Rezachek & Associates'
Energy & Environmental Resources

SOLAR THERMAL

SOLAR THERMAL POWER - What Is It?

Solar thermal power is heat energy obtained by exposing a collecting device to the rays of the sun. A solar thermal system makes use of the warmth absorbed by the collector to heat water or another working fluid, or to make steam. Hot water is used in homes or commercial buildings and for industrial processes. Steam is used for process heat or for operating a turbine generator to produce electricity or industrial power.

Advantages

Solar thermal power makes use of a renewable natural resource which is readily available in most parts of the world.
Solar power used by itself creates no carbon dioxide or other toxic emissions.
The heated fluid can be stored in insulated tanks allowing the process to be used during brief cloudy or overcast periods.
New equipment and technological advances have reduced the cost of solar thermal power to a level that can be competitive with fossil-fuel generated electricity in some areas.
Use of solar thermal power to heat water or generate electricity will help reduce Hawaii's almost complete dependence on imported fossil fuels.
The State of Hawaii offers a 35 percent tax credit for the cost of buying and installing a solar thermal device which supplies hot water for single family homes or multi-unit residential buildings.
Solar water heaters are an established technology, readily available on the commercial market, and simple enough to build, install and maintain by yourself.

Disadvantages

Solar thermal systems are not cost-effective in areas which have long periods of cloudy weather or short daylight hours. Efficiency is also reduced by atmospheric haze or dust.
In cooler climates, freezing can damage collecting system components, such as pipes.
The arrays of collecting devices for large systems cover extensive land areas. This land could be used for agricultural, residential, resort or commercial purposes.
The reflective surfaces of some collectors may be disturbing or undesirable in certain areas.
These systems only work with sunshine and do not operate at night or in inclement weather. Storage of hot water for domestic or commercial use is simple, using insulated tanks, but storage of fluids at the higher temperatures needed for electrical generation, or storage of electricity itself, needs further technical development.

The Basic Process

There are several basic kinds of solar thermal power systems including "flat plate" solar water heaters; concentrating collectors, such as central tower receivers; and parabolic trough and dish collectors. Although all of these use the sun's rays to heat a fluid, the equipment and, to some extent, the uses differ.

In solar water heaters, water flows through tubes which are attached to a black metal absorber plate. The absorber plate is enclosed in an insulated box with a transparent window to let in sunlight. The heated water is transferred to a tank where it is available for home, commercial or institutional use.

In order to produce steam and electricity with solar thermal energy, central receivers have a field of tracking mirrors called heliostats to focus sunlight onto a single receiver mounted on a tower. Water, or some other heat transfer fluid, in the tower is heated. This heat is used directly, or transferred to water, to generate steam for electricity.

Parabolic dishes or troughs are curved panels which follow the direction of the sun's rays and focus the sunlight onto receivers. A liquid inside the pipes at the receivers' focal point absorbs the thermal energy. The thermal energy received can be converted to electricity at each unit or transported to a central point for conversion to electricity.

Use of Solar Thermal Power in Hawaii

An estimated 60,000 single family homes, multi-unit dwellings and institutional facilities in Hawaii are served by solar water heaters. Tax credits for solar water heaters were increased to 35 percent by the Hawaii State Legislature in 1990.

A concentrating parabolic-trough collector system, formerly part of a combined concentrating solar thermal/photovoltaic system at Wilcox Medical Center in Lihue, Kauai, was moved to the Pacific Missile Range Facility at Barking Sands, Kauai, where it furnishes hot water for some of the buildings at that location.

In 1987 the state and the U.S. Department of Energy participated in a feasibility study of a 250-kilowatt (kW) solar thermal system to be built on the Island of Molokai. The system was planned to provide electricity to Molokai Electric Company. Federal funding for the project was reassigned, however, and the project was not continued.

The state, through a contract with Kearney & Associates, prepared a feasibility study to determine if a large solar thermal plant, such as the kind built by LUZ in California, would be applicable to Hawaiian conditions. Released in June 1993, the Kearney study reported that such a plant would not be profitable because there are too many clouds in Hawaii. Even in preferred areas such as the lava fields of South Kohala, the amount of direct sunlight is 25 percent to 30 percent lower than the Mojave Desert. This would result in a 40 percent to 50 percent reduction in power output. Even with tax credits, a LUZ-type solar plant would not be economical, the report said.

Most of information contained in this summary was excerpted from a Fact Sheet prepared by the staff of the Energy Division of the State of Hawaii-Department of Business, Economic Development, and Tourism (DBEDT) and published with funding from the U.S. Department of Energy, Grant No. DE-FG49-94R900023. This information does not necessarily reflect the views of the State of Hawaii, the United States Government, or any agency thereof.