Value energy

Heat and electrical energy should be used wisely

Solar Stirling Power System: converting solar energy to heat to electricity.

Solar Stirling Power System: converting solar energy to heat to electricity.

Photo courtesy of Sandia national labs

Sustainable Space columnists Lori Brown and Greg Kallio are professors in the College of Engineering, Computer Science and Construction Management at Chico State University.

The photovoltaic-thermal (PVT) system described in my last column provides more energy than I reported. My original statement that “a PVT Solar echo system will yield an additional 40 percent to 50 percent of energy capture in heat” understates the actual performance. The president of PVT Solar informed me that the system typically provides about 100 percent more usable energy than a basic PV system.

For example, if a basic PV system produces 3,000 kWh in electricity per year, the echo system would provide an additional 3,000 kWh in heat energy per year for space and water heating. Furthermore, since the echo system actually collects as much as 300 percent more energy in heated air, this figure can go even higher depending on thermal demands.

Heat vs. electric
This brings up an important question: Is heat energy as valuable as electrical energy? Thermodynamically, the answer is no. Electrical energy is regarded as a higher-quality energy because it can be used to perform work at a rate of 100 percent. For example, 1,000 watts of electricity can power a 1,000 watt motor. If the motor has an efficiency of say 90 percent, then the spinning shaft of the motor will provide 900 watts of mechanical power for doing work (power = energy per unit time).

Heat energy cannot be converted to work energy at a rate of 100 percent. This is a consequence of the Second Law of Thermodynamics—a law that sets maximum limits to the operation of all heat engines, such as gasoline engines and steam power plants. The best gasoline engines have efficiencies of about 30 percent, meaning that they will produce only 300 watts of mechanical power at the crankshaft for every 1,000 watts of heat released from the combustion of gasoline.

Valuable asset
Recognizing that electricity is a high-quality form of energy, it should be used wisely and judiciously. For example, using electricity to heat water or air by passing current through a resistance heating element is terribly wasteful. We do this all the time in water heaters, clothes dryers, coffeemakers, toasters, etc., for the sake of convenience.

It is wasteful because the electricity that is delivered to our households is hard-earned; often that electricity is generated from burning natural gas in steam power plants or from damming rivers and funneling water through hydraulic turbines. If it is produced from the sun’s rays in a PV system, that electricity is even more precious because PV modules are expensive and the conversion efficiency from the solar photons to electricity is only about 15 percent.

Erase waste
Using electricity to light our homes is a given, but often wasteful. In a well-designed home, there should be no need to use electric lights during the daytime. Through the use of efficient daylighting windows and skylights, free sunlight can significantly reduce electricity use. The electric lighting that is required should be fluorescent or LED-based. A 100-watt incandescent light produces only 18 lumens/watt, while a 26-watt compact fluorescent light produces 72 lumens/watt, and the newest white LEDs produce 100-150 lumens/watt.

The wisest use of electricity in homes is for electronic devices and appliances that have motors, compressors, fans and pumps. Water and space heating in homes should be done by renewable sources as much as possible. This includes solar thermal collectors (first choice) and clean combustion of renewable fuels (second choice). Heat pumps are the most responsible way to heat and cool our homes with electricity because they can transfer anywhere from three to six times more heat energy than the electrical energy used to power them!