Solar schoolin’

Golden gift
Solar energy is California’s most promising renewable energy resource. There are many solar technologies—usually divided into solar-electric and solar-thermal categories. The former includes photovoltaic (PV) modules, which directly produce electricity when the photons in sunlight strike semiconducting materials such as doped silicon. Solar-thermal technologies collect the thermal radiation from the sun to heat substances such as water, air or building materials. While sometimes under-appreciated, this heat can be converted into electricity by running a steam-turbine power plant, a Stirling engine generator, or even a wind turbine inside a “solar chimney.”

Unlimited resource
Producing electricity directly from sunlight still amazes me—even though the photoelectric effect has been known since 1839! Residential PV modules now have efficiencies as high as 19 percent, but when integrated into a home power system with an inverter to produce 120 volts AC, the overall efficiency will drop to 15 percent or less.

This means that a PV array that receives the maximum 1,000 watts of solar energy per square meter in full sun will produce about 150 watts of electrical power per square meter. That’s not very impressive when compared to conventional power technologies, but one must remember that the solar “fuel” is free and essentially limitless. Furthermore, static PV arrays require very little maintenance. Sure, PV capital costs are still relatively high, but technological advances are bringing these costs down.

Energy fallacy
The most common myth about PV—often cited by the fossil-fuel and nuclear industries and in the blogosphere—is that “the energy produced by a PV module over its normal lifetime is less than the energy required to create the module.”

This falsehood is surprisingly sticky. Do you really think that there would be so much global attention on PV systems if they could not pay back their energy investment? While this statement was probably true before the 1970s, there is plenty of evidence showing that today’s grid-tied PV systems produce five to 10 times more energy over their lifetime than the energy used to manufacture, install, operate and recycle them.

Good gauge
The most popular parameter used to quantify the life-cycle performance of PV is the Energy Payback Time (EPT), defined as the time (in years) in which the energy input during the PV life-cycle is compensated by the electricity generated. The EPT depends on several factors such as cell technology and solar irradiation (location). A recent study by researchers B.S. Richards and M.E. Watt at the University of New South Wales showed that a mono-crystalline silicon (mc-Si), rooftop-mounted, grid-tied PV system has an EPT of 2.7 years in Australia and an EPT of 4.9 years in Northern Europe.

A better measure …
While EPT is useful, it does not take into account the lifetime of the PV module. A more meaningful metric has been proposed by Richards and Watt: the Energy Yield Ratio (EYR), which represents how many times the energy invested in PV is paid back by the system over its entire life. A PV system or any energy product with an EYR of greater than one generates more energy over its lifetime than was required to create it, while a system with an EYR of less than one can be regarded as environmentally unsustainable.

This parameter provides a better way of comparing different systems since it implicitly includes the expected lifetime of the PV system, which is continually increasing as better materials and manufacturing methods are developed. The EYR for the mc-Si, rooftop PV system in Australia is 7.5 for a 20-year lifetime module and 11.2 for a 30-year module! Since Australia is comparable to California with regard to solar resources, these data are useful here.