The environmental value of solar thermal

Solar panels connect us to an inexhaustible energy source: the Sun. This energy is there ready on the roof and paradoxically, it is lost if it does not get used up. During its life span, which is usually more than 20 years, one square meter of solar collector (let us remind you that to obtain hot water an average family needs about 4-5 square metres of solar collectors) recovers a median, at our latitude, of around 12.000 kWh, which is the equivalent of, say, burning 3.000 m3 of methane gas. This fuel is a primary source of energy, which Italy imports from abroad. A widespread use of solar collectors by Italian families could make a considerable contribution to the balance of payment, as well as its own domestic saving. .

In the graph below we have shown the diffusion of the installation using solar collectors for the production of hot water across Europe, expressed in square metres of collectors for each 1.000 inhabitants. The data has been brought up to date in 1998. At any rate nothing particularly significant has happened since that date and our country remains one of the "small men" of Europe.

Solar energy

In astronomical terms the Earth is located about 150 million kilometres from the Sun, and this fact guarantees an irradiation of 1,37 kW/m2

This energy was made available around 5 million years ago and will continue to be so for as many more years. Considering that a part of this energy is reflected by the atmosphere and another part is dispersed in a different manner it is estimated that at our latitude and with clear skies, we have around 1 kW/m2 at our disposal.

The total energy received by the Earth from the Sun, referring solely to dry land surface, corresponds to 15,000 times the world's consumption of energy for the same amount of time.

The table below shows the average values of solar energy around Italy:

Efficiency and Performance

The performance of a solar collector here refers to the type of panel with thermal insulation and vitreous surface, which represents the most common collector, with a good relation to costs/performance.

The efficiency of a solar panel is related to the temperature of the water to be heated. The lower the temperature at the point of entry, the higher the quantity of heat which the panel intercepts and transfers to the exchange fluid.

One can reach up to 80% efficiency with an entry temperature of water equal or less than the temperature of the environment. This means that, for instance, if the solar irradiation measured with a solar meter reaches 1.000 W/m2, the solar collector manages to transfer to the fluid a quantity of heat corresponding to a power of 800 W for each square meter of surface exposed at right angles to the direction of the irradiation

As the temperature of the water that needs to be heated increases the efficiency drops until it is practically down to zero when the water reaches high temperatures of 80 - 100°C.

On installing correctly proportioned solar collectors one can expect to obtain almost 100% of requested hot water during summer (from May to September). During this period the temperature attainable is almost always higher than 50°C. During the remainder of the year the performance can vary considerably in relation to the meteorological condition: on average over the course of one year one can expect to obtain 70% of the necessary hot water in the Northern Italian regions and 80-90% in the Central and South Italian regions.

The traditional solar panel is generally installed facing South (differences of + / - 15° show no real performance differences) and with an inclination on the horizontal plane more or less corresponding to the angle of latitude of the place (in Italy the angle is 45°). This inclination corresponds to the best compromise between winter and summer situations. Italian roofs almost never have these inclinations. To be able to install the solar collectors in line with the roof tiles, to obtain better stability and an aesthetic look, it is necessary to take into consideration the non optimal plane of inclination and install a proportionally bigger surface of panels.

In the table below we show as an example the average solar energy, month by month expressed in kcal/m2 on a level surface with several inclinations in respect to the horizontal plane. The data refer to a locality in Northern Italy (Bologna)

Typical application of thermal solar energy

The most common method of using thermal energy from the Sun is the production of hot water for domestic use. This application gives large economical advantages, if the installation is planned correctly, of up to 70% of the hot water necessary in Northern Italy and up to 90% in Southern Italy, free from the Sun.
The typical family (4 people) requires usually a surface of well exposed collectors measuring from 4 to 6 square metres, with storage tanks from 200 to 300 litres.
Similar applications are used for the production of hot water for hotels, restaurants, communities, showers for sporting facilities, gymnasiums etc.
Solar energy also functions perfectly for heating water in pools (bathing-pools, swimming baths). Installation in covered swimming pools open all year are generally proportioned to cover about 50% of the required energy allowing considerable savings.
In open swimming pools, used mainly in summer, the solar collectors allow to lengthen the period of use of the establishment by 2-3 months.
.Other uses mainly during summer are showers in camping sites and on the beach.
.These applications are particularly suitable, as their main use takes place at the same time as the periods of greater availability of solar energy.
Another use of solar thermal energy is the winter heating of buildings. This is still little known because, contrary to the previous cases, a construction scheme giving fully satisfactory technical and economical results has not yet been identified. As we mentioned above, the efficiency of solar collectors is that much higher the lower the temperature of the fluid to be heated. Thus to produce heat it is always necessary to use a system at low temperature, like radiated panels under the floor or fixed to a wall.
The main difficulty for winter heating of buildings is given by the scarce availability of solar energy in the precise period in which it is needed. To resolve this problem several ways are currently in use:

• Very sophisticated installations with high efficiency collectors and electronic control panels which manage several accumulation tanks, are used to optimise the recuperation of all the available heat in winter. In any event it is necessary to have large areas of collectors, which are larger than usual and therefore unused in summer, and large reservoirs of fluid in the order of thousands of litres for building, which is not always possible to install. These installations are very expensive and guarantee rather low percentages of energy contribution, generally under 50%.

• Large stores of energy are built in the form of underground water tanks , which utilise also summer heat, therefore installing a reduced surface of solar collectors using them the whole year round. The difficulties in these systems are due to the dimensions that the storage capacity must have to be efficient. The tanks have volumes equal to the whole volume of the building that requires heating. In practice one must build enormous underground pools, generally put to the service of entire areas.

• Another method utilised in same cases consists of combining solar collectors with heat pumps. The heat pump improves the performance of the collectors, lowering the temperature of the fluid from which thermal energy is taken. It is also able to supply heat at contained costs taking it from other sources (for examples the air in the environment) when the solar collectors cannot perform well.

Solar plants

In its simplest form a solar plant for the production of hot water consists of the following elements (see fig 1):

• A solar collector, usually a flat metallic surface, black in colour, with channels for the transportation of fluid, contained by a glass pane in the upper part and by thermal insulating surfaces in the bottom and sides.

• A storage tank, called solar boiler, which serves to compensate for the variation of availability of solar energy, due to the alternation of day/night and limits of certain climatic conditions

• Piping to connect the installation elements together and the installation itself to the bulk of the domestic water installation.

Fig. 1

In this solution, the simplest, the installation works only if the boiler is placed higher than the collector and the latter placed inclined, so that the hot water can flow by natural convention from the panel (hotter) to the storage tank, pushing the cold water down to the panel. In this case the collector, that must be installed in the open air, has to be drained when the temperature can drop under zero.

In fig. 2 the storage tank, or boiler, is designed with a circular cavity for fluid with antifreeze. The circuit between the cavity and the collector is closed, so that it runs even when the temperature of the environment is below zero. With these solutions the storage tanks are left in the open, exposed to bad weather, and are not the right solutions for rough climates.

Fig. 2

A solution suitable for all climates is instead the following (fig 3)

Fig. 3

In this case the circuit for heat transport from the solar collector to the storage tank is separated from the water in the tank. The collector can therefore be filled with a blend of antifreeze and be used throughout the year. It is necessary to add a circulation pump and a control device of the pump (T), which will make it work only when the temperature of the fluid in the solar collector is higher than the water in the boiler.

Since the sun is not available every day of the year, the solar installation cannot supply hot water the whole year round, especially in our climatic zone.

The dimension of the boiler is usually tied to the dimension of the solar panels. A typical installation for a family (to obtain only hot water for domestic use) consists of approximately 4 square metres of panel and 200 litres of water as storage.

There are several solutions to produce hot water even when the sun is not out. The simplest consists of inserting an electric resistance in the boiler, which will intervene when the temperature of the water is too low. The resistance is placed at about 1/3 of the way from the top of the boiler. In this way, when the water is heated electrically, it tends to stay in the upper part of the tank, therefore a lesser quantity is heated, and the remaining water stays cold. When the sun is back, the panels heat the water contained in the bottom 2/3 of the tank.

Other possible solutions are shown in figures 4 and 5. In the first the boiler is also connected to the house's heating installation or to any other source of heating available. In that case during winter hot water can be had at very low cost, using, only when necessary, a portion of the heating supplied by the boiler of the heating system. In summer it is very rare to have to heat the water. In these cases the electric resistance can always be used.

Fig. 4

In the second the water that comes from the boiler is passed through a small wall boiler or a water heater (possibly a gas one). If the water heated by the solar panels is already sufficiently hot, the supplementary heating system does not intervene. If it does intervene, it will heat water that is already pre-heated, guaranteeing a saving in any case.

Fig. 5