain mill and a deep well pump. Linked directly to solar panels, these machines offer all the capabilities of modern grid-powered technology, with the exception that they can only be used during the day. ¹⁰

On a much smaller scale, I have used direct solar power for a soldering iron, glue gun and irrigation pump (for the balcony) at home. Other examples of appliances and machines that could be used only during the day include hoovers, sewing machines, washing machines, game consoles, laser cutters and 3D printers. It is not so difficult to imagine a modern society where activities such as vacuuming and DIY chores only take place during the day. It is certainly not a return to the Middle Ages.

Image: several workshop tools at the Living Energy Farm, most of them run on direct solar power. Image: Alexis Zeigler.

Image: Metal lathe running on direct solar power, Living Energy Farm. Image: Alexis Zeigler.

Image: Soldering with direct solar power. Photo: Marie Verdeil. [Watch the video](https://www.youtube.com/watch?v=qozZCJU4IOc).

Moreover, not all electrical appliances require constant attention. Washing machines or dishwashers that trigger automatically when the sun shines are often cited example applications of a “smart” power grid. But that approach relies on an extensive infrastructure of electricity transmission, communication networks, and electronics-packed appliances.

In contrast, in a decentralised direct solar approach, the intelligence is provided by the sun and the rotation of the planet. A direct solar-powered washing machine or dishwasher can be fully charged and switched on in the evening. The machine then starts up “automatically” in the morning. You can even use timers (electronic or mechanical) to run different appliances one after the other.

Whether clouds pose an additional limit to a direct solar installation, and to what extent, depends on the size of the solar panels. Doubling the area of solar panels guarantees sufficient solar power during moderate cloud cover, while the installation remains much cheaper and more sustainable than a system with batteries or other backup infrastructure.

An even larger area of solar panels could provide sufficient energy even during heavy cloud cover, but increasing the size of the system tenfold brings the cost back to the level of an autonomous system with batteries. Quadrupling the area makes the system equally dependent on fossil fuels again.

Many appliances already have batteries

Direct solar power does not rule out the use of electrical appliances after sunset either. As mentioned, the Living Energy Farm has a modest battery system, providing power for lights, fans, and electronic devices after sunset, among other things. ¹⁰ In addition, many modern appliances already have built-in energy storage. This is the case for all kinds of electric vehicles, for most electronic gadgets, and for older electrical appliances with AA batteries.

Consequently, these types of devices can be charged with direct solar energy during the day and then used for several hours after sunset thanks to the built-in battery. Combined with a lithium-ion power bank, a direct solar panel can also make it possible to charge USB devices after sunset. This strategy can even work for lighting, as there are many battery-powered lamps that you can use as modern torches, hung in different parts of rooms and buildings.

Image: A mobile phone on direct solar power. Photo: Marie Verdeil.

Of course, outsourcing chemical energy storage to the device is not the most sustainable option. The production of lithium-ion batteries requires fossil fuels, and (unlike lead-acid batteries) they are not recycled. The best solution, of course, is to reduce the use of electrical devices. But charging them with direct solar energy is a lot more sustainable and efficient than via other batteries or a fossil-fueled electricity grid. If we use high-tech devices, then preferably in the smartest way possible.

Non-electric energy storage

A third reason why direct solar power is more practical than it initially seems is that some electrical appliances can be used after sunset thanks to thermal energy storage. This is much cheaper and more sustainable than electrical energy storage. Thermal energy storage is already fairly well established for space and water heating systems, which store solar-heated water in an insulated boiler or (for space heating only) in the building envelope. It is no surprise that the Living Energy Farm has such systems, and solar thermal energy also provides hot water in my flat.

However, the same approach also works for two important household appliances that need to work after sunset and also consume a lot of electricity: the fridge and the cooker. Instead of storing electricity from a solar panel in a battery to then power a fridge or cooker after sunset, these appliances on the Living Energy Farm use thermal insulation. This keeps the heat inside (in the case of the cooker) or outside (in the case of the fridge) when there is no power supply. The thermal insulation also ensures very high energy efficiency, which means that each of these appliances can already operate on a solar panel of just 100-200 watts.

A direct solar-powered fridge

It is perfectly possible to connect a conventional fridge or freezer directly to a solar panel, but such an appliance would heat up very quickly at night. Even refrigerators with the most energy-efficient labels have a relatively limited insulation thickness (usually 2.5 cm). However, if that insulation thickness is increased to about 12.5 cm, the energy consumption of a refrigerator drops by a factor of four. ¹² ¹³ The passive cooling capacity of a refrigerator can be further increased by adding thermal mass in the form of a water tank inside the appliance. During the day, the solar panel cools the water or converts it to ice. At night, this cold water or ice slows down the heating of the refrigerator. ¹⁴

A direct solar-powered fridge also opens at the top, not at the front. Cold air is heavy, and so much less energy is lost that way when someone opens the door. All these design choices add up to spectacular energy efficiency. A study of direct solar refrigerators in very sunny regions (Texas and New Mexico, USA) showed that they maintained their cooling capacity for 6 or 7 days without power supply. The units operated year-round with solar panels of only 80W to 120W. ¹⁵ The Living Energy Farm powers its solar refrigerator with a 200W panel. ¹⁰

Image: The Sundanzer DDR165. A refrigerator designed specifically for direct solar power. Photo: Sundanzer.

Unlike solar heating, solar cooling is optimally tuned to seasonal variations in solar radiation. Cooling requires more energy in summer, when there is more solar energy. The aforementioned refrigerator in New Mexico recorded electricity consumption of 406 watt-hours per day in summer and only 230 watt-hours in winter. ¹⁶ Moreover, the technology can be used throughout the cold chain, of which the household refrigerator is only a small (but essential) part. Another application is air cooling, although this is less well researched and more challenging. ¹⁷

A direct solar electric cooker

In principle, a conventional cooker can also be connected directly to a solar panel, but as with a conventional fridge, it is not very practical. You can only cook during the day, and you have to install a lot of solar panels. A single hot plate needs 1,000 watts of electrical power. A solar electric cooker solves these problems by packing the cooktop with thermal insulation. The technology is basically a combination of an electric cooktop and a haybox.

Image: Test of an electric solar cooker. Photo: California Polytechnic State University (Cal Poly).

Thanks to thermal insulation, an electric solar cooker slowly accumulates heat during the day, which can then be used for cooking after sunset. In this way, a much lower power supply can be sufficient to achieve high temperatures. Think of it as “charging” your cooker, not with electricity but with heat.

Researchers at US California Polytechnic State University (Cal Poly) built the first solar electric cooker in 2015. Their 12-volt device, which has since been further developed, needs only a 100W solar panel to work. It boils a litre of water in an hour. With a full day of sunlight, it can cook almost 5 kg of beans, rice, stew or potatoes. ¹⁸

Cooking after sunset is possible by using a cooking pot with a much thicker bottom (5-10 kg). Cal Poly’s research team managed to bring the temperature of that solid heat storage to 250°C in five hours with a 100W solar panel. They were then able to boil a litre of water in three seconds after sunset. In another test, they stir-fried 1 kg of vegetables in two minutes. The ideal configuration consists of two cooking pots: one with and one without heat storage. Thus, an electric solar cooker can cook both slowly and quickly, depending on the time of day and the dish. ¹⁹

Image: The principle of a solar electric cooker with solid heat storage. Drawing: California Polytechnic State University (Cal Poly).

Thermal or electric?

Like solar water and space heating systems, cooking and cooling can work both with and without electricity – with PV panels on the one hand and solar thermal collectors on the other. But while solar space and water heating are more cost- and energy-efficient without electricity, for solar cooling and solar cooking it is just the opposite.

Space and water heating require relatively small temperature differences, which can be provided by low-cost solar thermal collectors made of glass plates and water pipes. In contrast, cooling and cooking require larger temperature differences, which require more sophisticated (vacuum tube or parabolic) solar collectors – and these are more expensive than PV panels. ²⁰ ²¹

The only exception is a simple solar cooker – an insulated box with a glass top – but it cannot achieve such high temperatures. Moreover, an electric solar cooker has some additional advantages. With a non-electric appliance, you have to cook outside, which is less practical but also less efficient, especially in winter: a thermal solar cooker will lose more heat to the environment. An electric solar cooker is also more energy-efficient because it is insulated on all sides. It also works better in cloudy weather and can be used after sunset. At the Living Energy Farm, the parabolic solar cooker is only used in optimal conditions – at full sun and high outdoor temperatures.

What are the technical challenges?

Although the Living Energy Farm is putting all these applications of direct solar energy into practice, there are some technical challenges for those who want to follow suit. Almost all our modern technology is designed to operate with a stable and uninterrupted power supply. It doesn’t have to be that way, but for now, direct solar power usually requires some tinkering. A direct solar system is much easier to build than an autonomous system with batteries, but it often requires modifications on the appliance side.

Some devices can be connected directly to a solar panel: it is enough to connect the positive and negative contacts of the solar panel and the device. For example, machines with a DC motor tolerate large fluctuations in the power supply. The metal workshop and agricultural machinery at the Living Energy Farm work this way. If clouds block the sun, the combined electrical load can become greater than the power supply from the solar panels, but this does not stop the machines. All the engines will slow down because they share the available energy, but they all continue to do useful work. ¹⁰ ²²

The same applies to all appliances that work on the basis of resistive heating elements, such as kettles, hotplates or electric heating systems. They work regardless of power or voltage, just slower or faster. A direct solar-powered fridge preferably operates on a variable DC compressor, which can adjust its speed according to the varying solar power production. ¹⁰ ²³

Many other devices need a specific and stable voltage input, which usually does not match what the solar panel produces. This can be solved by placing a DC-DC converter (a “buck” or “boost” converter) between the solar panel and the device. This is a small electronic module that converts the fluctuating voltage of a solar panel into a constant output voltage for a low-voltage device (5V, 12V or higher). ²⁴

Image: Experiments with direct solar power. Photo: Marie Verdeil.

If you use an inverter in addition to this, even mains appliances can operate directly on a solar panel. ²⁵ DC-DC converters are essential for all appliances that contain electronic components. This is the case for many appliances today, including those, such as washing machines or coffee machines, that until recently operated without electronics. That often gives you two options to run such appliances on direct solar power. You can either fit a DC-DC converter or modify the appliance by bypassing the electronics.

DIY manuals & commercial devices

Most direct solar power applications operate at low voltage, so you can safely do it yourself. Low-tech Magazine will soon publish a manual on this. However, the Living Energy Farm uses direct current with higher voltages for a number of applications. Examples are the machine tools in the metal workshop (90V) and a number of powerful electric solar cookers (48V, 180V). It is not a good idea to build these systems yourself unless you have the help of a qualified electrician, as these voltages can lead to fatal accidents.

Those wishing to build their own (low-voltage) electric solar cookers will find comprehensive manuals at both Living Energy Farm and Cal Poly. ²⁶ The devices can be made with simple materials. The insulation material should be fireproof. Example materials are rock wool, fibreglass, natural wool or clay.goed

Different technologies can be used for heating elements, but embedding nichrome wires in cement is the simplest option. These wires can be taken from a variety of appliances such as toasters, ovens and hotplates. In principle, the heating wires can be attached directly to the cooking pot, but it is more practical to make a heated “nest” in which a pot can be placed.

Image: Inspired by Cal Poly's work, Living Energy Farm also developed a number of electric solar cookers, one of which they [offer for sale through their website](https://livingenergylights.com/product/roxy-solar-electric-oven/). The Roxy Oven can be used as a hotplate or an oven, for example for baking bread. The door also remains closed when used as a hot plate. This solar cooker has no energy storage.

Image: The Roxy Oven without the door and with the glass wool insulation visible. The device - made in the metal workshop with direct solar power - runs on 48V and requires a solar panel of 200 to 500 watts. Living Energy Farm also offers Sunstar's solar refrigerator [for sale online](https://livingenergylights.com/product/sunstar-direct-drive-8-cuft-chest-style-refrigerator-freezer/).

Does direct solar power waste energy?

The sustainability of a solar installation depends not only on the energy required to produce and maintain the infrastructure, but also on the energy produced by the solar panels during their lifetime. Some people will argue that direct use of solar power is inferior to conventional grid-connected or battery-powered solar installations in this respect.

After all, the hoover, washing machine and power drill are not used every day, and if no electrical appliance is connected then a solar panel will not produce power either. Consequently, the amount of electricity produced by the panel will decrease over its lifetime, while the energy needed to manufacture the panel remains the same. This makes the power from a direct solar panel more carbon-intensive.

However, because energy storage in batteries (or the grid-connected alternative) accounts for such a large proportion of the total energy invested, a standalone solar panel can waste quite a lot of energy before it becomes less sustainable than its counterpart with battery storage or grid connection.

Moreover, direct use of solar power avoids the charging and discharging losses caused by batteries, or the energy losses in the transmission infrastructure for grid-connected systems. Both have to be offset by additional solar panels. Furthermore, solar panels connected to batteries or the grid also waste power – a consequence of the large difference in energy production between summer and winter.

Maximising direct solar power with collective services

Nevertheless, it is important to maximise the energy production of a direct solar panel. In that context, it is useful to return for a moment to the original example system located on my balcony. Direct solar power could be a nice addition to this system, especially for the fridge and cooker. It was because of these appliances that I concluded in 2016 that it was impossible to completely disconnect my flat from the grid.

However, the Living Energy Farm shows that it could be done: there is room for a further 200 watts of solar panels (4 x 50W) on the balcony, enough to power both a thermally insulated fridge and hob. Additional battery capacity would not be needed.

For other appliances, however, direct solar power is of little use in my case. It would not be very efficient to install an extra solar panel for the washing machine or the power drill, as they are only used occasionally. This seems to play into the hands of a “smart” electricity grid, because that way many households can use the same solar power – there is always someone who needs to wash clothes or drill a hole.

However, such a smart grid does require a lot of infrastructure, even if direct solar power were to be used at that scale. It may not require batteries or fossil fuels as backup, but it does require transmission and communication infrastructure.

Image: A record player on direct solar power. Photo: Marie Verdeil. [Watch the video](https://www.youtube.com/watch?v=_LjSigJv0-0).

The Living Energy Farm demonstrates an alternative solution: the communal organisation of household tasks and work. Instead of a communal power grid distributing energy to many indidvidual households, we can set up collective services with decentralised energy production.

In the Living Energy Farm communal workshop, direct solar power can be used much more efficiently than in an individual workshop that is only used occasionally. A collective laundry in each street would also use direct solar power much more efficiently. Moreover, we save a lot of energy on building appliances this way, and gain a lot of space.

Direct wind power?

This strategy becomes even more important if we choose not direct solar power but direct wind power – or a combination of both. The Living Energy Farm is located in a sunny region, but the same approach could also work in windy places.

However, there is an important difference between solar power and wind power. The efficiency of a solar panel does not depend on its size, which makes solar power ideal for decentralised energy production. In contrast, the efficiency of a wind turbine increases more than proportionally as the rotor diameter increases. Much better than one wind turbine per household, therefore, is a somewhat larger wind turbine for a community of households, e.g. for powering a collective laundry or workshop.

The service life of lead-acid batteries depends on many factors. If they are discharged too deeply or are not fully charged regularly, the service life can be shorter than three years. On the other hand, a lead-acid battery that is hardly used or not discharged at all can last much longer than five years. However, the academic literature states a life expectancy of three to five years and this has also been my experience with the batteries I have used since 2016. See, for example, “Optimal Sizing and Life Cycle Assessment of Residential Photovoltaic Energy Systems With Battery Storage”, A. Celik, in “Progress in Photovoltaics: Research and Applications”, 2008. & “Energy pay-back time of photovoltaic energy systems: present status and prospects”, E.A. Alsema, in “Proceedings of the 2nd World Conference and Exhibition on photovoltaics solar energy conversion”, July 1998. ↩︎
Manufacturing a lead-acid battery (based on largely recycled materials) takes about 1 MJ of energy per watt-hour of storage capacity. My 100 amp-hour battery equates to a storage capacity of 1,200 watt-hours, and so the embedded energy equals 1,200 MJ. Over a 30-year lifespan, I need six of these batteries at best, so 7,200 MJ in total. Source: “Energy Analysis of Batteries in Photovoltaic systems. Part one (Performance and energy requirements)” and “Part two (Energy Return Factors and Overall Battery Efficiencies)” (PDF). Energy Conversion and Management 46, 2005. ↩︎
Not much research has been done on the embedded energy of charge controllers. The most relevant data I found is a value of 1 MJ per watt maximum power: Kim, Bunthern, et al. “Life cycle assessment for a solar energy system based on reuse components for developing countries.” Journal of cleaner production 208 (2019): 1459-1468. For a capacity of 120W (my charge controller has a maximum capacity of 10A x 12V = 120W), this amounts to 120 MJ. For the estimated lifetime, I found values of 7 and 12.5 years: same reference as above, as well as: Kim, Bunthern, et al. “Second life of power supply unit as charge controller in PV system and environmental benefit assessment.” IECON 2016-42nd Annual Conference of the IEEE Industrial Electronics Society. IEEE, 2016. I therefore made the calculation on an estimated lifetime of 10 years. ↩︎
Nawaz, I., and G. N. Tiwari. “Embodied energy analysis of photovoltaic (PV) system based on macro-and micro-level.” Energy Policy 34.17 (2006): 3144-3152. According to this widely quoted source, it takes 3,500 MJ to produce 1 m2 of solar panel. My two solar panels together measure 0.65 m2, representing a total energy cost of 2,275 MJ. A more recent literature review puts the energy cost for producing different types of solar panels at between 1,034 and 5,150 MJ/m2. The most recent studies of silicon solar panels in this review put the energy cost at around 1,000 MJ/m2, much lower than the figure I am using. See: Ludin, Norasikin Ahmad, et al. “Prospects of life cycle assessment of renewable energy from solar photovoltaic technologies: A review.” Renewable and Sustainable Energy Reviews 96 (2018): 11-28. ↩︎
Lithium-ion batteries are a lot more expensive than lead-acid batteries, but unlike lead-acid batteries, they can be discharged deeper (up to 15% of their total capacity) and have a longer lifespan (7 to 10 years). Consequently, fewer and smaller batteries are needed. Taking these factors into account, the lifetime cost of the battery is €750, compared with €1,020 for lead-acid batteries. On the other hand, lithium-ion batteries require a more sophisticated and more expensive charge controller: a 10A charge controller costs between 200 and 600 euros, depending on the quality. Assuming a price of 400 euros for the charge controller and a 10-year lifetime for both the battery and the charge controller, battery storage accounts for 95% of the total lifetime cost (a total of 2,070 euros, much more than the total cost for the system with lead-acid batteries). Sources: https://www.lithiumion-batteries.com/products/product/12v-50ah-lithium-ion-battery & https://www.lithiumion-batteries.com/products/12v-lithium-ion-battery-chargers/ ↩︎
Although the production of a lithium-ion battery costs more energy than the production of a lead-acid battery (1.4-1.9 MJ/Wh versus 1 MJ/Wh), this is offset by a longer lifespan and greater discharge capacity. The energy cost of lithium-ion batteries over a 30-year lifetime is then about 3,000 MJ, significantly less than a comparable lead-acid battery system. In contrast, the charge controller contains more complex electronics. Unfortunately, no data is available for the energy cost of such a charge controller. So there is no alternative but to estimate the energy cost based on the financial cost, which is four to twelve times more expensive than a charge controller for a lead-acid battery. Assuming a four times higher cost, the embedded energy of the charge controller increases to 480 MJ, or 1,440 MJ over a 30-year period. The total energy cost for the system is then 6,685 MJ, less than a comparable system with lead-acid batteries. Of this, almost 70% is attributable to battery storage. ↩︎
Nickel-iron batteries are even bigger and heavier than lead-acid batteries and they need regular maintenance. But they can be fully discharged and have a very long service life (20 years). Moreover, they can be used with the same charge controllers as lead-acid batteries. The lifetime cost over 30 years for the battery is €750, cheaper than the six lead-acid batteries of similar capacity. The total lifetime cost for a nickel-iron battery system with 100W solar panels is €1,020, of which 85% goes to energy storage. Unfortunately, nickel-iron batteries are hard to find, especially the smaller models. Sources: https://beyondoilsolar.com/product/nickel-iron-battery-industrial-series/ & https://beyondoilsolar.com/product-category/batteries/nickel-iron/ ↩︎
Actually, the price of solar panels in a somewhat larger solar installation would be proportionally even smaller. This is because solar panels with small sizes (such as 50W) are proportionally more expensive per watt of peak capacity than solar panels with more conventional sizes (from 250W onwards). More or less the same applies to the energy cost. ↩︎
https://livingenergyfarm.org ↩︎
Alexis Zeigler, founder of the Living Energy Farm, wrote a book about the project, which is available in full online: Empowering Communities. A Practical Guide to Energy Self Sufficiency and Stopping Climate Change. It can also be ordered in hard copy. ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎
Since direct solar power does not require a charge controller for each separate system, splitting up a solar system does not involve any additional costs or energy consumption. ↩︎
Research shows that doubling the insulation thickness from 2.5 cm (standard insulation) to 5 cm reduces the annual electricity consumption of a refrigerator (50 litre capacity) from 250 to 125 kilowatt hours. ¹³ With an insulation thickness of 10 to 12.5 cm, electricity consumption halves again to around 60 kilowatt hours per year. Even thicker insulation brings a smaller reduction in electricity consumption and is no longer attractive because thicker insulation also increases the cost and size of the refrigerator. The study concerns a solar-powered AC fridge that operates thanks to an inverter and a battery, which is less energy-efficient than a direct solar-powered fridge. ↩︎
Gupta, B. L., Mayank Bhatnagar, and Jyotirmay Mathur. “Optimum sizing of PV panel, battery capacity and insulation thickness for a photovoltaic operated domestic refrigerator.” Sustainable Energy Technologies and Assessments 7 (2014): 55-67. ↩︎ ↩︎
This thermal mass can literally be a container of water placed inside the fridge. or some water bottles for drinking. But the water can also be stored in reservoirs along the side of the appliance, behind an inner lining that keeps them in place and hides them from view. Water has a higher heat storage density than air, keeping the temperature stable for longer. ↩︎
Ewert, M., et al. “Photovoltaic direct drive, battery-free solar refrigerator field test results.” Proceedings of the solar conference. American solar energy society; American institute of architects, 2002. ↩︎ ↩︎
This advantage only applies if the fridge is set up in an unheated room. The modern habit of placing a fridge in a heated kitchen when the outside temperature in winter is equal or lower than that in the fridge is obviously absurdly wasteful. But neither is this advantage valid in tropical countries, where temperatures are high all year round. ↩︎
The use of direct solar power for space cooling has not been analysed as thoroughly as for domestic refrigerators. See: Luerssen, Christoph, et al. “Life cycle cost analysis (LCCA) of PV-powered cooling systems with thermal energy and battery storage for off-grid applications.” Applied energy 273 (2020): 115145. Moreover, it is unlikely to achieve equally large energy savings. A refrigerator is always insulated, but in the case of an air-cooled room or building, this is not necessarily the case. Moreover, a refrigerator is set up in a room where there is a stable temperature. A building is subject to greater temperature fluctuations and can also be heated by direct solar radiation. So direct solar air cooling is a lot more complicated. See: Qi, Ronghui, Lin Lu, and Yu Huang. “Parameter analysis and optimisation of the energy and economic performance of solar-assisted liquid desiccant cooling system under different climate conditions.” Energy conversion and management 106 (2015): 1387-1395. ↩︎
Solar Electric Cooking, Pete Schwartz, Cal Poly Physics. See also this PowerPoint by the same author. ↩︎
Insulated Solar Electric Cooker with Solid Thermal Storage, Andrew McCombs et al., 2022. See also this video. ↩︎
See: Ferreira, Carlos Infante, and Dong-Seon Kim. “Techno-economic review of solar cooling technologies based on location-specific data.” International Journal of Refrigeration 39 (2014): 23-37. ///// Riffat, James, et al. “Development and testing of a PCM enhanced domestic refrigerator with use of miniature DC compressor for weak/off grid locations.” International Journal of Green Energy 19.10 (2022): 1118-1131. ///// Du, Wenping, et al. “Dynamic energy efficiency characteristics analysis of a distributed solar photovoltaic direct-drive solar cold storage.” Building and Environment 206 (2021): 108324. ///// Alsagri, Ali Sulaiman. “Photovoltaic and photovoltaic thermal technologies for refrigeration purposes: an overview.” Arabian journal for science and engineering 47.7 (2022): 7911-7944. ↩︎
For lack of research, whether the same applies to embedded energy consumption is not clear. ↩︎
In both cases, however, it is necessary to bypass the device’s switch, because DC electricity produces more heat than AC electricity. Instead, a suitable external switch can help, but in doing so you bypass the device’s safety mechanism, which is obviously a risk. ¹⁰ Again, this does not have to be the case: it is technically possible to make devices suitable for direct solar power. ↩︎
A fixed-speed compressor can only use 50% of the solar power produced in a useful way, while a variable-speed compressor uses about 75% in a useful way. ¹⁵ A capacitor is needed to provide the compressor with an energy boost during the start-up phase. ↩︎
Instead of a DC-DC converter, you can also install a small “buffer battery” and a charge controller. Like a DC-DC converter, the charge controller will ensure a stable output voltage. In addition, the small battery can provide limited energy storage that can be useful to handle short spikes in power consumption. For example, some devices have a current spike when charging. The disadvantage of a buffer battery is that the cost and embedded energy increase, and additional components can fail. A capacitor is an alternative technology to absorb power peaks. ↩︎
However, using low-voltage direct current devices is a lot more energy-efficient because solar panels also produce low-voltage direct current: https://qelnixcor.cloud/2016/04/slow-electricity-the-return-of-dc-power/ ↩︎
Insulated Solar Cooker Construction Manual, Living Energy Farm. Insulated solar electric cooker manual, Pete Schwartz, Cal Poly Physics. Roxy Oven Manual, Living Energy Farm. Video presentation manual solar electric cookers, Alexis Zeigler, Living Energy Farm. Video manual for making heating wires. Thermal heat storage: Insulated Solar Electric Cooker with Solid Thermal Storage, Andrew McCombs et al., 2022. Also see this video. ↩︎

Keeping Some of the Lights On: Redefining Energy Security

Sun, 09 Dec 2018 00:00:00 +0000

Maintaining a steady supply of something that’s finite is impossible. Image: [Camilla MP](https://www.flickr.com/photos/dieknochenblume/8454004839/in/photolist-nJrNa3-z9St6d-vicpX8-bjNYMa-CNWajb-PKUbFu-8TqWZX-qzaoch-r3Gb3J-28jYUV3-p3gMD1-snwVj-2chyArN-4ehCVH-cWuLz-dT3Z78-pnFKK9-5qGDSP-hxU2d7-24uoKVs-f7CoCe-93ZqZQ-jPMVaK-T4yoN-4HiX59-97Kq68-23hFdSw-jE59uD-9aFpr7-68DbEo-NvymKZ-335BtT-8RtT65-a6Jut4-nt2zNy-qrkSGP-HPM9ee-bcdyA2-5Fy731-FGSpvq-eqKSpH-8jGFmq-qcFSw4-6USSog-dJEYby-jk3JQ2-7BMzWV-jetX2F-hLnHJy-5SHzAW).

As a society depends more on energy sources for its daily functioning, it becomes more vulnerable if the supply of energy is interrupted. This obvious fact is ignored in current strategies to achieve energy security, making them counter-productive.

What is Energy Security?

What does it mean for a society to have “energy security”? Although there are more than forty different definitions of the concept, they all share the fundamental criterium that energy supply should always meet energy demand. This also implies that energy supply needs to be constant – there can be no interruptions in the service. ¹ ² ³ ⁴ For example, the International Energy Agency (IEA) defines energy security as “the uninterrupted availability of energy sources at an affordable price”, the US Department of Energy and Climate Change (DECC) defines the concept as meaning that “the risks of interruption to energy supply are low”, and the EU defines it as a “stable and abundant supply of energy”. ⁵ ⁶ ⁷

Historically, energy security was achieved by securing access to forests or peat bogs for thermal energy, and to human, animal, wind or water power sources for mechanical energy. With the arrival of the Industrial Revolution, energy security came to depend on the supply of fossil fuels. As a theoretical concept, energy security is most closely related to the oil crises from the 1970s, when embargoes and price manipulations limited oil supply to Western nations. As a result, most industrialised societies still stockpile oil reserves that are equivalent to several months of consumption.

Although oil remains as vital to industrial economies as it was in the 1970s, mainly for transportation and agriculture, it’s now recognised that energy security in modern societies also depends on other infrastructures, such as those supplying gas, electricity, and even data. Furthermore, these infrastructures increasingly interconnect and depend on each other. For example, gas is an important fuel for power production, while the power grid is now required to operate gas pipelines. Power grids are needed to run data networks, and data networks are now needed to run power grids.

Power grids are needed to run data networks, and data networks are needed to operate power grids.

This article investigates the concept of energy security by focusing on the power grid, which has become just as vital to industrial societies as oil. Moreover, electrification is seen as a way to decrease dependency on fossil fuels – think electric vehicles, heat pumps, and wind turbines.

The “security” or “reliability” of a power grid can be measured precisely by indicators of continuity such as the “Loss-of-Load Probability” (LOLP), and the “System Average Interruption Duration Index” (SAIDI). Using these indicators, one can only conclude that power grids in industrial societies are very secure. For example, in Germany, power is available for 99.996% of the time, which corresponds to an interruption in service of less than half an hour per customer per year. ⁸ Even the worst performing countries in Europe (Latvia, Poland, Lithuania) have supply shortages of only eight hours per customer per year, which corresponds to a reliability of 99.90%. ⁸ The US power grid is in between these values, with supply interruptions of less than four hours per customer per year (99.96% reliability). ⁹

How Secure is a Renewable Power Grid?

In the current operation of infrastructures, the paradigm is that consumers could and should have access to as much electricity, gas, oil, data or water as they want, anytime they want it, for as long as they want it. The only requirement is that they pay the bill. Looking at the power sector, this vision of energy security is quite problematic, for several reasons. First of all, most energy sources from which electricity is made are finite – and maintaining a steady supply of something that’s finite is of course impossible. In the long run, the strategy to maintain energy security is certainly doomed to fail. In the shorter term, it may disrupt the climate and provoke armed conflicts.

The International Energy Agency (IEA), which was set up following the first oil crisis in the early 1970s, encourages the use of renewable energy sources in order to diversify the energy supply and improve energy security in the long term. A renewable power system is not dependent on foreign energy imports nor vulnerable to fuel price manipulations – which are the main worries in an energy infrastructure that is largely based on fossil fuels. Of course, solar panels and wind turbines have limited lifetimes and need to be manufactured, which also requires resources that could come from abroad or which can become depleted. But, once they are installed, renewable power systems are “secure” in a way and for a period of time that fossil fuels (and atomic energy) are not.

Renewable energy sources pose fundamental challenges to the current understanding of energy security

Furthermore, solar and wind power provide more security concerning physical failure or sabotage, even more so when renewable power production is decentralised. Renewable power plants also have lower CO2-emissions, and the extreme weather events caused by climate change are a risk to energy security as well. However, in spite of all these advantages, renewable energy sources pose fundamental challenges to the current understanding of energy security. Most importantly, the renewable energy sources with the largest potential – sun and wind – are only intermittently available, depending on the weather and the seasons. This means that solar and wind power don’t match the criterium that all definitions of energy security consider to be essential: the need for an uninterrupted, unlimited supply of power.

Image: [Eduard Bezembinder](https://www.flickr.com/photos/bezembinder/3560945758/in/photolist-6qEM7w-7urQui-iSeKZ-8VjqeD-dUgKQ-e4ybCy-eke2Zk-ekeCdc-eke4NV-qBE1z-6Dfw5n-68EJKh-ekk6Rs-qBE2V-NqkS-oWp8Du-psYQc1-pCDop-5JSFFH-9fr321-oguPbE-6pZ6MT-dZ9YLx-vhpHJb-3oeLdu-69J2h1-7hatWp-d26CpQ-27dVzAC-5BEpZz-sUBfz-7B8zeq-HkygG-bHhG5R-2UoYjD-bRCZnx-o1e2oL-4LcBmy-69vhwD-ekz9ec-bLqreV-5jtvAp-2GUCLK-GpCny7-s36gn-dy6aBU-8moRHP-8rrRxd-5BJJyC-8KdmGR).

The reliability of a power grid with a high share of solar and wind power would be significantly below today’s standards for continuity of service. ¹⁰ ¹¹ ¹² ¹³ ¹⁴ In such a renewable power grid, a 24/7 power supply can only be maintained at very high costs, because it requires an extensive infrastructure for energy storage, power transmission, and excess generation capacity. This additional infrastructure risks making a renewable power grid unsustainable, because above a certain threshold, the fossil fuel energy used for building, installing and maintaining this infrastructure becomes higher than the fossil fuel energy saved by the solar panels and the wind turbines.

Renewable energy sources like wind and sun have advantages that current definitions of energy security don’t capture

Intermittency is not the only disadvantage of renewable energy sources. Although many media and environmental organisations have painted a picture of solar and wind power as abundant sources of energy (“The sun delivers more energy to Earth in an hour than the world consumes in a year”), reality is more complex. The “raw” supply of solar (and wind) energy is enormous indeed. However, because of their very low power density, to convert this energy supply into a useful form solar panels and wind turbines require magnitudes of order more space and materials compared to thermal power plants – even if the mining and distribution of fuels is included. ¹⁵ Therefore, a renewable power grid cannot guarantee that consumers have access to as much electricity as they want, even if the weather conditions are optimal.

How Secure is an Off-the-Grid Power System?

Today’s energy policies related to electricity try to reconcile three aims: an uninterrupted and limitless supply of power, affordability of electricity prices, and environmental sustainability. A power grid that is mainly based on fossil fuels and atomic energy cannot achieve the aim of environmental sustainability, and it can only achieve the other goals as long as foreign suppliers do not cut off supplies or raise energy prices (or as long as national or international reserves are not depleted).

However, a renewable power grid cannot reconcile these three goals either. To achieve an unlimited 24/7 supply of power, the infrastructure needs to be oversized, which makes it expensive and unsustainable. Without that infrastructure, a renewable power grid could be affordable and sustainable, but it could never offer an unlimited 24/7 supply of power. Consequently, if we want a power infrastructure that is affordable and sustainable, we need to redefine the concept of energy security – and question the criterium of an unlimited and uninterrupted power supply.

If we look beyond the typical large-scale central infrastructures in industrial societies, it becomes clear that not all provisioning systems offer a limitless supply of resources. Off-the-grid microgeneration – the local production and storage of electricity using batteries and solar PV panels or wind turbines – is one example. In principle, off-the-grid systems can be sized in such a way that they are “always on”. This can be done by following the “worst-month method”, which oversizes generation and storage capacity so that supply can meet demand even during the shortest and darkest days of the year.

Matching supply to demand at all times makes an off-the-grid system very costly and unsustainable, especially in high seasonality climates

However, just like in an imaginary large-scale renewable power grid, matching supply to demand at all times makes an off-the-grid system very costly and unsustainable, especially in high seasonality climates. ¹⁶ ¹⁷ ¹⁸ Therefore, most off-the-grid systems are sized according to a method that aims for a compromise between reliability, economic cost and sustainability. The “loss-of-load probability sizing method” specifies a number of days per year that supply does not match demand. ¹⁹ ²⁰ ²¹ In other words, the system is sized, not only according to a projected energy demand, but also according to the available budget and/or the available space.

Image: [Stephen Yang / The Solutions Project](https://www.flickr.com/photos/149368236@N06/33068752693/in/photolist-Sob15v-bBnpyx-keyKG-cuaVX3-nuP1zk-U2eVh7-cuaWEf-pskKMf-cuaswE-p27cJW-cu9SQu-cuaMky-mCLFCt-ajiCfB-4AFrsp-943usV-TyoqrN-pu9HK-erKVcJ-aYHgDT-7zrUXc-tQv77b-6xot6g-baF4gg-Xjymka-qHgAkg-ii2jys-9eD7tj-9fJDFi-Ge2Mn-guUowg-amvdKB-cvDZ15-79wfLn-c6XjSS-ddFjjF-9KYuQV-8Zp8z6-guV3wK-9P1nHp-q5c2cz-9RCRVu-cD8w4d-9YDNzC-7ehy1e-4obYkG-8tkNMS-cvDZru-4obYtN-23Aqhr).

Sizing an off-the-grid power system in this way generates significant cost reductions, even if “reliability” is reduced just a little bit. For example, a calculation for an off-the-grid house in Spain shows that decreasing the reliability from 99.75% to 99.00% produces a 60% cost reduction, with similar benefits for sustainability. Supply would be interrupted for 87.6 hours per year, compared to 22 hours in the higher reliability system. ¹⁶

According to the current understanding of energy security, off-the-grid power systems that are sized in this way are a failure: energy supply doesn’t always meet energy demand. However, off-gridders don’t seem to complain about a lack of energy security, on the contrary. There’s a simple reason for this: they adapt their energy demand to a limited and intermittent power supply.

In their 2015 book Off-the-Grid: Re-Assembling Domestic Life, Phillip Vannini and Jonathan Taggart document their travels across Canada to interview about 100 off-the-grid households. ²² Among their most important observations is that voluntary off-gridders use less electricity overall and routinely adapt their energy demand to the weather and the seasons.

Voluntary off-gridders use less electricity overall and routinely adapt their energy demand to the weather and the seasons.

For example, washing machines, vacuum cleaners, power tools, toasters or videogame consoles are not used at all, or they are only used during periods of abundant energy, when batteries can accommodate no further charge. If the sky is overcast, off-gridders act differently to draw less power and have some more left over for the day after. Vannini and Taggart also observe that voluntary off-gridders seem to feel perfectly happy with levels of lighting or heating that are different from the standards that many in the western world have come to expect. Often, this shows itself in concentrating activities around more localised sources of heat and light. ²²

Similar observations can be made in places where people – involuntarily – depend on infrastructures that are not always on. If centralised water, electricity and data networks are present in less industrialised countries, they are often characterised by regular and irregular interruptions in the supply. ²³ ²⁴ ²⁵ However, in spite of the very low reliability of these infrastructures – according to common indicators of continuity – life goes on. Daily household routines are shaped around disruptions of supply systems, which are viewed as normal and a largely accepted part of life. For example, if electricity, water or Internet are only available during certain times of the day, household taks or other activities are planned accordingly. People also use less energy overall: the infrastructure simply doesn’t allow for a resource-intensive lifestyle. ²³

More Reliable, Less Secure?

The very high “reliability” of power grids in industrial societies is justified by calculating the “value of lost load” (VOLL), which compares the financial loss due to power shortages to the extra investment costs to avoid these shortages. ¹¹⁰ ²⁶ ²⁷ ²⁸ ²⁹ However, the value of lost load is highly dependent on how society is organised. The more it depends on electricity, the higher the financial losses due to power shortages will be.

Current definitions of energy security consider supply and demand to be unrelated, and focus almost entirely on securing energy supply. However, alternative forms of power infrastructures like those described above show that people adapt and match their expectations to a power supply that is limited and not always on. In other words, energy security can be improved, not just by increasing reliability, but also by reducing dependency on energy.

Image: Natural gas storage terminal. [Jason Woodhead](https://www.flickr.com/photos/woodhead/7150825737/in/photolist-bTTRmV-85JomL-jysSQn-fw7gTZ-5Jkm2T-eDueWy-ohYc4x-fFxZCm-eD8VG8-eDfhqy-8pCnxZ-qPTdqx-22WNtVf-fFybmb-fFxRVG-fFyhCf-mGNU1p-24mDPG2-8efS2s-fFguSX-nN4pMi-fFgpjT-6br69i-hVGdgU-9DSQQ5-cDwVt-EqVP-dp7vJX-fwmwQh-oHAfHH-fFy6QS-fFgvS8-aaCofJ-fFxW5L-agEkAL-eDfonE-fFgrrn-eD9m9a-PLLffy-fFggcX-fFgka6-nRdzs-fFgwFH-88JrU8-nN4epz-2atchc9-nN523B-24mDNL4-2atciAb-GFzRM).

Demand and supply are also interlinked, and mutually influence each other, in 24/7 power systems – but with the opposite effect. Just like “unreliable” off-the-grid power infrastructures foster lifestyles that are less dependent on electricity, “reliable” infrastructures foster lifestyles that are increasingly dependent on electricity.

Industrial societies with “reliable” power grids are in fact the weakest and most fragile in the face of supply interruptions

In their 2018 book Infrastructures and Practices: the Dynamics of Demand in Networked Societies, Olivier Coutard and Elizabeth Shove argue that an unlimited and uninterrupted power supply has enabled people in industrial societies to adopt a multitude of power dependent technologies – such as washing machines, air conditioners, refrigerators, automatic doors, or 24/7 mobile internet access – which become “normal” and central to everyday life. At the same time, alternative ways of doing things – such as washing clothes by hand, storing food without electricity, keeping cool without air-conditioning, or navigating and communicating without mobile phones – have withered away, or are withering away. ³⁰

As a result, energy security is in fact higher in off-the-grid power systems and “unreliable” central power infrastructures, while industrial societies are the weakest and most fragile in the face of supply interruptions. What is generally assumed to be a proof of energy security – an unlimited and uninterrupted power supply – is actually making industrial societies ever more vulnerable to supply interruptions: people increasingly lack the skills and the technology to function without a continuous power supply.

Redefining Energy Security

To arrive to a more accurate definition of energy security requires the concept to be defined, not in terms of commodities like kilowatt-hours of electricity, but in terms of energy services, social practices, or basic needs. ¹ People don’t need electricity in itself. What they need, is to store food, wash clothes, open and close doors, communicate with each other, move from one place to another, see in the dark, and so on. All these things can be achieved either with or without electricity, and in the first case, with more or less electricity.

Defined in this way, energy security is not just about securing the supply of electricity, but also about improving the resilience of the society, so that it becomes less dependent on a continuous supply of power. This includes the resilience of people (do they have the skills to do things without electricity?), the resilience of devices and technological systems (can they handle an intermittent power supply?), and the resilience of institutions (is it legal to operate a power grid that is not always on?). Depending on the resilience of the society, a disruption of the power supply may or may not lead to a disruption of energy services or social practices.

For example, although our food distribution system is dependent on a cold chain that requires a continuous power supply, there are many alternatives. We could adapt refrigerators to an irregular power supply by insulating them much better, we could reintroduce cold cellars (which keep food fresh without electricity), or we could relearn older methods of food storage, like fermentation. We could also improve people’s skills in terms of fresh cooking, switch to diets based on ingredients that don’t need cold storage, and encourage local daily shopping over weekly trips to large supermarkets.

To improve energy security, we need to make infrastructures less reliable.

If we look at energy security in a more holistic way, taking into account both supply and demand, it quickly becomes clear that energy security in industrial societies continues to deteriorate. We keep delegating more and more tasks to machines, computers and large-scale infrastructures, thus increasing our dependency on electricity. Furthermore, the Internet is becoming just as essential as the power grid, and trends like cloud computing, the Internet of Things, and self-driving cars are all based on several interconnected layers of continuously operating infrastructures.

Image: An abandoned power line. [Miura Paulison](https://www.flickr.com/photos/paulisson_miura/10318768955/in/photolist-gHQovz-kCLi9r-82pqq6-f4539G-6i3Aih-5m5G9b-6RkZvr-6V6k85-2b9wdNP-4DvxJx-WfvmJT-5CGLgF-5C1ojh-eANWrM-kjDG4Z-9QKWz-DnnTH9-ntvKWL-82sxbf-UssMS3-deJRBD-d6qh1S-5C1ooU-tkcYLj-MpbqCB-84zF9u-5CM5d7-5CM51J-82ppX6-a1H2sr-Rd9o59-a1LEed-6W3He9-VCD56X-bg3vgT-5BW5CT-82sxDb-2b1hTxi-6hpZ1g-8d19tj-qm9Cy-cgpx3-gszM15-eANtbt-MpbCWK-98h2dj-7HyrGe-5md8aD-d9fLdq-2cyGoSv).

Because demand and supply influence each other, we come to a counter-intuitive conclusion: to improve energy security, we need to make the power grid less reliable. This would encourage resilience and substitution, and thus make industrial societies less vulnerable to supply interruptions. Coutard and Shove argue that “it would make sense to pay more attention to opportunities for innovation that are opened when large network systems are weakened and abandoned, or when they become less reliable”. They add that the experiences of voluntary off-gridders “provide some insights into the types of configuration at stake”. ³⁰

Arguing for a less reliable power supply is sure to be controversial. In fact, “Keeping the lights on” is a phrase that is often used to justify energy reforms such as building more atomic plants, or keeping them in operation past their planned lifetimes. To achieve real energy security, “keeping the lights on” should be replaced by phrases like “keeping some of the lights on”, “which lights should we turn off next?”, or “what’s wrong with a bit more dark?”. ³¹ Obviously, a less reliable energy supply would bring fundamental changes to routines and technologies, whether it is in households, factories, transport systems, or communications networks – but that’s exactly the point. Present ways of life in industrial societies are simply not sustainable.

This article was originally written for the UK Demand Centre.

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Pillai, N. “Loss of Load Probability of a Power System.” (2008). https://mpra.ub.uni-muenchen.de/6953/1/MPRA_paper_6953.pdf ↩︎
Al-Rubaye, Mohannad Jabbar Mnati, and Alex Van den Bossche. “Decades without a real grid: a living experience in Iraq.” International Conference on Sustainable Energy and Environment Sensing (SEES 2018). 2018. https://biblio.ugent.be/publication/8566224 ↩︎
Telson, Michael L. “The economics of alternative levels of reliability for electric power generation systems.” The Bell Journal of Economics (1975): 679-694. https://www.jstor.org/stable/3003250?seq=1#page_scan_tab_contents ↩︎
Schröder, Thomas, and Wilhelm Kuckshinrichs. “Value of lost load: an efficient economic indicator for power supply security? A literature review.” Frontiers in energy research 3 (2015): 55. https://www.frontiersin.org/articles/10.3389/fenrg.2015.00055/full ↩︎
Ratha, Anubhav, Emil Iggland, and Goran Andersson. “Value of Lost Load: How much is supply security worth?.” Power and Energy Society General Meeting (PES), 2013 IEEE. IEEE, 2013. https://www.ethz.ch/content/dam/ethz/special-interest/itet/institute-eeh/power-systems-dam/documents/SAMA/2012/Ratha-SA-2012.pdf ↩︎
De Nooij, Michiel, Carl Koopmans, and Carlijn Bijvoet. “The value of supply security: The costs of power interruptions: Economic input for damage reduction and investment in networks.” Energy Economics 29.2 (2007): 277-295. https://s3.amazonaws.com/academia.edu.documents/40102922/The_Value_of_Supply_Security_The_Costs_o20151117-24458-1eo081r.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1544213977&Signature=d01qoyIcopj1rE5HpSWkCGcQzRk%3D&response-content-disposition=inline%3B%20filename%3DThe_value_of_supply_security.pdf ↩︎
Coutard, Olivier, and Elizabeth Shove. “Infrastructures, practices and the dynamics of demand.” Infrastructures in Practice. Routledge, 2018. 10-22. https://www.routledge.com/Infrastructures-in-Practice-The-Dynamics-of-Demand-in-Networked-Societies/Shove-Trentmann/p/book/9781138476165 ↩︎ ↩︎
Demand Dictionary of Phrase and Fable, seventeenth edition. Jenny Rinkinen, Elizabeth Shove, Greg Marsden, The Demand Centre, 2018. http://www.demand.ac.uk/wp-content/uploads/2018/07/Demand-Dictionary.pdf ↩︎

Ditch the Batteries: Off-Grid Compressed Air Energy Storage

Wed, 16 May 2018 00:00:00 +0000

Small-scale compressed air energy storage. Image in the public domain.

Going off-grid? Think twice before you invest in a battery system. Compressed air energy storage is the sustainable and resilient alternative to batteries, with much longer life expectancy, lower life cycle costs, technical simplicity, and low maintenance. Designing a compressed air energy storage system that combines high efficiency with small storage size is not self-explanatory, but a growing number of researchers show that it can be done.

Compressed Air Energy Storage (CAES) is usually regarded as a form of large-scale energy storage, comparable to a pumped hydropower plant. Such a CAES plant compresses air and stores it in an underground cavern, recovering the energy by expanding (or decompressing) the air through a turbine, which runs a generator.

Unfortunately, large-scale CAES plants are very energy inefficient. Compressing and decompressing air introduces energy losses, resulting in an electric-to-electric efficiency of only 40-52%, compared to 70-85% for pumped hydropower plants, and 70-90% for chemical batteries.

The low efficiency is mainly since air heats up during compression. This waste heat, which holds a large share of the energy input, is dumped into the atmosphere. A related problem is that air cools down when it is decompressed, lowering electricity production and possibly freezing the water vapour in the air. To avoid this, large-scale CAES plants heat the air prior to expansion using natural gas fuel, which further deteriorates the system efficiency and makes renewable energy storage dependent on fossil fuels.

Why Small-scale CAES?

In the previous article, we outlined several ideas — inspired by historical systems — that could improve the efficiency of large-scale CAES plants. In this article, we focus on the small but growing number of engineers and researchers who think that the future is not in large-scale compressed air energy storage, but rather in small-scale or micro systems, using man-made, aboveground storage vessels instead of underground reservoirs. Such systems could be off-the-grid or grid-connected, either operating by themselves or alongside a battery system.

The main reason to investigate decentralised compressed air energy storage is the simple fact that such a system could be installed anywhere, just like chemical batteries. Large-scale CAES, on the other hand, is dependent on a suitable underground geology. Although there are more potential sites for large-scale CAES plants than for large-scale pumped hydropower plants, finding appropriate storage caverns is not as easy as was previously assumed.¹²³

Experimental set-up of small-scale compressed air energy storage system. Source: [^27]

Compared to chemical batteries, micro-CAES systems have some interesting advantages. Most importantly, a distributed network of compressed air energy storage systems would be much more sustainable and environmentally friendly. Over their lifetimes, chemical batteries store only two to ten times the energy needed to manufacture them. ⁴ Small-scale CAES systems do much better than that, mainly because of their much longer lifespan.

Compared to chemical batteries, a distributed network of compressed air energy storage systems would be much more sustainable and environmentally friendly

Furthermore, they do not require rare or toxic materials, and the hardware is easily recyclable. In addition, decentralised compressed air energy storage doesn’t need high-tech production lines and can be manufactured, installed and maintained by local business, unlike an energy storage system based on chemical batteries. Finally, micro-CAES has no self-discharge, is tolerant of a wider range of environments, and promises to be cheaper than chemical batteries. ⁵

Although the initial investment cost is estimated to be higher than that of a battery system (around $10,000 for a typical residential set-up), and although above-ground storage increases the costs in comparison to underground storage (the storage vessel is good for roughly half of the investment cost), a compressed air energy storage system offers an almost infinite number of charge and discharge cycles. Batteries, on the other hand, need to be replaced every few years, which makes them more expensive in the long run. ⁵⁶

Challenge: Limiting Storage Size

However, decentralised CAES also faces important challenges. The first is the system efficiency, which is a problem in large- and small-scale systems alike, and the second is the size of the storage vessel, which is especially problematic for small-scale CAES systems.

Both issues make small-scale CAES systems unpractical. Sufficient space for a large storage vessel is not always available, while a low storage efficiency requires a larger solar PV or wind power plant to make up for that loss, raising the costs and lowering the sustainability of the system.

To make matters worse, system efficiency and storage size are inversely related: improving one factor is often at the expense of the other. Increasing the air pressure minimizes the storage size but decreases the system efficiency, while using a lower pressure makes the system more energy efficient but results in a larger storage size. Some examples help illustrate the problem.

Compressed air energy storage tanks. [Source](http://www.screwtypeaircompressors.com/sale-8108163-vertical-compressed-air-tank-natural-gas-tank-2000l-air-receiver-tank.html).

A simulation for a stand-alone CAES aimed at unpowered rural areas, and which is connected to a solar PV system and used for lighting only, operates at a relatively low air pressure of 8 bar and obtains a round-trip efficiency of 60% – comparable to the efficiency of lead-acid batteries. ⁷

However, to store 360 Wh of potential electrical energy, the system requires a storage reservoir of 18 m3, the size of a small room measuring 3x3x2 metres. The authors note that “although the tank size appears very large, it still makes sense for applications in rural areas”.

System efficiency and storage size are inversely related: improving one factor is often at the expense of the other.

Such a system may indeed be beneficial in this context, especially because it has a much longer lifespan than chemical batteries. However, a similar configuration in an urban context with high energy use is obviously problematic. In another study, it was calculated that it would take a 65 m3 air storage tank to store 3 kWh of energy. This corresponds to a 13 metre long pressure vessel with a diameter of 2.5 metres, shown below. ⁸

Furthermore, average household electricity use per day in industrialised countries is much higher still. For example, in the UK it’s slightly below 13 kWh per day, in the US and Canada it’s more than 30 kWh. In the latter case, ten such air pressure tanks would be required to store one day of electricity use.

Small-scale CAES systems with high pressures give the opposite results. For example, a configuration modelled for a typical household electrical use in Europe (6,400 kWh per year) operates at a pressure of 200 bar (almost 4 times higher than the pressure in large-scale CAES plants) and achieves a storage volume of only 0.55 m3, which is comparable to batteries. However, the electric-to-electric efficiency of this set-up is only 11-17%, depending on the size of the solar PV system. ⁹

Two Strategies to Make Micro CAES work

These examples seem to suggest that compressed air energy storage makes no sense as a small-scale energy storage system, even with a reduction in energy demand. However, perhaps surprisingly to many, this is not the case.

Small-scale CAES systems cannot follow the same approach as large-scale CAES systems, which increase storage capacity and overall efficiency by using multi-stage compression with intercooling and multi-stage expansion with reheating. This method involves additional components and increases the complexity and cost, which is impractical for small-scale systems.

The same goes for “adiabatic” processes (AA-CAES), which aim to use the heat of compression to reheat the expanding air, and which are the main research focus for large-scale CAES. For a micro-CAES system, it’s very important to simplify the structure as much as possible. ⁵¹⁰

This leaves us with two low-tech strategies that can be followed to achieve similar storage capacity and energy efficiency as lead-acid batteries. First, we can design low pressure systems which minimize the temperature differences during compression and expansion. Second, we can design high pressure systems in which the heat and cold from compression and expansion are used for household applications.

Small-scale, High Pressure

Small-scale compressed air energy storage systems with high air pressures turn the inefficiency of compression and expansion into an advantage. While large-scale AA-CAES aims to recover the heat of compression with the aim of maximizing electricity production, these small-scale systems take advantage of the temperature differences to allow trigeneration of electrical, heating and cooling power. The dissipated heat of compression is used for residential heating and hot water production, while the cold expanding air is used for space cooling and refrigeration. Chemical batteries can’t do this.

Small-scale, high pressure systems use the dissipated heat of compression for residential heating and hot water production, while the cold expanding air is used for space cooling and refrigeration.

In these systems, the electric-to-electric efficiency is very low. However, there are now several efficiencies to define, because the system also supplies heat and cold. ¹⁰¹¹ Furthermore, this approach can make several electrical appliances unnecessary, such as the refrigerator, the air-conditioning, and the electric boiler for space and water heating. Since the use of these appliances is often responsible for roughly half of the electricity use in an average household, a small-scale CAES system with high pressure has lower electricity demand overall.

A typical air compressor. [Source](https://www.thomasnet.com/articles/machinery-tools-supplies/Air-Compressors).

High pressure systems easily solve the issue of storage size. As we have seen, a higher air pressure can greatly reduce the size of a compressed air storage vessel, but only at the expense of increased waste heat. In a small-scale system that takes advantage of temperature differences to provide heating and cooling, this is advantageous. Therefore, high pressure systems are ideal for small-scale residential buildings, where storage space is limited and where there is a large demand for heat and cold as well as electricity. The only disadvantages are that high pressure systems require stronger and more expensive storage tanks, and that extra space is required for heat exchangers.

Experimental set-up of a micro CAES system. Source: [^30]

Several research groups have designed, modeled and built small-scale combined heat-and-power CAES units which provide heating and cooling as well as electricity. The high pressure system with a storage volume of only 0.55 m3 that we mentioned earlier, is an example of this type of system. ⁹ As noted, its electrical efficiency is only 11-17%, but the system also produces sufficient heat to produce 270 litres of hot water per day. If this thermal source of energy is also taken into account, the “exergetic” efficiency of the whole system is close to 70%. Similar “exergy” efficiencies can be found in other studies, with systems operating at pressures between 50 and 200 bar. ¹¹¹²

Heat and cold from compression and expansion can be distributed to heating or cooling devices by means of water or air. The setup of an air cycle heating and cooling system is very similar to a CAES system, except for the storage vessel. Air cycle heating and cooling has many advantages, including high reliability, ease of maintenance, and the use of a natural refrigerant, which is environmentally benign. ¹¹

Small-scale, Low Pressure

The second strategy to achieve higher efficiencies and lower storage volumes is exactly the opposite from the first. Instead of compressing air to a high pressure and taking advantage of the heat and cold from compression and expansion, a second class of small-scale CAES systems is based on low pressures and “near-isothermal” compression and expansion.

Below air pressures of roughly 10 bar, the compression and expansion of air exhibit insignificant temperature changes (“near-isothermal”), and the efficiency of the energy storage system can be close to 100%. There is no waste heat and consequently there is no need to reheat the air upon expansion.

Isothermal compression requires the least amount of energy to compress a given amount of air to a given pressure. However, reaching an isothermal process is far from reality. To start with, it only works with small and/or slowly cycling compressors and expanders. Unfortunately, typical industrial compressors are not made for maximum efficiency but for maximum power and thus work under fast-cycling, non-isothermal conditions. The same goes for most industrial expanders. ¹³¹⁴

Below air pressures of 10 bar, compression and expansion of air exhibit insignificant temperature changes and the efficiency can be close to 100%.

The use of industrial compressors and expanders explains in large part why the low pressure CAES systems mentioned at the beginning of this article have such large storage vessels. Both systems are based on devices which are operated outside of their optimal or rated conditions. ¹⁵ Because inefficiencies multiply during energy conversions, even relatively small differences in the efficiency of compressors and expanders can have large effects. For example, a variation in device efficiency from 60% to 80% results in a system efficiency from 36% to 64%, respectively.

New Types of Compressors and Expanders

Because the performance of a compressor and an expander significantly impact the overall efficiency of a small-scale CAES system, several researchers have built their own compressors and expanders, which are especially aimed at energy storage. For example, one team designed, built and examined a single-stage, low power isothermal compressor that uses a liquid piston. ¹³ It operates at a very low compression rate (between 10-60 rpm), which correspond to the output of solar PV panels, and limits temperature fluctuation during compression and expansion to 2 degrees Celsius.

The low-cost device has minimum moving parts and obtains efficiencies of 60-70% at 3 to 7 bar pressure. ¹³ This is a very high efficiency for such a simple device, considering that a sophisticated three-stage centrifugal compressor, used in large-scale CAES systems or in industrial settings, is roughly 70% efficient. Furthermore, the researchers state that the efficiency is limited by the off-the-shelf motor that they use to power their compressor. Indeed, another research team achieved 83% efficiency. ¹⁶

A scroll compressor. Source: [^30]

Another novelty is the use of scroll compressors, which are the types of compressors that are now used in refrigerators, air-conditioning systems, and heat pumps. Both fluid piston and scroll compressors have a high area-to-volume ratio, which minimizes heat production, and can easily handle two-phase flow, which means that they can also be used as expanders. They are also lighter and less noisy than typical reciprocating compressors. ¹⁴

Varying Air Pressure

Although compressors and expanders are the most important determinants of system efficiency in small-scale CAES systems, they are not the only ones. For example, in every compressed air energy storage system, additional efficiency loss is caused by the fact that during expansion the storage reservoir is depleted and therefore the pressure drops. Meanwhile, the input pressure for the expander is required to vary only in a minimal range to assure high efficiency.

Image: air pressure meter.

This is usually solved in two ways, although neither is really satisfactory. First, air can be stored in a tank with surplus pressure, after which it is throttled down to the required expander input pressure. However, this method — which is used in large-scale CAES — requires additional energy use and thus introduces inefficiency. Second, the expander can operate at variable conditions, but in this case efficiency will drop along with the pressure while the storage is emptied.

During expansion the storage reservoir is depleted and therefore the pressure drops.

With these problems in mind, a team of researchers combined a small-scale CAES with a small-scale pumped hydropower plant, resulting in a system that maintains a steady pressure during the complete discharge of the storage reservoir. It consists of two compressed air tanks that are connected by a pipe attached to their lower portions: each of these have separate spaces for air (below) and water storage (above). The configuration maintains a head of water by means of a pump, which consumes 15% of the generated power. However, in spite of this extra energy use, the researchers managed to increase both the efficiency and the energy density of the system. ¹¹

Off-the-Grid Power Storage

To give an idea of what a combination of the right components can achieve, let’s have a look at a last research project. ¹⁷ It concerns a system that is based on a highly efficient, custom-made compressor/expander, which is directly coupled to a DC motor/generator. Apart from its efficient components, this CAES project also introduces an innovative system configuration. It doesn’t use one large air storage tank, but several smaller ones, which are interconnected and computer-controlled.

The setup consists of the compression/expansion unit coupled to three small (7L) cylinders, previously used as air extinguishers, and operates at low pressure (max 5 bar). The storage vessels are connected via PVC pipework and brass fittings. To control the air-flow, three computer-controlled air valves are installed at the inlet of each cylinder. The system can be extended by adding more pressure vessels. ¹⁷

Small-scale CAES with modular storage tanks. Image by Abdul Hai Al-Alami.

A modular configuration results in a higher system efficiency and energy density for mainly two reasons. First, it helps more effective heat transfer to take place, because every air tank acts as an additional heat exchanger. Second, it allows better control over the discharge rate of the storage reservoir. The cylinders can be discharged either in unison to satisfy a demand for high power density (more power at the cost of a shorter discharge time), or they can be discharged sequentially to satisfy a demand for high energy density (longer discharge time at the cost of maximum power).

By discharging modular storage cylinders sequentially, the discharge time can be greatly increased, making the system comparable to lead-acid batteries in terms of energy density.

By discharging the cylinders sequentially, the discharge time can be greatly increased, making the system comparable to lead-acid batteries in terms of energy density. Based on their experimental set-up, the researchers calculated the efficiencies for different starting pressures and numbers of cylinders. They found that 57 interconnected cylinders of 10 litre each, operating at 5 bar, could fulfill the job of four 24V batteries for 20 consecutive hours, all while having a surprisingly small footprint of just 0.6 m3.

Interestingly, the storage capacity is 410 Wh, which is comparable to the 360 Wh rural system noted earlier, which requires an 18 m3 storage vessel — that’s thirty times larger than the modular storage system.

Computer-controlled air valves. [Source](http://www.jaksa.si/compressed-air-solenoid-valves.html).

The electric-to-electric efficiency for the 3-cylinder set-up reached a peak of 85% at 3 bar pressure, while the estimated efficiency for the 57-cylinder set-up is 75%. These are values comparable to lithium-ion batteries, but adding more storage vessels or operating at higher pressures introduces larger losses due to compression, heat, friction and fittings. ¹⁷¹⁸

Nevertheless, when I e-mailed Abdul Alami, the main author of the study, thinking that the results sounded too good to be true, he told me that the figures were actually overly conservative: “We stuck to low pressures to achieve near-isothermal compression and to ensure safe operation. Operating at pressures higher than 10 bar would create serious thermal losses, but a pressure of 7-8 bar may be beneficial in terms of energy and power density, though maybe not in terms of efficiency.”

Build it Yourself?

In conclusion, small-scale compressed air energy storage could be a promising alternative to batteries, but the research is still in its early stages — the first study on small-scale CAES was published in 2010 — and new ideas will continue to shed light on how best to develop the technology. At the moment, there are no commercial products available, and setting up your own system can be quite intimidating if you are new to pneumatics. Simply getting hold of the right components and fittings is a headache, as these come in a bewildering variety and are only sold to industries.

However, if you’re patient and not too unhandy, and if you are determined to use a more sustainable energy storage system, it is perfectly possible to build your own CAES system. As the examples in this article have shown, it’s just a bit harder to build a good one.

There’s more ideas for small-scale CAES systems in the previous article: History and Future of the Compressed Air Economy.

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There is increasing competition for potential CAES geologic units, as many are also well suited to the storage of natural gas or sequestered carbon. Furthermore, cavern storage imposes harsh requirements on the geographical conditions. For example, the originally planned Iowa CAES project in the US was terminated due to its porous sandstone condition. ² ↩︎
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The small-scale system aimed at urban environments, which has a storage reservoir of 18 metres long, is based on a compressor that “had been in service for 30 years on building sites to run various air tools and had little maintenance done”. ⁸ This is detrimental to system efficiency, because a compressor that is not maintained well easily wastes as much as 30% of its potential output through air leaks, increased friction, or dirty air filters. This small-scale system also used a highly inefficient expander. All together, this explains why it combines a very large storage volume with a very low electric-to-electric efficiency (less than 5%). ↩︎
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Abdul Alami, e-mail conversation. ↩︎