As discussed in Chapter I, modem civilization consumes thousands of megawatts of electrical power. If solar cells are to supply a significant portion of this energy requirement, we must give some thought to overall system organization. Fundamentally, we require a source of heat energy and a source of electric energy to power our homes, factories and businesses. A single solar cell supplies direct current at approximately one volt. If this electrical energy is to be used efficiently we must electrically "stack" several solar cells in order to reach the 12 to 200 volts line voltages in common usage. In addition, many kinds of electrical equipment require considerably more than a single ampere of current, so that several series "stacks" must be wired in parallel to provide for needed voltage and current values.
When we consider the dc aspect of solar cell system output, most modem electrical equipment can be converted to direct current operation, though the use of inverters to "condition" dc solar cell system power outputs and convert them to ac is widespread. If the solar-electric power is generated close to the point of use, then low voltage and dc power transmission is feasible. There are, however, many locations in the world where weather and geography discourage year around photovoltaic energy generation. As a result, we must consider long distance transmission of solar cell derived energy. The existing electrical power transmission network is available, but requires the use of inverters on solar cell systems in order to produce ac power. Once ac power is available, then transformers can be employed to produce the required high voltages for long distance transmission.
Rather than transport the electrical energy, an alternative is to use the solar cell generated electrical energy to produce hydrogen. In turn, the hydrogen can be locally stored or transmitted by tanker or pipeline to the point of use, where it can be burned or used in a fuel cell. Presumably this hydrogen could also be used as an automotive fuel (compressed, liquified or adsorbed in some material) and as feed stock in a number of chemical processes, resulting in materials ranging from cooking oils to nylon stockings.
In practice, all of these options will most likely be employed depending on location (of both solar cell system and of the consumer) and circumstances. For example, consider the possible applications of a solar energy conversion/storage system to an on site usage. Figure X.4 portrays such a system using both thermal and photovoltaic conversion systems.
Figure X.4. An integrated domestic solar power system. Copyright Earthlab, Santa Barbara, CA, USA. Reproduced by permission.
Note that heat is stored and used for low quality heating of water and building space. Electrical energy is directly used (lighting, cooking, cooling, entertainment, communication, etc.) and is also stored in batteries (for high current, short term applications) as well as being converted
to hydrogen by electrolysis of gray water. The hydrogen is then stored on site until required for burning to produce heat energy or being reconverted to electricity via fuel cell. Provision is also made for interconnection of both produced hydrogen and electricity to public utilities. Note that the entire system is computer controlled. Whether such a system applies to a single family residence a shopping center, a village, an industrial or commercial plant or an entire geographic region, the solar energy supplying system must have the same assortment of components. Efficient operation is made possible by the dedicated computer which is responsible for minute-to-minute control of the energy demands and supplies. The computing power required for such an operation is well within the range of the modem personal computer. Clearly, systems such as those described in Figure X.4 are not inexpensive and the cost will, in general, be beyond the resources of the average individual [61, 62]. This leads to a potential future utility company; a company composed of many small, widely scattered generating systems, some of which are leased to individuals or groups of consumers. Such a utility company will act more as an "energy broker" assisting in the transfer of energy from one part of its service area to another, than it will as a centralized producer of electrical energy. A considerable number of studies have been conducted examining the aspects of merging conventional electric utilities with small photovoltaic energy producers. To provide the reader with a start on this topic, the following references are given [63-65].