One way of utilizing solar energy is to use it directly as a source of thermal energy, either to provide space heating for residential and commercial buildings, or to generate electricity using a conventional Rankine steam cycle. As we have seen, a great deal of energy is used to provide basic comfort in buildings, and in the populous mid-latitude countries this is primarily used for heating during the winter months. The use of both "active" and "passive" solar thermal energy systems for these applications could provide a significant reduction in the need for non-renewable primary energy sources. Passive solar heating simply refers to architectural design techniques which enable the building structure to absorb as much solar energy as possible during daylight hours in the winter months, and then using this "stored" energy to replace heat that would normally be provided by a fossil fuel-fired furnace, or by electric heating. Design concepts can be as simple as ensuring that windows are minimized on north-facing building walls, and enlarged on south-facing walls so that as much sunlight as possible will enter the building and heat up structural elements such as internal walls and floors. More complex design ideas have also been utilized to increase this passive heating, including the use of ''Trombe walls,'' for example. These are heavy, usually black-painted concrete walls placed just behind south-facing glass that are used specifically to absorb as much heat as possible from the sun's rays, so that this thermal energy can be released over periods of several hours. The glass just in front of the wall acts as a greenhouse to trap as much solar energy as possible, and then air is allowed to circulate through the gap between the glass and the concrete. The circulating air then absorbs heat which has been stored in the wall and transfers this to other parts of the room, or even to other parts of the house. The massive wall structure is able to absorb sufficient energy so that heat can be transferred to the circulating air for several hours after the sun has gone down. Some installations have even included blinds just inside the glass which are automatically closed on cold nights in order to reduce the energy which would otherwise be lost by being re-radiated back out through the window.
Active solar heating uses ''solar collectors," usually mounted on rooftops for residential buildings, to heat water, or another fluid which is then circulated to other parts of the building. These active solar collectors can also be used as a source of domestic hot water, or to provide heat directly to a swimming pool. The outdoor swimming pool application is particularly attractive, since these are usually used during the warm summer months when the maximum amount of solar radiation is available. The economics of solar water heating are obviously affected by the cost of alternative energy sources used for this purpose, principally electricity and natural gas, and by the building location. In the USA, for example, solar heating of swimming pools is particularly attractive in sunny states like California and Florida in which there are many outdoor swimming pools. In most installations, whether they are used for domestic hot water or for swimming pools, a conventional water-heating system using natural gas or electricity is installed to provide back-up energy during cloudy periods or when cool weather results in extra demand for hot water. In many cases, however, more than half of the cost of traditional sources of energy can be saved over the course of a year using solar energy, and in some cases much more than this. The solar system costs are also reasonably modest, so that financial ''payback'' times can be less than 10 years, making solar energy an attractive investment.
Finally, the ''concentrating solar collector'' is an active solar thermal energy installation usually used to generate electricity on a fairly large scale. These systems use one or more reflecting mirrors to concentrate a beam of solar energy onto a focal point in order to provide a source of high temperature heat. The use of a large number of mirrors over a wide area can provide a relatively low-cost source of concentrated energy, suitable for heating water or other fluid to a high temperature. This high temperature heat can then be used either to run a hot-air, or "Stirling" engine, or to provide steam for use in a conventional steam-generating plant, both of which are used to drive an electric generator. Of course, this type of system can only be used to provide a source of thermal energy during daylight hours, although some large systems incorporate a thermal storage system so that they can continue to generate electricity for some time during cloudy periods, or even at night. If a source of firm electricity is required then some form of back-up system may also be required for stand-alone applications. In large-scale demonstration plants built to date in the USA, a "hybrid" system using natural gas as a back-up fuel has been used to provide continuous generation, even during the night. However, one of the advantages of using solar-based systems in hot sunny climates is that the period of maximum electrical output corresponds closely with the period of maximum demand for air-conditioning. Smaller systems using a parabolic mirror with a Stirling engine at the focal point have also been suggested as a possible way to provide electricity for small rural communities in developing countries, and particularly for those in regions with high levels of solar insolation.
Larger installations, using an array of mirrors covering a wide area have usually been funded by government departments or research agencies, and have been built in desert or near-desert conditions to demonstrate the technology. These systems have been built primarily to demonstrate the technology, using two different approaches; either a solar "power tower" concept, or a "trough" concept. A solar power tower thermal plant uses a large number of mirrors, or "heliostats" which are able to automatically track the sun and focus the reflected rays onto a "receiver" on top of the central tower. The receiver is heated to a very high temperature by the highly concentrated solar radiation, and this is used to heat water to produce steam directly, or in some cases to heat molten salt which has a greater capacity to carry this heat away and then transfer it to water in a secondary boiler. In either case the steam which is ultimately produced is then used in a conventional Rankine cycle to drive a steam turbine-powered generator. Figure 7.1 (US Department of Energy, 2005) shows the Solar Two demonstration plant located in the Mojave desert, near the town of Barstow, California. This plant uses molten salt as an intermediate heat transfer fluid, and is a retrofitted version of the original Solar One plant, which
heated water directly in the tower to produce steam. The original Solar One plant operated between 1982 and 1988, with a peak output of 10MWe under clear sunny skies. The molten salt heat transfer fluid used in the Solar Two plant increases the ability to store energy for use during cloudy periods and at night. Successful operation of this plant over a 3-year period from 1996 to 1999 confirmed the benefit of increased energy storage capacity. These results then led to plans for the construction of a similar plant in Spain, the "Solar Tres'' (Solar Three) plant, where a substantial renewable energy subsidy makes this an economically attractive option. This first commercial plant is designed to have a peak solar energy input of around 40 MW, and will use molten salt thermal energy storage so that a 15 MW turbine can be operated for 24 hours per day during the summer, with an annual capacity factor approaching 65%.
A newer technology now being demonstrated in the USA, is the solar "trough" concept, which uses an array of parabolic mirrors which focus the sun's rays on a receiver pipe which runs along the complete length of each mirror at the focal point. This concept then does not require a tower, since the heat is collected continuously by the hot oil heat transfer fluid piped around the complete linear mirror array. A heat exchanger is used to transfer heat from the hot oil to boil water with the resulting steam again being used to generate electricity using a conventional Rankine cycle. The parabolic mirrors are all aligned along a North-South axis, and are automatically tilted to follow the
sun as it traverses the sky from east to west. The mirrors are therefore focused on the sun for the maximum possible time to maximize the amount of solar energy collected. Nine of these solar electricity generating systems (referred to as "SEGS") have been built in the Mojave desert, ranging in size from 14 to 80 MWe peak power. In total these provide a peak electrical output of some 350 MWe, with the power being fed into the California grid. Natural gas is used in a "hybrid" fashion so that firm electricity can be generated during extended cloudy periods, or at night, but to date abut 75% of the total electrical energy produced has been generated from solar energy. These plants have a lower capital cost than the solar power tower design, and have shown that they can provide the cheapest form of solar-generated electricity, at around $0.12 per kWh. A partial view of the Kramer Junction solar trough power station operating in California, which consists of five individual plants (SEGS 3 to 7) generating a total of 150 MWe, is shown in Figure 7.2 (US DOE, 2005). This gives an indication of the scale of the individual solar concentrating troughs and shows the receiver pipe running along the focal point of each trough.
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