Medium-temperature technology includes systems which are able to reach temper­atures between 100°C and 250°C. The most common application of the medium — temperature solar thermal system is represented by the solar oven (see Fig. 77): a parabolic reflector (composed of aluminium sheets mounted on a zinc-plated steel structure) concentrates the solar radiation towards a single point which works as a cooking-stove. At this point, a pot is placed which warms itself and cooks the food contained inside. Using a solar oven it is possible to reach the same temperature as a traditional cooking-stove (about 200°C).

A solar oven with a diameter of 1 m takes nearly 18 minutes to boil 1 L of water, while it takes only 9 minutes if the diameter is wider (1.4 m). The reflector can be oriented on the basis of the Sun’s position so that it is possible to cook from morn­ing to afternoon and even to exploit the shortest moment of radiation. In Italy, the use of solar ovens is not common; they represent a very small slice of the market and their use is restricted to the those who consider it a hobby. In countries where lack of energy resources is a daily problem (such as Africa), the solar ovens can have good applications [12, 13, 18, 41].

In spite of the various advantages offered by thermal solar systems, their great potential has not been exploited much in the industrial sector. Thermal solar sys­tems can partially meet the heat demand for low — and medium-temperature (up to 250°C) processes, which are typical of a few industrial sectors such as the chemi­cal, food and textile industry. The thermal solar collectors that are now available now in the market, which we analysed when we talked about low-temperature solar thermal systems (par. 2.2), can reach temperatures of 100°C. As regards applications which need higher temperature (up to 250°C), the experiences are limited and suitable collectors do not exist. In 2003, the International Energy Agency (IEA) started a research project called Task 33/IV which aims to find more promising industrial applications in the thermal solar field and also to calculate the overall potential of thermal solar applications for the production of medium-tem­perature process heat. One of the Task 33/IV activities is research, developed together with the industry, on new collectors which can produce processed heat between 100°C and 250°C (a temperature range that is consistent with several industrial processes) [42-44].

At present, the collector typologies which are more promising in the medium- temperature field are:

• high efficiency glazed flat plate collectors: these are flat collectors with double

antireflection glass;

• linear parabolic collectors, similar to the ones used in the high temperature field

but much smaller (these will be analysed in par. 4.4.1).

• static concentration solar collectors: these are flat plate collectors or more frequently evacuated tube collectors characterized by static mirrors (fixed) for the concentration of solar radiation.

Figure 78: Double-glazed flat plate collector with antireflection glass.