Local solar: today's sustainable energy

Don't wait for cheap solar cells - there are effective ways to use solar energy today.

by Skip Fralick, P.E.

et us pay due respect for the en- ergy benefits provided by the sun. Electromagnetic energy from the sun provides the life force for our earth. Without it, our planet would quickly assume room temperature that is, the cold "room temperature" of outer space! Amory Lovins, energy guru and founder of the Rocky Mountain Institute (www.rmi.org), said the best energy source is a nuclear power plant located 93 million miles away. How true!

Solar energy comes in many forms. The forms most commonly thought of as "solar" are thermal, thermal electric, photovoltaic, wind, hydro, and biomass. In all of these, solar energy plays an important function.

This article will discuss solar thermal energy, for which Southern California is the best location in the nation. Indirect solar energy sources wind, hydro, and biomass are not as available here, except that modern wind generators can be cost effective at sites with less wind than previously thought (most wind generator designs need an average wind speed of at least 12mph to be cost effective).

In hot water

The most common application of solar technology in San Diego is heating water for domestic use, as part of a two-stage hot water system.

The first stage consists of a solar collector panel, a storage tank and a circulating pump. During collection hours when the water in the solar collector is hotter than the water in the storage tank the pump circulates the water from the storage tank through the collector and back to the tank. The water in the tank provides a large heat reservoir, for hours when the sun is not shining and for times of high demand.

The water from this first "pre-heat" stage may not always be able to provide the 120 - 140 degrees required for domestic use. Therefore, water from the storage tank is fed to the second stage: a "backup" water heater that takes it up to the required temperature. In 85 percent of San Diego homes, the backup heater is fired by natural gas. Because the water has been pre-heated by the sun, the second phase uses a lot less energy than a conventional, non-solar system.

The installation of a solar water heater typically costs $2,000 to $3,000. This is more expensive than the solar heaters typically used for swimming pools, because the hotter temperatures require more expensive collectors with glass covers and an insulated metal enclosure (to create a greenhouse effect). Compared to the cost of a conventional natural gas heater, energy savings will usually pay for the system in seven to 15 years.

If you currently have an electric water heater, you should rush down to your local solar companies and get quotes, because heating water with electricity costs about 4 times as much as with gas! Get two or three quotes, ask a lot of questions, and follow-up with references. If the salesperson says no maintenance is required go elsewhere. I know many of the solar contractors who survived the lunacy of the early 80's solar tax credit boom/bust cycle, and for the most part, they are very professional.

Sizing a solar water heating system is fairly straightforward if your family is normal and uses the average 20 gallons per person per day. But if you have a teenager, throw away the sizing book. They either don't wash (a badge of honor), or you can't get them out of the shower.

There's another category that uses a lot of hot water. I was monitoring hot water consumption of 35 apartment buildings when I noticed one building's consumption being way off-scale: over 100 gallons per person per day! Not believing my instrumentation, I drove to the building. Once I stepped into the courtyard, the reason for the high consumption was readily apparent - I was in Hooker Heaven! Beautiful, friendly women leaning over the railings, GI's coming and going... I got out of there, fast. I had discovered a new category for the hot water statisticians.

In the swim

The most simple use of solar energy is for heating swimming pools. If you can afford the luxury of heating a pool, you should seriously consider buying a pool cover, then a solar pool heating system (in order of cost effectiveness), rather than burning fossil fuel.

When I had a pool in Tierrasanta, I experimented with both a pool cover and solar panels that I'd installed. With neither solar nor cover, the pool temperature was above 80 degrees for only about 5 months. With a cover, the 80 degree season was extended by about 3 months. Solar had the same effect, but cost much more. With both the cover and solar, the 80 degree temperature was good for 9 or 10 months.

A typical pool solar system consists of metal or plastic solar panels on a south-facing roof with an area of 50 to 100 percent of the pool surface area. Pool water is pumped through the collectors when energy is needed and available for collection. Risk of freeze damage to the collectors is minimized by automatic draining after use. Cost effectiveness depends on many factors. Most simulations I've run show a payback of 3 to 6 years, compared to heating with gas year round.

Space conditioning

Space conditioning means heating and/or cooling, and can include ventilation. Space conditioning can be done actively (with solar collectors and pumps or fans), or passively (using simple building design techniques). Because of San Diego's mild weather, active solar heating or air-conditioning is generally not cost-effective. The collection and storage components are too expensive for the relatively short heating or cooling seasons.

Efficient building design coupled with good solar orientation makes economic sense, and can cost little more than a standard Title 24 building (Title 24 is the code that sets the allowable annual energy consumption budget for new California buildings). Work with an architect or engineer familiar with passive solar design.

In passive solar systems, it is important to pay heed to proper collection, storage and distribution. Typically, solar radiation passing through south-facing windows warms up a thermal mass (slab, stucco, brick, etc.). The heat is reradiated, blown or conducted into the space to be heated. A greenhouse on a southern exposure is an example of a simple passive heating system.

But remember to put in appropriate ventilation or cooling controls. Months after I'd held an open house at SDG&E's Pala Passive Solar Test Facility, I got a call from a desperate woman (no, not my ex-wife). This lady had liked the greenhouse so much that she had a contractor install one on the south wall of her home. However, they forgot to include a masonry wall between the greenhouse room and the living room. As a consequence, she said the living room temperature was approaching 120 degrees! A thermal wall would have intercepted the solar radiation, storing it for reradiation into the living room at night, when needed.

In a future article, I'll show how we plan to integrate many of the above features, along with strawbale walls and photovoltaic electricity, into a cost-effective demonstration "green building" at U.S. International University.

You are invited to our free monthly program on sustainable development and the green building demo project, held on the first Thursday of each month at 6:30 pm at Green Hall on the U.S. International University campus in Scripps Ranch. Call me evenings at (619) 565-2603 for more information.

Skip Fralick is an energy engineer at San Diego Gas &Electric and a volunteer for Habitat for Humanity