Strange Article on Solar Thermal and Energy Storage
I keep an eye out for articles like this that talk about concentrated solar power (CSP –aka solar thermal) and the use of energy storage to make that power dispatchable. Normally, they contemplate molten salt, as the cost of storing energy as heat is so much lower than other technologies, e.g., batteries.
Here, however, the conversation, for some strange reason, revolved around compressed air energy storage (CAES). Yes, as the spokesperson said, “the energy to compress the air could come from a solar thermal turbine. “
I’m not saying that it couldn’t, but it strikes me that it wouldn’t, given the horrendous inefficiencies that the project would suffer. Let me ask frequent commenter Glenn Doty (and anyone else): Am I wrong here?
I think you are right on that. Given the inherent capability to design in thermal storage using mixtures of molten salts, or even concrete and liquid glass, why would you use a steam driven turbine to compress air which will give you at best 70 to 80% of the energy used to compress it and that with the added complexity of storing the heat given off by compression and reintroducing it during expansion? CAES would almost certainly lose more energy than simply storing heat for later, and add a higher level of costs and complexity.
The compressed air energy storage option is in my view far better suited to wind power or photovoltaic energy generation which do not have such a convenient designed in energy storage option.
Compressed air storage does tend to be very inefficient. However, capacity is also important.
Thermal storage, using a mixture of KCl and NaCl, has been used with moderate success, but only for providing enough storage for a few hours. It doesn’t seem practical for storing enough energy to last for prolonged intervals when there is no generation. Also, while it would be theoretically possible to use thermal storage to store power from wind farms, that would seem impractical.
It may be possible to increase considerably the efficiency of compressed air storage. The main reason it is inefficient is that, in accordance with gas laws, when a gas is adiabatically compressed, its absolute temperature increases with the decreased volume, i.e., if the compression cuts the volume in half, the absolute temperature will be doubled. That generated heat is generally lost during storage. But what the heat of compression could be used to recover part of the power used to compress the air? That could be done using a heat engine of some sort. Then, when the stored compressed air is being used to recover the power, it could be expanded isothermally instead of adiabatically using atmospheric air or another heat source to make the expansion isothermal. Over-all, that approach should significantly improve round-trip efficiency. Actually, the idea is sufficiently obvious that it may already have been considered.
Whether the efficiency could be improved to make compressed air storage practical I don’t know. However, experts in the field should be able to calculate the improved efficiency fairly accurately.
Lifting some mass against gravity is fairly efficient – over 90% or so. That said, highly distributed generation and the associated transmission mechanisms has not even come close to an in depth exploration.
Craig,
You aren’t wrong.
But I would take your question even further: why on Earth is anyone trying to force storage capacity onto thermal solar?
Alternative energy SHOULD BE reduceable to basic economics. The value of inputs + O&M + amortized capital vs. value of outputs. Solar’s inherent value lies in it’s production curve – It produces most of its power in the middle of the day when its power is most needed. Due to the time frame in which the power is generated, that power has maximum value. Why would you add costly infrastructure – of any kind – in order to realize an efficiency penalty only to sell that energy during a time of lower demand/lower value?
It’s just stupid. There are no kinder words that can be applied.
If you’re going to build storage, it should be built for dealing with low-value energy. Energy is at its lowest value when it is produced in excess. With wind energy, there are often times when excess energy is produced, and that energy can be stored for later use (most economically via pumped hydropower or – ecology permitting – dammed hydropower).
But building a dedicated energy storage system for an industrial solar field is just moronic. That energy is high value. Let it be sold as is.
Typically, storage is discussed with respect to CSP because people are infatuated with high efficiency numbers, and since CSP is a thermal system the thought of storing thermal energy is exciting because you have a theoretical limit of 100% – you’re storing energy before its first conversion. But molten salt flowing through tons of granite blocks happens to be dreadfully expensive. This is why fossil fuel or nuclear power (all thermal power systems) choose to take the hit on efficiency by first converting the thermal energy to electricity, then using the electricity to pump water uphill, then re-converting the potential energy of the water mass…
Efficiency might not be ideal, but the final costs are still lower than trying to use the molten salt/granite structure trick.
CAES itself is pure garbage, with no plausible path towards competitive storage between very high system costs and extreme thermal losses (you lose 100% of the thermal energy of compression), it just doesn’t work. CAES doesn’t compete with a simple lead battery, much less an advanced PbC battery system.
I meant to add – and of course an advanced PbC battery system cannot compete with pumped hydrostorage, which is nowhere near as economical as dammed hydrostorage, which is nowhere near as economical as WindFuels (storing the energy as gasoline, diesel, and jet fuel).
😉
I agree with Glenn that it seems odd, but I’m not sure how stupid it is. One problem CAES has is that when you decompress the air for power, its temperature drops, and they need some way to apply heat. Normally natural gas is used for this.
It’s just possible that this excess cold could be stored until the next day and be used to reduce the cooling temperature of the thermal turbine, increasing its efficiency. CSP has to dispose of excess heat while CAES has to dispose of excess cold. A match made in heaven? Seem tricky from an engineering POV, but the physics is interesting.
Although it is true that commonly CH4 has been used to apply heat when compressed air is expanded to recover stored power, CH4 would not need to be used. Atmospheric air could be used instead to prevent the expanding air from dropping much below atmospheric temperature, or water from a lake or river could be used. Surely during the compression phase, a use could be found for the heat generated.
Glen, perhaps I am just being cynical, but this sounds like one of the research programs working up a semi-plausible project like the periodic interest in Zinc-air batteries which work so wonderfully in hearing aids but simply can’t be scaled up to the levels of current and voltage for electric cars, and who periodically we read about yet another grant for tens of millions of dollars going to that research. Maintaining some researcher in a nice home and good salary while diverting most of the funds to another project which generates other cash flow. perhaps we will soon see some Political figure is involved in fundraising for this seemingly ineffective system of energy storage and they will receive some many-million dollar grant and the politician will earn a percentage for their “War Chest” and several research “Chairs” at some University will pay the researcher a nice salary and divert the remainder for other projects. It is the way “Research Works.” Most effective research comes from manufactures R&D Labs and little from Universities but you know who gets the most publicity…
One of the other energy boards I am on had a guy that wanted to use an air conditioner as a dump load for his wind turbine. He was going to put the compressor and condenser in duct in a box full of Glauber’s salt to heat the salt for the storage of energy to heat the house. His idea was to take advantage of the transition temperature point of the salt to gain the maximum energy storage and release. He was going to just vent the cold from the evaporator into the atmosphere. I thought it might be an improvement to use actual refrigeration components and add a box of water or some other material to the system and save the cold from the evaporator by freezing the water. I figured the cold box could be on the bottom because cold falls and the hot box could be on the top because heat rises and that would save floor space. A system of valves would allow routing of air from the heating/cooling system for the house through whatever was needed at the time. Heating and cooling are large energy uses. Around here heating is either oil or electricity mostly and saving either would be an advantage. I figured in wintertime during the day solar air heaters could help heat the house and at night heat could be pulled from the hot box. In summer the house AC could be run by PV and/or wind and at night cool could be pulled from the cold box. In either case the refrigeration unit could be run to store excess energy whenever it occurred. I figured rather than save only heat or cold and vent the other to the atmosphere I would save both by simply moving heat from one box to the other. I had it pretty much worked out but stopped thinking about it for the same reason I stopped working on my hydrogen generator which was, none of my systems are generating more energy than I can use at this point. I decided I should focus my efforts on being more efficient and generating more energy and go back to these things when I am finally generating enough excess energy to make it worth it.