Solar thermal collectors and applications

Industrial process heat

Beyond the low temperature applications there are several potential fields of application for solar thermal energy at a medium and medium-high temperature level (80-240 °C). The most important of them is heat production for industrial processes. The industrial heat demand constitutes about 15% of the overall demand of final energy requirements in the southern European countries. The present energy demand in the EU for medium and medium-high temperatures is estimated to be about 300 T W h/yr [117].

From a number of studies on industrial heat demand, several industrial sectors have been identified with favour­able conditions for the application of solar energy. The most important industrial processes using heat at a mean temperature level are: sterilising, pasteurising, drying, hydrolysing, distillation and evaporation, washing and cleaning, and polymerisation. Some of the most important processes and the range of the temperatures required for each are outlined in Ref. [20].

Large-scale solar applications for process heat benefit from the effect of scale. Therefore, the investment costs should be comparatively low, even if the costs for the collector are higher. One way to cause economically easy terms is to design systems without heat storage, i. e. the solar heat is fed directly into a suitable process (fuel saver). In this case the maximum rate at which the solar energy system delivers energy must not be appreciably larger than the rate
at which the process uses energy. This system however cannot be cost-effective in cases, where heat is needed at the early or late hours of the day or at nighttimes when the industry operates on a double shift basis.

The types of industries that spent most of the energy are the food industry and the manufacture of non-metallic mineral products. Particular types of food industries, which can employ solar process heat, are the milk and cooked pork meats (sausage, salami, etc.) industries and breweries. Most of the process heat is used in food and textile industry for such diverse applications as drying, cooking, cleaning, extraction and many others. Favourable conditions exist in food industry, because food treatment and storage are processes with high energy consumption and high running time. Temperature for these applications may vary from near ambient to those corresponding to low-pressure steam, and energy can be provided either from flat-plate or low concentration ratio concentrating collectors.

The principle of operation of components and systems outlined in the previous sections apply directly to industrial process heat applications. The unique features of the latter lie in the scale on which they are used, and the integration of the solar energy supply system with the auxiliary energy source and the industrial process.

The two primary problems that need to be considered when designing an industrial process heat application concern the type of energy to be employed and the temperature at which the heat is to be delivered. For example, if a process requires hot air for direct drying, an air heating system is probably the best solar energy system option. If hot water is needed for cleaning in food processing, the solar energy will be a liquid heater. If steam is needed to operate an autoclave or sterilizer, the solar energy system must be designed to produce steam probably with concentrating collectors. Another important factor in determination of the most suitable system for a particular application is the temperature of the fluid to the collector. Other requirements concern the fact that the energy may be needed at particular temperature or over a range of temperatures and possible sanitation requirements
of the plant that must also be met as for example in food processing applications.

The investments in industrial processes are generally large, and the transient and intermittent characteristics of solar energy supply are so unique that the study of options in solar industrial applications can be done by modelling methods (Section 4.6) at costs that are very small compared to the investments.

Many industrial processes use large amounts of energy in small spaces. If solar is to be considered for these applications, the location of collectors can be a problem. It may be necessary to locate the collector arrays on adjacent buildings or grounds, resulting in long runs of pipes or ducts. Where feasible, collectors can be mounted on the roof of a factory especially when no land area is available. In this case shading between adjacent collector rows should be avoided and considered. However, collector area may be limited by roof area and orientation. Existing buildings are generally not designed or orientated to accommodate arrays of collectors, and in many cases structures to support collector arrays must be added to the existing structures. New buildings can be readily designed, often at little or no incremental cost, to allow for collector mounting and access.

In a solar process heat system, interfacing of the collectors with conventional energy supplies must be done in a way compatible with the process. The easiest way to accomplish this is by using heat storage, which can also allow the system to work in periods of low irradiation and/or nighttime.

The central system for heat supply in most factories uses hot water or steam at a pressure corresponding to the highest temperature needed in the different processes. Hot water or low pressure steam at medium temperatures (< 150 °C) can be used either for preheating of water (or other fluids) used for processes (washing, dyeing, etc.) or for steam generation or by direct coupling of the solar system to an individual process working at temperatures lower than that of the central steam supply (Fig. 38). In the case of water preheating, higher efficiencies are obtained due to the low input temperature to the solar system, thus low-technology

collectors can work effectively and the required load supply temperature has no or little effect on the performance of the solar system.

A number of research papers on the subject have been presented recently by a number of researchers. Norton [156] presented the most common applications of industrial process heat. In particular the history of solar industrial and agricultural process applications were presented and practical examples were described.

A system for solar process heat for decentralised applications in developing countries was presented by Spate et al. [157]. The system is suitable for community kitchen, bakeries and post-harvest treatment. The system employs a fix-focus parabolic collector, a high temperature FPC and a pebble bed oil storage.

Benz et al. [158] presented the planning of two solar thermal systems producing process heat for a brewery and a dairy in Germany. In both industrial processes the solar yields were found to be comparable to the yields of solar systems for domestic solar water heating or space heating. In another paper, Benz et al. [159] presented a study for the application of non-concentrating collectors for food industry in Germany. In particular the planning of four solar thermal systems producing process heat for a large and a small brewery, a malt factory and a dairy are presented. In the breweries, the washing machines for the returnable bottles were chosen as a suitable process to be fed by solar energy, in the dairy the spray-dryers for milk and whey powder production and in the malt factory the wither and kiln processes. Up to 400 kW h/m2 per annum were delivered from the solar collectors, depending on the type of collector.