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Solar Thermal Energy

The solar thermal energy has experienced a very important development in our country.

On the one hand, the current situation, thanks to the legislation in force that gives incentives to the installations of solar energy for different uses and, on the other hand, the citizen’s awareness concerning the need to minimize the dependence on energy of fossil fuels and also the need to reduce the emissions of gaseous pollutants creates an appropriate environment to develop solar energy in Spain, not to speak of the constant crude oil price fluctuations due to political instability of producing countries.

Among the applications of solar thermal energy, we can underline the following:

We may not forget about Spain’s privileged geographical situation for the use of this energy, since it receives a higher radiation compared to other European countries.

Nowaydays in our country only some Town halls (Barcelona, Madrid, Rivas-Vaciamadrid, Seville, Valencia and Molina de Segura among others) had a specific legislation reflecting the obligation for new work buildings to install solar energy systems in order to produce sanitary hot water.

These solar regulations together with other technical documents have served as inspiration for ellaborating the Technical Building Code (TBC), and after two years of revisions it will see the light shorlty. In the TBC, the obligation to install renewable energy systems in order to produce SHW will be extended to the whole national territory.

Basic Concepts

The running system of solar thermal energy is very easy. Generally, it can be explained as follows: the sun heats the heat-transfer liquid circulating across the sensors (normally a mixture of water and anti-freeze). The heat “absorbed” by this liquid is given to the cold water of the network by means of an exchanger, so that we get hot water to be used, meanwhile the heat-transfer liquid returns to the sensors with a lower temperature in order to be heated again.

The energy contribution obtained from the installation of solar energy depends on certain factors, such as the direction of sensors, their angle, the water temperature, the appearance of shadows on sensors (this should be avoided whenever possible, shadowed locations are always discarded when designing the installations), etc. All these factors are taken into account when designing the installation, as well as to studying the percentage obtained from every particular case.

Compact Systems

The compact systems or natural circulation systems, also known as thermal syphonic systems, are used to cover small demands of H.S.W. in one-family houses generally. These systems do not need to run any active system such as pumps, electrovalves, etc.

The main components of these systems are the sensors, the tank and the pipes.

The accumulation volumes used in compact systems are around 200 or 300 litres.

The are many ways to install this kind of systems:

• Flat cover installation (roof/terrace/ground).
• Installation in an inclined roof.

The compact systems are only used whether there is a demand of S.H.W. in one-family houses.

If there is the need to meet a very high demand of hot sanitary water (for instance, 8-10 persons) or different demands at once such as H.S.W. and heating, it will be necessary to use forced circulation systems, which allow to work with higher flows, higher accumulation volumes and one storage tank for each application.

- Forced Circulation Systems

The Solar Thermal Energy systems by means of forced circulation are used in the event of a great demand of hot water.

The advantages of these facilities compared to compact systems are, among others, a greater control of the installation and a higher performance.

The Solar Thermal Energy systems by means of forced circulation are made up of a series of components which can be summarized as follows:

• Sensors
• Exchanger
• Storage tanks. In the event of a swimming pool, there will be no storage tank. If there are many demands, a tank will be installed for every one of them (excluding the swimming pool)
• Pumps
• Controller
• Energy counters
• Electrical sytem-valves, manometers, pipes, etc

- Demand of S.H.W. over 300 litres

This kind of demands are produced in the event of one-family houses with more than 7 occupants, buildings of houses or buildings with special uses such as hospitals, hotels, etc.

- Heating

In the event of a demand to support heating, either radiant floor heating or by means of radiators, it shall always be used forced circulation systems.

If the installation is provided with radiant floor heating, there will be less needs to be met, since the temperature of water is lower than in the event of using radiators.

Concerning situations where the contribution of solar energy for heating is secondary, giving priority to the production of H.S.W., the heating coverage will decrease slightly, thus obtaining a total contribution in H.S.W.

In these type of installations, as well as in the rest of the solar thermal energy installations, according to the Code for Thermal Installations in Buildings and the Technical Specifications, it is compulsory to have an auxiliary energy system in order to meet the requirements which are not covered by solar systems and in the event of an occasional need it may assume all energy requirements.

- Extension of bath season in swimming pools

This is one of the most common demands covered by forced circulation systems. The coverage of these systems depends on many factors such as the area of the swimming pool, indoor or outdoor, the use of thermal blanket, etc.

- Industrial Uses

Many Spanish companies, such as food companies use hot water in their processes. The volume of water needed for theses processes is very high compared to the use of hot water for household use.

The amount of energy used by these companies in order to warm water means a sufficient percentage of expenses for the installation of solar energy to be very cost-effective. The solar energy system would carry out a process for pre-heating the water, thus saving part of the current expenses of the company concerning fossil fuels, and also decreasing the local and global environmental pollution.

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