10- 1 Overview
Solar power, when absorbed by a surface, converts to either thermal or electrical power. This chapter focuses on conversion to thermal power (or energy), primarily for water and space heating. Solar collectors are described along with their design, testing, and performance leading into a presentation of the f-chart method for characterizing expected flat-plate solar collector output. The analysis method is then extended to designing optimum systems. Passive solar heating systems are described and the load: collector ratio (LCR) method is presented to show how solar glazing and thermal storage can be coordinated to develop optimum passive heating designs that are sensitive to geographic locations as well as system parameter values. A brief introduction to passive thermal ventilation is included, which can be adapted, with modification, for solar chimney design.
10- 2 General Comments
As mentioned at the beginning of Chap. 9, the amount of energy reaching the earth from the sun over the period of 1 day or 1 year is very large, and in fact is much larger than the energy used by humans for an equivalent period. Although the solar energy resource is large, it is not immediately useful at large scale at most locations on earth. It must be captured at a useful temperature, transported, and perhaps stored for later use. Solar energy installations are obviously better suited to locations with the best solar availability, but well-designed systems can be useful even in locations with significant cloudiness during parts of the year.
The most obvious use of solar energy is for direct heating. Solar heating has been applied to crop drying in agriculture, space heating of homes and businesses, heating water, growing food (and other crops) in greenhouses, producing salt from evaporating seawater, cooking food, and driving heat engines in power cycles, as examples. The diffuse and temporally variable availability of solar energy requires large collection areas to capture the energy and convert it to heat, suitable storage means to hold the heat until it is needed, well-insulated distribution methods, and highly effective controls to avoid wasting the energy after it is collected.
Any method of solar energy capture and use must be viewed within the context of the system of which it is a part as well as the stochastic nature of weather. Flat plate solar collector systems to heat water, for example, must be based on actual need, be compatible with the energy storage capability of the water heating system, and work in concert with a conventional backup system. A passive system for home space heating
must be balanced against insulation of the house, and air exchange to create a healthy environment, to determine the best combination.
Solar energy is diffuse, more available in some areas than other areas, and nowhere continuously available or predictable, except in space. This chapter is an overview of some of the technologies available today that are in various states of maturity, and provides analysis tools useful for designing water and space heating systems of near optimality based on economic return. Economic return is the evaluation metric to which most people can relate. Life cycle analysis and assessment can provide a different metric that balances embodied energy against captured energy over the life of the renewable energy system, but such analyses are beyond the scope and level of detail of this book.