Storing Energy from Solar Arrays

Storing Energy from Solar Arrays

Even in the case there is heavy daytime use for solar power, e.g. air conditioning, there will always be a need for power when the sun can’t provide it.  Ideally, the solar generation system will be over-specified so that peak usage can be handled, in which case there is a need to store the excess energy for “after sun” hours.  In addition, peak energy consumption is in the afternoon, and solar arrays are effective in the morning hours, which might generate excess power in the mornings.

Now the best “storage” device is usually the grid.  Dump your excess energy onto the grid so that someone else can use it, and then use the grid whenever (day or night) your solar power isn’t adequate.

The problem is that the grid has only minuscule renewable storage capacity – most of it behind dams that contain hydro-electric generators. Thus “storing” excess energy on the grid assumes that someone can use it at essentially that moment.  Utility companies try to shape demand by turning on and off not only hydro-electric generators, but also (ugh) fossil fuel generators and by adjusting the various components of energy pricing.  For the excess producer of solar or other renewable energy, this means that putting electricity onto the grid at non-peak times, may provide low or even negative prices for that energy.

That said, what if you can’t (or don’t want to) dump your excess power onto the grid for some reason?  Here are some thoughts to be elaborated on in subsequent postings:

  • Store the energy as heat, e.g. as hot water.  This can be done in hot water tanks, or even in a swimming pool.  A more sophisticated method is to store the heat in a heat sink such as molten salt (very hot), which can be reused in various ways.  There are various salts besides NaCl – common table salt – but the idea is to heat the salt to over 500 degrees Celsius.  Well insulated, liquid salt can keep 90% of its heat for over 24 hours.
  • Storing heat in gravel has merit.  Isentropic’s Pumped Heat Electricity Storage (PHES) system is based on the First Ericcson cycle and uses a heat pump to store electricity in thermal form. The storage system uses two large (7m high  x 8m in diameter) containers of gravel, one hot (500C) and one cold (-150C) with a heat pump machine between them. Electrical power is input to the machine which compresses/expands air to 500C on the hot side and -150C on the cold side. The air is passed through the two piles of gravel, where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is reversed with a round trip efficiency of 70-80 percent. The temperature difference is used to run the system as a heat engine.
  • Compressed Air Energy Storage (CAES) is very interesting; however,  don’t think of a tank of compressed air, but rather think of an underground cavern filled with it.  In the middle of the night when the price of electricity is low, utilities can run compressors and pump air into a cavern at around 750 psi.  When the price of electricity goes up, the compressed air is then used to power a turbine generator. Often this portion of the design is supplemented by the use of natural gas either to heat the air or to mix with the pressurized air to burn.  Investigations by EPRI indicate that up to 80 percent of the U.S. has geology suitable for CAES. A single 300 megawatt CAES plant would require 22 million cubic feet of storage space — enough to store eight hours’ worth of electricity. Ridge Energy has a nice diagram here. Whether CAES systems can be commercially viable at a utility level remains to be seen.  One negative argument is here.
  • Store it as kinetic energy, e.g. in a fly wheel.  Well, there are horror stories of large fly wheels disintegrating with hunks flying around causing much damage.  Not a pretty picture.  Fly wheels in general lose a lot of energy to friction, and really are only good to transition use from one source of energy to another.  Lots of small fly wheels are too expensive to maintain. The company Velkess (founded 2007) is developing new flywheel technology.  Keep an eye on them.
  • Pump water up hill.  This isn’t as silly as it sounds, especially if what is needed is water pressure.  Think of those old fashioned city water tanks, which are decidedly useful. On a large scale, filling a reservoir that was above a hydro-generation facility would be a good idea.  For example, the TVA’s Racoon Mountain pump-up facility has been operating for a number of years.  Some estimate that about 10% of the hydro-electric dams are suitable for this.
  • Freeze water.  If you don’t have enough use for ice, e.g., for cooling, then run the freezer during the day and let it “coast” at night.  Some commercial refrigerators have logic for this, expecting both power and usage to be nil at night and to start up in the morning.  Even when commercial electricity costs less at night, this is a win for solar.  For wind, making ice at night and using it to cool a building during the day is done by CALMAC.
  • Use electrolysis to make hydrogen, then burn the hydrogen later to make electricity.  This approach is costly, and it takes considerable energy to compress the hydrogen into a liquid form so that it can be shipped or stored easily.
  • A more sophisticated version of the preceding is to use electrolysis to make MgH2 – magnesium hydroxide.  This is a relatively stable liquid, and the hydrogen can be released either by mixing the MgH2 with water or by heating it.  The released hydrogen can then be burned to make electricity.  The byproduct after adding water, Mg(OH), is essentially Milk of Magnesia. which can be recycled to recover the magnesium and start the process all over again. In addition to storing energy in the form of hydrogen, this “liquid energy” and its byproduct can be transported easily in trucks, boats, etc..
  • Of course batteries are the standard answer, and battery technology is getting better every year.  Here, you get a little extra mileage bypassing the inverter(s) and using DC appliances, but I’m not sure what the trade-offs are.  For example, using LED’s for DC lighting would be a good idea.  It is pretty easy to bypass the power supplies in most computer equipment.  DC fans would work.  But the cost of DC ovens, air conditioners, etc. is probably prohibitive.  At the scale of utilities, EOS Aurora and Revolt Technology both claim to have rechargeable Zinc-Air batteries that will scale hold 6 MWh each.  Zinc-air batteries aren’t as good as Lithium-air, but they are vastly cheaper.  The battery researchers are lately hot on Vanadium Redox batteries which you can read about here.

As I’ve thought about putting solar arrays on my house’s rooftop, some of the above ideas may work for me, e.g. heating water, and getting the freezer and refrigerator extra cold during the day.  On the other hand, I’ll also connect to the grid and take the pittance that the electric company gives me for my “donations.”

Finally, for you wind farm fans, most of these ideas apply – with the exception of storing liquid salt.  Not that a wind mill can’t make liquid salt, but solar arrays essentially start there.



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