Thermo-Electric Solar Collection Technology (multi-sun)

This variation on solar collection centers on the idea of using heat to make electricity. The oldest form of this technology is to simply boil water by concentrating sun light on a boiler which is piped to a steam engine that drives an electrical generator. More modern versions of this use concentrating parabolic dishes to run stirling engines. Regardless of the specifics, this technology requires relatively high temperatures be generated by the solar collectors. This means collectors must concentrate solar light by using mirrors or lenses and this means the collectors must track the sun.

Solar thermal electric is definitely more complicated than PV solar technologies, but it offers the promise of a few very key benefits. It is these benefits that make solar thermal electric technologies worth the complexity. In fact, almost all large scale solar generation facilities in use today implement some form of solar thermal technology.

First, light from the sun is very efficiently converted to heat. common PV cells only convert about 15 percent of the sun's light to electricity, the rest if reflected or heats up the PV cells. So thermal solar collectors can easily achieve 90% conversion of light to heat. Leading edge experimental solar cells are only reaching 25% efficiency. So a very simple and cheap thermal collection system can be many times more efficient in terms of energy converted per area of collection, than current expensive PV systems.

Second, because heat can be generated so efficiently and collector expense is relatively low. It is possible to store that heat in the form of oil tanks, or liquid sodium tanks. This creates the possibly of around the clock power generation even when the sun is not shinning. A PV system could do the same thing with batteries, but batteries are also hugely expensive compared to a storage tank. Therefore the potential exists to make solar thermal electric systems that efficiently collect large amounts of heat from the sun when the sun is shinning and then store heat for conversion to electricity around the clock.

The weak link of solar thermal is the step where the thermal energy (heated liquid) is converted to electricity. The only practical option for this conversion step are heat transfer engines. This include phase change engines like steam engines, Air engines like stirling engines, or more esoteric technologies like TEG cells (Seebeck effect) that are like a huge number of thermocouples. All of these transfer technologies generate electrical or mechanical work as heat moves from a hot reservoir to a cool reservoir. Furthermore there efficiency is best when the temperature differences between these reservoirs is greatest.

The theoretical best efficiency for a heat transfer engine is given by the Carnot equation:

efficiency = 100 * ( 1 - T_cold / T_hot ) Where T_cold is the temperature of the cold reservoir and T_hot is the temperature of the hot reservoir in degrees Kelvin. For reference water freezes at 0 degrees Celsius, 32 degrees Fahrenheit, and 273 degrees Kelvin. Water boils at 100 degrees Celsius, 212 degrees Fahrenheit, and 373 degrees Kelvin. For common temperatures involved in solar collection the following table shows what peak efficiencies might be achieved at given temperatures.

Keep in mind that the table above assumes the cold side cooling is 100% effective and that there is no heat loss on the hot side. Thus, you can probably reduce the efficiency numbers 4% or more based upon non-ideal real world conditions. The main point to take away from the table above, is that it is theoretically possible for a residential scale system to achieve 30% efficiency on a round the clock basis. Compare this to PV systems which are typically 15% efficient for only 1/4 of a day. This suggests that a thermal solar system can generate 8 times the power of a PV collection system with the same collector size.

The challenge for thermal solar electric is to make the system affordable, reliable, safe, and easily maintained. These four attributes are opposed to each other thus it is an engineering challenge for the DIYer to come up with a practical system.