Direct Solar Energy
thermal energy storage.This technology uses a natural underground layer (e.g., sand, sandstone or chalk) as a storage medium for the temporary storage of heat or cold. The transfer of thermal energy is realized by extracting groundwater from the layer and by re-injecting it at the modi- fied temperature level at a separate location nearby. Most applications are for the storage of winter cold to be used for the cooling of large office buildings and industrial processes. Aquifer cold storage is gain- ing interest because savings on electricity bills for chillers are about 75%, and in many cases, the payback time for additional investments is shorter than five years. A major condition for the application of this technology is the availability of a suitable geologic formation.
Active solar heating and cooling applications
For active solar heating and cooling applications, the amount of hot water produced depends on the type and size of the system, amount of sun available at the site, seasonal hot-water demand pattern, and instal- lation characteristics of the system (Norton, 2001).
Solar heating for industrial processes is at a very early stage of develop- ment in 2010 (POSHIP, 2001). Worldwide, less than 100 operating solar thermal systems for process heat are reported, with a total capacity of about 24 MWth (34,000 m² collector area). Most systems are at an exper- imental stage and relatively small scale. However, significant potential exists for market and technological developments, because 28% of the overall energy demand in the EU27 countries originates in the industrial sector, and much of this demand is for heat below 250°C. Education and knowledge dissemination are needed to deploy this technology.
In the short term, solar heating for industrial processes will mainly be used for low-temperature processes, ranging from 20°C to 100°C. With tech- nological development, an increasing number of medium-temperature applications—up to 250°C—will become feasible within the market. According to Werner (2006), about 30% of the total industrial heat demand is required at temperatures below 100°C, which could theoreti- cally be met with solar heating using current technologies. About 57% of this demand is required at temperatures below 400°C, which could largely be supplied by solar in the foreseeable future.
In several specific industry sectors—such as food, wine and beverages, transport equipment, machinery, textiles, and pulp and paper—the share of heat demand at low and medium temperatures (below 250°C) is around 60% (POSHIP, 2001). Tapping into this low- and medium- temperature heat demand with solar heat could provide a significant opportunity for solar contribution to industrial energy requirements. A substantial opportunity for solar thermal systems also exists in chemi- cal industries and in washing processes.
Among the industrial processes, desalination and water treatment (e.g., sterilization) are particularly promising applications for solar thermal energy, because these processes require large amounts of
medium-temperature heat and are often necessary in areas with high solar irradiance and high energy costs.
Some process heat applications can be met with temperatures deliv- ered by ‘ordinary’ low-temperature collectors, namely, from 30°C to 80°C. However, the bulk of the demand for industrial process heat requires temperatures from 80°C to 250°C.
Process heat collectors are another potential application for solar thermal heat collectors. Typically, these systems require a large capac- ity (hence, large collector areas), low costs, and high reliability and quality. Although low- and high-temperature collectors are offered in a dynamically growing market, process heat collectors are at a very early stage of development and no products are available on an industrial scale. In addition to ‘concentrating’ collectors, improved flat collectors with double and triple glazing are currently being devel- oped, which could meet needs for process heat in the range of up to 120°C. Concentrating-type solar collectors are described in Section 3.3.4.
Solar refrigeration is used, for example, to cool stored vaccines. The need for such systems is greatest in peripheral health centres in rural communities in the developing world, where no electrical grid is available.
Solar cooling is a specific area of application for solar thermal tech- nology. High-efficiency flat plates, evacuated tubes or parabolic troughs can be used to drive absorption cycles to provide cooling. For a greater coefficient of performance (COP), collectors with low con- centration levels can provide the temperatures (up to around 250°C) needed for double-effect absorption cycles. There is a natural match between solar energy and the need for cooling.
A number of closed heat-driven cooling systems have been built, using solar thermal energy as the main source of heat. These systems often have large cooling capacities of up to several hundred kW. Since the early 2000s, a number of systems have been developed in the small-capacity range, below 100 kW, and, in particular, below 20 kW and down to 4.5 kW. These small systems are single-effect machines of different types, used mainly for residential buildings and small com- mercial applications.
Although open-cooling cycles are generally used for air conditioning in buildings, closed heat-driven cooling cycles can be used for both air conditioning and industrial refrigeration.
Other solar applications are listed below. The production of potable water using solar energy has been readily adopted in remote or isolated regions (Narayan et al., 2010). Solar stills are widely used in some parts of the world (e.g., Puerto Rico) to supply water to households of up to 10 people (Khanna et al., 2008). In appropriate isolation conditions, solar detoxification can be an effective low-cost