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Combined Panels for hot water and heating

Solar thermal panels for combined systems that produce hot water and heating for spaces.

Traditionally, solar thermal panels have been used either for meeting hot water needs during the course of the year or to heat swimming pools during the summer, since the demand for heat occurs during periods with increased radiation.

In recent years, however, there is an increasing tendency to design and install combined solar systems, which satisfy hot water needs and which partially integrate heating for internal spaces.

The most commonly encountered interest in these systems lies in the creation of Task 26, a working group within the ‘Solar Heating and Cooling Program’ developed by the International Energy Agency (IEA).

The objective of Task 26 studies is to develop and optimize the combined solar system’s performance. In this context the Altener project is worth noting. It collects technical data, monitoring data and practical experiences on installations carried out in all of Europe.

What is a combined thermal system?

A combined thermal solar system must deal with two different types of needs:

1) hot water, which tends to be constant throughout the year,

2) heating, found only during the winter months.

This is where modifications in the design phase begin, compared to a single system which only produces hot water.

  • increase in the absorbing surface size and therefore the number of solar collectors;
  • increase of the solar panels’ inclination angle in order to take advantage of as much sun as possible during the winter and limit the energy produced during the summer. Optimum inclination exceeds 50°;
  • the need for a system that can provide water at two different temperatures for two independent circuits.

There are considerable advantages in this type of system.

The ability to meet the needs of two utilities guarantees flexibility: if, for example, there is no demand for hot water, the heat supplied by the panels will be shifted to the radiators.

Furthermore, compared to a system which only generates hot water, some cost items such as (control unit, antifreeze, the primary circuit piping, insulation) remain unchanged while others (support structure, installation) take advantage of economies of scale. A combined system is well-suited for homes where energy saving elements, such as a good level of surface insulation and a system for distributing heat at a low temperature (heated floor, wall and baseboard), have already been implemented.

A solar panel system creates a synergistic effect compared to the above for two reasons:

  • reducing the home’s energy requirements increases the impact of savings made through the solar system;
  • The efficiency of thermal solar panels drops if the average temperature of the heater fluid increases (external dispersions increase), thus a low temperature heating process has a positive effect on the system’s overall performance.

Types of combined systems

Simplified hydraulic diagram of a solar thermal system “tank in tank”

As far as the plant is concerned there are two most commonly used solutions:

1.Combined system with ‘tank in tank’ type accumulation.

The hot water tank (smaller in size) is located inside the water tank used for the heating circuit. The characteristic shape of the hot water tank is that of a ‘mushroom’. The mushroom shape enables it to take advantage of the convective motion of the hot water that rises naturally upwards and makes it available for sampling.   This solution is very advantageous from an energy point of view, as heat dispersion is minimized, and at the same time it’s quite compact, allowing all energy sources to be directed towards a single element, thus simplifying the hydraulic circuit.

2. Combined system with hot water storage tank and heat exchanger for heating circuit

By adopting this solution, the energy collected by the solar panels is sent mainly towards the tank. When the water reaches the desired temperature, the control unit will divert the thermal energy, using a 3-way valve, to the heater.

In this configuration, the amount of accumulation is reduced and at the same time the plant costs drop. The overall space is reduced, but at the same time the absorbing surface is substantial compared to the accumulation, thus increasing the risk of overheating in the summer months. Alternatively you can use an instant heat exchanger for hot water and use the tank for heating, but the above-mentioned technical aspects don’t change. Care should be taken in managing the thermal energy produced during the months when the heating is turned off.

As a matter of fact during the summer months, the heating produced by the solar panels is greater than the hot water requirements. It would therefore be appropriate to find a way to use the energy produced, such as heating an outdoor swimming pool. Should this not be possible, you can check the stagnation of the heat transfer fluid, found in the thermal collectors, that evaporates and as a consequence expands.

This condition does not pose a problem: each system component is guaranteed for high temperatures and the expansion duct is properly sized so that it can sustain excess amounts.

The monitoring of combined solar thermal systems requires a very high level of detail in order to determine how much heat is used to produce hot water and how much is used for the heating circuit.

This solar thermal application was launched, as often is the case, in Northern Europe and is finding widespread use in Italy.