PV Gap

Ramona Said:

it least efficient to store solar energy in what? hot water, hot liquid salt, a battery, oil?

We Answered:

It depends what the energy is to be used for. This comes down to higher or lower forms of energy. Charging a battery with a photovoltaic system returns about 7% of the suns energy intercepted by the panels when overall losses are taken into account, but the energy is much more available or usable as we can just plug electrical appliences into the outlet. On the other hand, using the sun to heat a mass of water as in solar hot water systems might store more of the available energy, but it is a lower form of energy, useful for hot water only, and would be very inefficient at producing higher forms of energy like electricity (with a heat engine).

Water has the highest specific heat of all, meaning it stores more energy per Kg, but other materials may work better at temperatures outside the liquid water range. The energy could be used to slowly heat or cool soil well undergrpound and recover this months later in winter or summer. Each system has different pros and cons, and it would need a careful analysis for the particular circumstances. When generating electricity a good option is to sell the energy back to the electricity supply company, using the so called "grid connect" system. This means less energy loss, because no storage is required. SOmone else uses the energy immediatley, through the wider netwrok of users.

A solar thermal system using concentrators and tracking the sun can achieve much higher temperatures, and is useful to provide heat to produce electricity using steam or stirling of some other form of heat engine with reasonable efficiency. The overall efficiency is higher, for example a currently available 11 meter diameter dish produces 25KW which is about 25% efficiency overall. The storage of this in a battery would reduce to about 15% efficiency. The steam or other working fluid can be at several hundred degrees C, so a higher Carnot efficiency follows for the heat engine. It doesn't work on overcast days, so should be located in a desert region.

For comparison, photosynthesis used in growing plants returns about 8% of the energy. Wood, charcoal and dried dung are still widely used fuels.

Methods of storing higher energy forms like electricity all involve energy conversions and inevitable losses. Typical methods are to convert water to hydrogen and oxygen by electrolysis, to pump water back into the reservoir in a hydro electric system, or quite a large variety of electro chemical batteries (look up vanadium battery). Each of the methods has advantages and disadvantages. A lead acid battery takes 140% of the energy it can produce to charge it, and is readily available and well understood in existing designs. The sustainability is questionable, when the replacement costs and resources to make them are considered. The hydrogen system is well suited to transport systems (except air planes) and is highly sustainable, but the system is not yet widely available. The hydro electric system is of course only useful to utility companies.