Solar Energy System of the Month:

Solar Thermal Combination Systems: An Asset for Every New Building

Solar thermal systems can cover a large part of the needed energy to heat potable water. In the summer the heating remains off: The solar heating system delivers enough heat for bathing, showering and washing machines from May to August. Throughout the remainder of the year the energy demand for warm water remains about the same. Even small solar thermal heaters reach efficiencies from 50 % to 65 %.

Together with the BINE Information Service (Bonn, Germany), issued by the Fachinformationszentrum (FIZ) in Karlsruhe, Germany, we will inform you of solar thermal combination systems used for backup, or support heating.


Solar collectors

HYBRID-layered hot water tank
Solar collectors used to heat potable water and support heating needs: Thermal solar heaters can reduce the yearly energy demand of a modern single-family home by about
25 %. Photo: SET
HYBRID-layered hot water tank for potable water and heating support. The layered hot water tank places solar-heated water in the layer having exactly the same temperature. Photo: Sailer Solarsysteme GmbH

During cooler months solar rays can also be efficiently used to cover a considerable part of the heating demand, which in the spring and autumn turns out to be relatively low. This is made possible by solar heating systems that can both heat potable water and support other heating needs. Low energy or passive buildings are especially qualified for solar heating support, as well as heating systems with low flow temperatures (maximum flow temp. 70ºC, maximum return flow 40ºC). Heating systems with high lay out design temperatures reduce solar yield less than expected, as proved in tests. For this reason combination systems are also of interest for the renovation of existing buildings, especially if they can be combined with boilers without seriously reducing their efficiency.

Exactly how combination systems must be designed is being investigated by work groups from the International Energy Agency (IEA), ensuring that solar circulation, space heating circulation, secondary heating, and storage tank all function together optimally. The IEA's work groups are developing storage tank parameters and testing methods to lay the groundwork for international standardization. A study of some system concepts delivered some surprising results: Solar combination systems reduce the total energy needs of a modern single-family home by about 20 - 30 %, dependent on the corresponding construction design, yet independent of which concept planners and building owners decide on. The system's output depends much more on its design and the quality of its components. The most important aspects concerning storage tanks are their optimal size and the high transfer performance of the heat exchanger.

Different System Concepts, Almost Equal Energy Savings

German institutions working on the procedures to rate solar combination systems in the IEA's "Solar Heating and Cooling Programm" include the Institute for Thermodynamics and Heat Technology (Institut für Thermodynamik und Wärmetechnik (ITW)) in Stuttgart, Germany and the Physics Department of the University of Marburg. They answer the questions that arise due to the many varieties of system concepts: Is a "tank-in-tank system" better than two separate storage tanks for space heating and potable water? What influence does conventional heating have on the solar heating output? Is it worth installing a complex system or is a system design that is as simple as possible good enough?

The answer is clear: Solar combination systems differ only slightly with respect to their energy saving potentials. At most, tested systems show differences of 0.5 %. Therefore, planners and building owners can concentrate on the building engineering and thereby take into consideration more aspects of this: the parameters of the boiler (output, timing behavior), the available buffer storage volume, the heat source for auxiliary heating, the uninterrupted supply of warm water (nominal temperature value and quantity) as well as the operating strategy for the solar circulation (low-flow or high-flow).

Single storage tank combination system

Single storage tank combination system: The tank serves additionally as a buffer storage for the boiler. Therefore, such systems are suited for use with wood-burning boilers.

Graphic: BINE Information Service (BINE Informationsdienst).

The engineers of ITW tested the various combination systems by means of a simulated single-family house constructed according to the Heat Conservation Provision of 1995 (Waemreschutzverordnung von 1995) on site at Würzburg, Germany. The model calculation assumed a four-person household, 128 m² of living area and a heating demand of 93 kWh/m² per year. The energy savings for all system variants were similar. Combination systems with 10 m² of flat-plate collector area and a storage tank volume of about 750 liters reduced the energy use for potable water heating and space heating of this 'typical' single-family home by about 25 %. According the specific needs of a building, planners and building owners can decide between systems with one or two tanks, with or without built-in heat sources, as well as for or against a return flow increase.

Compact systems with just one tank that also acts as buffer storage for the boiler dominate the German market. In conjunction with wood-burning boilers there remain no alternatives since such a buffer volume is urgently needed for their use. However, combination tanks with integrated gas or oil burners use a large storage tank to replace the boiler and its tank for heating domestic water.

Schematic diagram of a solar thermal combination system with a built-in gas burner

Schematic diagram of a solar thermal combination system with a built-in gas burner.

Graphic: BINE Information Service (BINE Informationsdienst).

Storage Tanks and Energy Yield

Like a complete system's concept, different storage tank concepts also affect no large deviations concerning the yearly energy savings: The deviations of four standard tanks of differing concepts were all between the boundary conditions of 19.7 % and 21 %. The total difference amounts to about 240 kilowatt-hours per year. Detailed tests have also shown that a storage tank's rate of heat loss and the standby volume's nominal temperature barely affect energy conservation.

The heat given off to the environment by a tank, despite its insulation, is calculated as a part of the rate of heat loss. "Good" tanks reach values of 2.5 watts per degree temperature difference (watt / Kelvin), and in practice tanks with volumes of 750 liters demonstrated heat loss rates of 4 - 5 W/K. In this respect, the tested tanks had an impact on the overall energy conservation of up to 5 %. Improvement of the insulation by the manufacturer would be one low-cost, easy solution-for example avoiding heat bridges due to poor workmanship and/or construction in areas with connections, flanges, and temperature gauges.

Influence of the heat loss rate on energy conservation

Graph: Influence of the heat loss rate on energy conservation.

Graphic: BINE Information Service (BINE Informationsdienst)

Building owners and planners can positively influence the solar energy yield by choosing a small volume and a low temperature for standby potable water. The amount of heated water that can be taken from the storage tank depends on the nominal temperature value of the standby volume. Next to uninterrupted supply, energy savings should also be considered: Even at a shutdown temperature of 43° C for the secondary heating (with the temperature gauge located at the lower level of the standby volume) the potable water demand is completely covered-higher temperatures noticeably decrease energy savings without offering further advantages.

Energy savings / shutdown temperature of secondary heating of the standby volume


Graph: Energy savings / shutdown temperature of secondary heating of the standby volume.

Graphic: BINE Information Service (BINE Informationsdienst).

Overall the system and storage tank concepts have very little effect on the solar yield and the heat costs of combination systems. They are by all means comparable to those used for potable water heating. When making a decision on a solar heating system for support heating, however, other criteria change the balance, for example the space requirements, the integration of more heat sources or the buffer storage tanks available for boilers.

Material and pictures: ITW and BINE Information Service (BINE Informationsdienst).
Further information such as literature, addresses, contact people and Internet links are available at:
Solarserver Editor: Rolf Hug


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