Combined solar heating systems

by J Rekstad, B Bjerke and M Meir, SolarNor AS, Norway

 Example of a SolarNor combi-system in Norway.

Due to its cold climate, the development of solar systems in Norway has always naturally focused on combined domestic hot water and space heating systems - so-called 'combi-systems'. Compared to more conventional solar domestic hot water systems, combi-systems require larger solar collector areas and larger heat stores.

Heating systems

Conventional heating systems have been developed over a long period, and their development was influenced much more by the availability and costs of electricity, oil and gas than by energy efficiency. Solar collectors and related components have often been developed as auxiliary equipment to conventional heating systems. These collectors have to withstand the high temperatures in these conventional systems, and this has led to competition to achieve the best collector efficiency in the high-temperature range.

The introduction of solar systems with large collector areas, such as combi-systems, represents a major change which justifies a review of how to design such a heating system. The temperature dependency of the solar collector performance, the capacity of the heat store and the increasing heat losses with temperature are strong arguments for minimising the temperature level of the heat distribution system. By eliminating heat exchangers and increasing heating surfaces, the system temperature may be reduced considerably. Exchanging conventional radiators, which operate with 20-30C overtemperature, with floor- or wall heating systems operating at an overtemperature of 5-7C will in most cases have a significant impact on the solar fraction.

The combi-system

By permitting the system to operate down to a system temperature as close as possible to the room temperature, new and challenging demands on the controller system are introduced. This article features a combi-system manufactured by SolarNor, Norway and developed with financial support from the Norwegian Research Council.

The controller in the SolarNor system has three functions:

  • to secure the maximum energy output from the solar collector;
  • to regulate the thermostat in such a way that the auxiliary heat supply to the store is minimised and the state of readiness for receiving solar heat is maximised;
  • to stabilise the room temperature by regulating the energy supply to the floor heating system.

All of these are important in obtaining the best possible energy economy, but this article will concentrate on the third of these control functions.

Temperature regulation

Low-temperature heating systems, such as floor heating, require different regulation principles from conventional heating systems because the low-temperature system offers limited power for fast correction of the temperature. This is a condition for successful use of feed-back control governed by a room thermostat. Room thermostat control of floor heating systems often gives unwanted temperature oscillations around the desired set temperature and unneccessarily raises the heating demand. To solve this problem a new control strategy was developed.

The heat capacity of the floor is considerable, and this capacity will smooth out short-term variations in the temperature of the water in the floor heating tube. Hence, within a certain time-window dependent on this heat capacity, there is no measurable difference in the floor surface temperature whether the heat is transferred in one portion, or by continuous transfer of the accurate power. We therefore suggested that the circulation pump should operate at intervals rather than continuously, and that the interval is determined by a microprocessor from the store temperature (which may vary considerably) and the outdoor temperature. The controller also collects information about the solar radiation available, which offers the opportunity to compensate for passive solar heating through the windows. The principle and the coupling scheme are shown in figure 1.

The regulation principle has been tested over three years, in various houses in Norway, with convincing results. Figure 2 demonstrates the regulation principle by exhibiting the temperature of different parts of a typical wooden floor construction. The measurements were carried out in the solar laboratory at the University of Oslo's Department of Physics.

Figure 1: The floor temperature regulation is based on information on store temperature,
 ambient temperature and solar intensity, and is performed by varying
 the operation time of the pump.

Figure 2: Temperature variation in a wooden floor with heat tubes and
 heat distribution plates in aluminium.

Energy use

This operational principle also offers an attractive method of monitoring the energy use in multi-apartment buildings and blocks. Conventional monitoring systems, based on measuring the flow and the inlet and outlet temperatures of the floor-heating loop have serious difficulties in determining the energy use because of the small temperature differences. The SolarNor controller stores information about the heat pulses delivered to the floor in a register. Since the circulation time and energy delivery through the floor are nearly proportional quantities in a low-temperature floor heating system, this information offers a new and simple way to determine the energy use.

In the development of solar systems the main focus has been on the solar collector itself. However, the performance of solar energy installations depends on all major system parts and the interplay between them. The temperature level and the control strategy for low-temperature systems have particular importance for the solar fraction as well as the comfort of customers and hence their acceptance of solar energy systems.

For more information contact the CADDET Norwegian National Team at Rud.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

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