composed primarily of inedible cellulose fibers that form the stems and branches of most plants. Dedicated energy crops, such as switchgrass, are also promising cellulose sources that can be produced in many regions of the United States. Heating In those countries where growing dependence on imported gas is a pressing energy security issue, renewable energy technologies can provide alternative sources of electric power production as well as displacing electricity demand through production of direct heat. The IEA suggests that the direct contribution that renewable energy can make to domestic or commercial space heating and industrial process heat should be examined more closely. Heat from solar, geothermal sources, and heat pumps, is increasingly economic but is often overlooked in government programmes that promote public acceptance and provide incentives for renewable electricity and energy efficiency. Solar heating systems are a well known technology and generally consist of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and a reservoir or tank for heat storage. The systems may be used to heat domestic hot water, swimming pools, or homes and businesses. The heat can also be used for industrial process applications or as an energy input for other uses such as cooling equipment. In many warmer climates, a solar heating system can provide a very high percentage (50 to 75%) of domestic hot water energy. Electricity generation As the electricity grid becomes increasingly vulnerable to faults from equipment failure, willful attack or even sunspot activity, the risk of a major national scale grid failure is rising. The deployment of renewable technologies usually increases the diversity of electricity sources and, through local generation, contributes to the flexibility of the system and its resistance to central shocks. The IEA suggests that attention in this area has focused too much on the issue of the variability of renewable electricity production. However, this only applies to certain renewable technologies, mainly wind power and solar photovoltaics, and its significance depends on a range of factors which include the market penetration of the renewables concerned, the balance of plant and the wider connectivity of the system, as well as the demand side flexibility. Variability will rarely be a barrier to increased renewable energy deployment. But at high levels of market penetration it requires careful analysis and management, and additional costs may be required for back-up or system modification. Renewable electricity