# Solar Heating and Cooling

Heating and cooling of a space is needed when the temperature falls below or rises above a desired value. To determine how much heating or cooling is necessary, we must determine how much heat enters or leaks out of a building. A simple procedure for the rough calculation of thermal load is presented in the box below. The more detailed calculation is outside the scope of this book, but the interested reader is referred to texts and journals dedicated to the topic (a).

## Passive Heating

Figure 1 Passive and active solar systems.

Passive heating refers to the collection of solar energy without use of electrical or mechanical power. The simplest form of passive heating is the direct gain of solar energy as it passes through window glass (Figure 1). In a more complex system, a medium with a large thermal mass (such as a concrete wall, or a drum of oil or water) stores the solar energy before releasing it at a later time when it is needed. Because they have fewer (if any) moving parts, passive designs are simpler, more reliable, more durable, and cost less than active systems. Passive solar systems can be designed in a variety of forms (1).

Solar rooms are south-facing rooms with large windows, thick walls, and well-insulated roofs. Large windows allow maximum gain of direct solar radiation. Double and triple glazing of windows can significantly reduce heat losses from the room, while only moderately reducing the solar gains. Thick walls have large masses and high thermal storage capacity. Solar chimneys allow cool ambient air into the bottom of a glass collector where it is subsequently heated by incident sunlight and then rises by natural convection to the top before being released into a room. Solar chimneys are advantageous in that they can eliminate direct sunlight and glare and reduce heat losses during the night. More elaborate designs allow the amount of circulating air (and thus heat input) to be adjusted as necessary.

## Active Heating

The main component of most active systems is rooftop solar collectors, where solar energy heats a working fluid like water and either stores it in a hot water reservoir or distributes it directly to interior spaces through pipes or ducts. For most applications, temperatures of about 100°C are sufficient and flat plate collectors are the most convenient. For applications that require higher temperatures, energy must be concentrated and solar concentrators such as lenses and mirrors are necessary.

Flat plate collectors consist of a number of tubes through which a working fluid (such as water or air) is heated by solar energy. Tubes are arranged in parallel inside an airtight collector box that is covered by a sheet of glass or plastic and insulated on the back. To absorb the maximum amount of energy possible, the back-plate is painted black (2). The collectors are placed on rooftops or in open areas and tilted at an angle that maximizes solar insolation. Ideally, collector plates should be perpendicular to the sun’s rays at all times. This requires costly and complex tracking devices, however, so fixed flat plate collectors are often used instead. For optimal efficiency, collectors are faced south (north if in the southern hemisphere) and tilted at an angle equal to the latitude of their location. Evacuated tube collectors are similar to flat plate collectors except that the glass tubes are replaced with two concentrating tubes. Fluid flows inside the inner one while the outer tube is evacuated. This arrangement eliminates conduction and convection losses, which increases the collection efficiency.

The most common uses of flat plate collectors are for domestic hot water systems, pools, spas, and space heating (b). Usually a gas or an electric-powered heater is added as a backup for periods when sufficient solar energy is not available. Evacuated-tube solar collectors can be used under cloudy conditions, so their year-round efficiency is higher. They are considerably more expensive and maintenance costs are higher. A typical domestic hot water system is shown in Figure 1.

## References

(1) Anderson, B., and Wells, M., Passive Solar Energy, Brick House Publishing, New Hampshire, 1981.

(2) Hoagland, W., “Solar Energy,” Scientific American, V. 273, pp 170-173, September 1995.

(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

(a) See for example ASHRAE Journal of Heating, Ventilating, and Air Conditioning.

(b) Solar water heaters, sometimes called domestic hot water systems, can be either passive or active and open or closed loop. Passive systems rely on the principle that water in the collector becomes lighter and rises as it heats, while cooler water in the tank sinks, causing circulation by natural convection (thermosiphon). No pump is needed for water circulation through the collectors, making the system simpler and less expensive. For passive systems to work, the tank must be above the collector. Active systems use pumps to assist circulation. Open-loop systems are popular in mild climates where there is no danger of freezing. These systems require a circulation pump and are therefore inherently active. Closed-loop systems use a mixture of water-glycol antifreeze mixture and thus can be used in area where freezing is a possibility.

Markvart, T., and Castanar, L., Solar Cells: Materials, Manufacture and Operation, Elsevier Publishing Company, 2005.

Galloway, T., Solar House, Elsevier Publishing Company, 2004.

Stine, W. B., and Harrington, R. W., Solar Energy Systems Design, John Wiley and Sons, Inc., 1985.

Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society, covers solar, wind and biomass energies.

National Renewable Energy Laboratory: Solar Research (http:// www.nrel.gov/solar).

Energy Efficiency and Renewable Energy: Solar Energy, US Department of Energy (http://www.eere.energy.gov).

American Solar Energy Society (http://www.ases.org).

Solar Electric Power Association (http://www.solarelectricpower.org).

California Solar Center (http://www.californiasolarcenter.org).