The easiest way to use solar energy is to put a panel on the roof to heat water. This is already done in many countries. Such panels can be seen as one rides on a train in Japan. In a place like Hawaii, the panel does not have to be very big at all; 1 m2 is more than adequate. A panel can be just a flat box with a glass top and a black bottom to absorb all the sunlight (Fig. 3.23). The panel is connected to the usual water heater with two pipes. A small pump circulates the water up to the solar panel and back down to the water heater. The gas or electricity driven heater then does not have to turn on as often to keep the hot water at the set temperature. No fancy electronics are needed, so the cost is low. Solar swimming pool heaters are even more economical. The same pump used for the water filter can pump the water up to panels on the roof, from where the water siphons down without further pumping energy. Since the temperature rise in each pass is only a couple of degrees, no high-temperature materials are needed. Black plastic panels, about one by two meters, are used. Each has many small channels to flow the water in parallel. Such panels have lasted over 30 years.

The fossil footprint of rooftop solar thermal collectors has been analyzed by the Italians [1]. As with the life-cycle analyses described in the previous section on

Wind, all the energy used in producing the materials used and in installation, operation, and maintenance is added up; and the energy recovered in the recycled materials at the end of life is subtracted. The energy comes from conventional sources, mainly fossil fuel plants. This is then compared with the solar energy pro­duced during the lifetime of the equipment. The resulting energy payback time lies somewhere between 1.5 and four years. However, the systems considered include an insulated tank on the roof, and this is the main contributor to the weight of the galva­nized steel component, which accounts for 37% of the energy used. For systems without a rooftop tank, the energy payback time should be closer to the lower limit of 1.5 years. All the solar heating collected after that is real “green” energy. There is really no reason for every house not to collect the solar energy that falls on its roof.

Photovoltaic (PV) solar panels on the roof are another matter. These are expen­sive, but they provide electricity, not just heat. It costs about $5 a watt to have PV installed on the roof. Since the electricity use per home in the USA is about 1.2 kW averaged over the whole year, one would need about 5 kW to cover the peak hours. The cost is then 5000 x $5 = $25,000. People usually pay between $20,000 and $40,000 for their systems, but there is a 30% federal rebate and some­times also a state rebate in the US. PV systems are usually guaranteed to lose no more than 20% of their efficiency after 25 years. States with net metering will allow the electric meter to count only the external energy used and to run back­wards if the solar cells produce more energy than is used. The savings in electricity

bills can payback the PV cost in about 15 years without rebates or about eight years with rebates.27 This presumes that there is a large roof area with an unobstructed view to the south (in the northern hemisphere) (Fig. 3.24).

Whether PV solar can pay for itself of course depends on where you live. The number of Peak-Equivalent Hours per Day is a measure of how much usable sunlight is available in a given place. The average in the USA is 3.5-6.5 hours. Winter in the Northwest would give only 1.5-2.5 hours, while summer in the Southwest can give 8 hours.27 At 2 hours of intense sun equivalent, a 5-kW PV system would yield 10 kWh of electricity. Remembering that the average use per home is 1.2 kW, amounting to 1.2 x 24=28.8 kWh/day, we see that a large system can supply about a third of the electricity requirements even in the Northwest. The good news is that even on cloudy days, 20-50% of solar energy can still be obtained.

Of course, the sun does not shine when we need electricity the most; namely, at night when the lights are on and we are watching TV. The energy has to be stored. In the Southwest, the peak power is so large that it cannot be used right way; it has to be stored. This requires batteries, which increases the cost of solar energy beyond that for the panels themselves. The most economical batteries available today are the lead-acid batteries used in cars. A whole bank of them will have to be installed in the house. There are larger, more compact lead-acid batteries available. These are used, for instance, in African safari camps in case diesel fuel for their generators cannot be delivered. A 20-feet (6 m) row of these can supply the minimal needs of a camp for three days. PV power, stored or otherwise, cannot run appli­ances because they produce direct current (DC) power. An inverter has to be used to convert the DC to AC at 60 cycles/s in the USA and 50 elsewhere. This is an addi­tional expense that must be counted.

There are other impediments to local solar power that are not widely known. Shadows, for instance, can completely shut off a solar panel. This is because each solar cell produces only 0.6 V of electricity. The cells in a panel are connected in series to buildup the voltage to at least 12 V, which the batteries and inverters need.

If one cell is in shade, it cuts off the current from all the cells. This is like the old strings of Christmas tree lights which were connected in series instead of parallel. If one bulb burns out, the entire string goes out.