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Solar heating, cooling and storage

By Dave Elliott

Solar thermal and solar electricity technologies are inevitably in competition for roof space. In terms of cost/kW, PV cost more, but, at the domestic scale, heat may have lower value than electricity – electricity prices are high and heat can at present be more economically provided conventionally by natural gas. It’s a similar story when looking at an up and coming idea – solar cooling. In hot climates that is beginning to make a lot of sense. But PV electricity to run air con units is often competitive with mains power at peak cooling times, whereas using solar thermal heat to run absorption chillers is more expensive – they need much higher temperatures (well over 100 C) than for space or water heating. See Sun and Wind Energy 7-8-13.

Set against all that, although you can store PV electricity in large batteries (e.g. see www.sunbat.co.uk), solar thermal does have the option of much cheaper storage of heat. That makes more sense on a larger scale – with community based systems. In terms of efficiency, the larger the better – the surface area to volume ratio (and hence the heat loss) decreases with scale.

There are already a lot of large solar heat stores around, in Denmark and elsewhere, linked to district heating networks. However, there is also the option of storing the cold. Ice has in the past been stored for cooling, and some modern air-conditioning systems do something similar, using mains power. Heat pumps can be used to generate ice at night for daytime cooling, using excess night time wind-derived electricity, or even more advanced, how about interseasonal ice storage using winter excess wind power, for summer cooling?

Obviously it would be neater to make use of daytime solar for direct real-time cooling, but as well as the option of storing solar heat, you could perhaps store excess domestic or grid PV (and wind) electricity as heat, for power generation later, to run air con at night. But maybe more interestingly, there are now systems for storing cold and using it to generate power later, although so far only on a large scale. In theory a heat engine can be made to work on any temperature difference, so cold is as good as heat for electricity generation. For example, in Highview’s 300kW prototype system, electric power is used to liquefy air, which is stored in a cryogenic tank. When electricity is needed, it’s pressurized and warmed to produce a high-pressure gas that drives a turbine. Unlike pumped-storage hydro, which requires large reservoirs, the cryogen plants can be located anywhere, so it may well spread as a bulk storage option.

So of course you may convert excess electricity from wind and solar into hydrogen gas for later use, when needed, for power generation in a turbine or fuel cell.  This ‘power to gas’ idea is now being taken very seriously in Germany, as a way to balance variable wind and solar. Some of the systems convert the hydrogen to methane, using CO2 from the atmosphere or from power plant exhausts. The green methane can then be added to the gas grid, or used directly as fuel for power generation. It’s good to see that a British company, ITM, is among the leaders in this field, with a prototype being built near Frankfurt.

It will be interesting to see which larger scale energy storage option wins out. Hydrogen is a relatively low-density energy storage media. Methane is a bit better. But hot water can hold about 3.5 times as much energy by volume as natural gas at atmospheric pressure and temperature. Though it’s also claimed that a single gasometer-style tank of liquid air could make good the loss of 5GW of wind power for three hours – nearly 10% of the UK’s peak electricity needs.

All this is for large-scale grid systems, but there are some smaller technologies for hydrogen energy storage. For example, the Fronius Energy Cell system, in which any excess electricity from a domestic PV cell is used to make hydrogen by electrolysis. The hydrogen is then stored ready to be converted back into electricity in a fuel cell when it is needed.

However, operating at this scale may be suboptimal in efficiency terms: it may make more sense to set up a full hydrogen economy, with excess electricity from PV and other renewables, large and small, feeding into large efficient high temperature electrolysers, and hydrogen then being stored centrally, distributed possibly via the gas grid and used to generate electricity in a large fuel cell, with full heat recovery, via high efficiency CHP generation.

That’s a way off. And at the small scale, for the moment, there are no special incentives for domestic users to store energy – the FiT system assumes they export any excess from their roof top PV arrays and rewards them accordingly with an export tariff. For heat, however, there may be attractions in small scale storage, but of course as yet no incentive (or means) to export any excess, just the new domestic Renewable Heat Incentive of 19.2p/kWh for solar heating. That is based on a ‘deemed’ amount of energy generated, not actual measurement. So maybe you could store some and still count it. https://www.gov.uk/government/news/new-scheme-offers-cash-incentive-to-households-using-renewable-heating-systems-in-their-homes

Take-up of PV has accelerated rapidly in the UK under the FiT to over 2GW so far. The RHI may have the same impact for solar heat collectors. In which case storage issues may become important, and so may competition for roof space. We might see garden based units proliferate! That could open up a micro-scale version of the land-use battle going on in relation to large solar farms. And if solar cooling also catches on, as global warming bites, then the competition will become even more intense. However, there are hybrid double-layered solar thermal/PV systems (‘PV-T’), which have the advantage that PV cells work more efficiently if the heat is extracted. So maybe technology will come to the rescue. For a good review of PV-T, see http://www.renewableenergyfocus.com/view/10922/solar-pv-and-thermal-a-marriage-made-in-heaven/.

However, leaving the technology aside, if PV and other microgen systems spread, and consumers become major energy suppliers, there may be some interesting corporate conflicts. Southern California Edison recently rejected an application for grid connection under their ‘net metering’ programme from a consumer who had installed a battery storage system for their domestic PV array. Maybe Edison was worried that homeowners would charge batteries from the grid during off-peak hours and then sell excess power back to the grid during peak hours at a higher price. That would never do! www.renewableenergyworld.com/rea/news/article/2013/08/solar-battery-backup-under-attack-in-california?cmpid=SolarNL-2013-08-0

*A new Palgrave book,  Keeping Cool in Southeast Asia by Marlyne Sahakian, explores the aircon issue as well as the wider issue of taming energy demand, linking in to a broader discussion what is meant by affluence and what sort of lifestyles are sustainable.

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