Category Archives: EuroSun2008-10

How to reach success?

More money within the trades is required to increase the availability, marketing efforts and information to the public. This money can be supplied by stronger actors: “companies with larger monetary assets help more, and when there is more money, the development will be faster”. But the government could also contribute, both for information campaigns and by introducing different types of instruments of control. Another necessity is to involve the installers and retailers, make them interested and willing to promote the systems. To simplify installation, but also enable larger product quantities and cheaper systems, they have to be standardized. Because, to actually reach success the combined solar and pellet systems have to compete and be comparable to heat pumps, both in the economic sense and the requirements of comfort and security from the costumer.

The different actors involved in a system installation often meet first at the actual installation, which is too late to enable highly efficient and well-planned systems. A joint trade organisation for solar and pellet could be the appropriate forum to advance in the development of a concept solution. But it would also constitute a stronger lobbyist against authorities, which in turn could generate more governmental support and thereby information to the public. The ball would then be set rolling.

The systems are already close to reaching out, and considered being “on the right track”; it is more or less only a matter of time. There are strong beliefs that as soon as the combined systems really reach the market, success will also come.

4. Conclusion

Some key words were recurring during the interviews, such as marketing, governmental standpoint, good examples or role models, attitude of the installers, easily operated systems and the power of the consumer. These issues are therefore assumed to be important factors in the development of a concept with combined solar and pellet heating, but probably also for other system solutions. The principal factor to further establish the concept was experienced to be increased marketing and information to

the public. Prerequisites for that are however increased monetary assets within the trade, but also long­term governmental instruments of control.

The attitude to combined solar and pellet systems is by mutual consent changing towards general acceptance, and it is happening right now. The informants agree that the change has just started, but the success of this type of system solution must come, and it is just a matter of time.

5. Future work

Further analysis of the interview material will be done in future work, focusing on for example knowledge about the systems, the attitudes towards each other within the trades, cooperation within the trades and between individual companies, as well as the role of the costumer. This will be reported in future journal articles.


The work has been carried out under the auspices of The Energy Systems Programme, which is primarily financed by the Swedish Energy Agency. Thanks to all the informants for being so open and sharing information and thoughts. You made this study possible.


[1] Merriam, Sharan B., Fallstudien som forskningsmetod (Org. title: Case Study Research in Education), Studentlitteratur, Lund, ISBN 91-44-39071-8, 1994. In Swedish.

[2] Kvale, Steinar, Den kvalitativa forskningsintervjun (Eng. title: InterWiews), Studentlitteratur, Lund, ISBN 91-44-00185-1, 1997. In Swedish.

[3] Statistics Sweden and Swedish Energy Agency, Energy statistics for one — and two-dwelling buildings in

2006, ISSN 1404-5869, 2007, available at www. scb. se. In Swedish.

[4] Swedish Ministry of Enterprise, Energy and Communication, Investeringsstod for konvertering fran direktverkande elvarme I bostadshus, Promemoria 2005-07-04, available at www. regeringen. se. In Swedish.

[5] Eriksson, L., Boom for varmepumpar nar oljepriset rusar i hojden, www. nyteknik. se/art/37399, Ny Tekniks webbtjanst, published 2004-11-10. In Swedish.

[6] Prockl, E., Forsaljningen rasar for svenska varempumpar, www. nyteknik. se/art/43645, Ny Tekniks webbtjanst, published 2005-12-07. In Swedish.

[7] Swedish Energy Agency (Energimyndigheten), Energilaget 2007, available at www. swedishenergyagency. se,

2007. In Swedish.

[8] Swedish Energy Agency (Energimyndigheten), Energilaget 2006, available at www. swedishenergvagencv. se, 2006. In Swedish.

[9] Svensk Solenergi (Solar Energy Association of Sweden), Solenergisystem i Sverige — Marknadsutveckling 1998-2007, March 2008, available at www. svensksolenergi. se. In Swedish.

[10] Fiedler, F, S. Nordlander, T. Persson and C. Bales, Thermal performance of combined solar and pellet heating systems, Renewable Energy 31 (2006) 73-88.

System Description

Подпись: Fig. 1. The demonstration site in Borlange, with (left) water jacketed stove, (middle) technical and store units in a bathroom under the stairs, and (right) view of the 10 m2 collector array on the roof.

The REBUS system concept consists of two compact units; the solar store unit and the technical unit that contain the hydraulic components including the pellet boiler and an auxiliary store for the hot water production (see also figure 2). Both units are built in cabinets of 60cm x 60cm (width, depth) which are the standard dimensions for kitchen and bathroom units like refrigerators and washing machines. The system uses a 12 kW water jacketed pellet stove with an internal water volume of 20 litre. The stove heats an auxiliary standby store and the upper part of the solar buffer store. The solar store comprises a water volume of 360 litre and has, due to high efficient vacuum insulation at the hottest parts of the store, a low UA-value of about 1.8 W/K. The solar system can provide heat to both stores. Instead of the separate water jacketed pellet stove, an integrated or separate pellet boiler can be used without any changes in the hydraulic layout. The system is equipped with one central controller with a specifically developed software for the control for all system components except the pellet stove. A detailed description can be found in [2].

In July 2006 a prototype of the REBUS system was installed in a single family house in Borlange/Sweden that had earlier been heated with electrical radiators. In the main house these heaters were replaced by water based radiators. First the two units were installed without a pellet heater. In the middle of October 2006 the water jacketed pellet stove was added in the living room where a chimney was accessible. Up until this point a 6 kW electrical heater in the 80 litre standby store in the technical unit was used as the auxiliary heater. The stove has an integrated 38 kg pellet store which is fed manually by the owners (see Figure 1, left). The technical and store units were installed in a small room that earlier was used as a second bath room. (see Figure 1, middle). The collector field of 10 m2 (Figure 1, right) consists of four modules of Svesol premium AR with a standard rated output of 465 kWh in Stockholm at constant 50°C. It was placed on the main roof facing 40°E with a slope of 40°.

Heat rejection technologies for Solar Combi+ systems: dry cooler and wet cooling tower

F. Besana12*,

J. Rodriguez1, W. Sparber1

1 EURAC Research, Institute of Renewable Energies, Viale Druso 1, 39100 Bolzano, Italy
2 Universita degli Studi di Bergamo, Viale Marconi 5, 24044 Dalmine (BG), Italy
Corresponding Author, francesco. besana@eurac. edu


In the present work the attention is focused on the heat rejection part of solar combi+ systems looking at the performance of two different technologies: air-cooled heat exchanger and wet cooling tower. A deck for dynamic simulations in TRNSYS of a small Solar combi+ system has been prepared, where a thermally driven chiller with both the technologies runs. The air-cooled heat exchanger is simulated by an EES based code. In this investigation the effects of three different climates, resulting in different available dry and wet bulb temperatures, on the performance of the heat rejection equipment and of the solar combi+ system are studied. In particular the specific primary energy consumption per unit of rejected heat and the specific costs of the heat rejection per unit of cooling energy produced by the chiller are calculated. For the geometries of the cooling tower and dry cooler here considered and without implementing any fan speed control strategy, the results show that the wet cooling tower has a lower primary energy consumption and specific cost compared to the dry cooler in all the three different locations.

Keywords: heat rejection, dry-cooler, wet cooling tower, solar combi+ systems, absorption chiller.

1. Introduction

Since the beginning of the 1980s, the growth rate of the utilization of solar collectors for domestic hot water production has shown that solar heating systems are both mature and technically feasible. However for several years, solar thermal systems seemed to be restricted to this application. When the first systems for combined domestic hot water production and space heating, called solar combisystem, appeared on the market, complex and individually designed systems were the rule. Especially in the Mediterranean regions the design had to take into account the seasonal displacement between the solar energy availability peak and the heating demand peak of the building. This, in summertime, could lead to overheating phenomena (stagnation temperature) causing thermal stress for most of the components and driving down the overall efficiency of the system. For these reasons the typical size of systems for single-family houses doesn’t exceed 15 m2 of solar collector area [1].

An integration of a small thermally driven chiller in a solar combisystem is a suitable solution to manage in a better way the heat production by solar collectors [2]. The share of building loads met by
solar energy can be increased, thereby reducing conventional energy consumption and giving to this technology a better possibility to penetrate the market. A system of this kind, which provides space cooling as well as space heating and domestic hot water, is called Solar combi+ system.

Next to the installation of a thermally driven chiller, cooling towers are often used in this kind of applications to reject heat coming from the cooling circuit of the chiller. In southern region for part of the day, this requirement needs evaporative cooling processes to cool down the water below the ambient temperature. For this reason beside the fossil fuels saving, drinkable water consumption has to be reduced especially in the regions with low water availability [3]. Moreover in few European countries there are law restrictions for this technology due to the possible legionella disease proliferation.

An air-cooled heat exchanger can be a possible solution for replacing a cooling tower. With this technology a good compromise between the efficiency of the absorption chiller and the electricity consumption is achieved.

Many articles treating this theme can be found in literature and some of them were used as starting point for this paper. Although the investigation was conducted for big capacity and for industrial processes purposes, overviews on heat rejection technologies and useful approach to this theme are presented by Jaggi/Gtintner manufacturer and University of Stellenbosch in South Africa, [4,5]. A contribution to these overviews in the direction of residential purposes with small heat rejection capacities and a different approach with dynamic simulations is given below.

The Housing Authority Initiative

Подпись: Fig. 1. Outer building envelope of Tal-Ftieh Social Housing Energy Saving Project, Birkirkara, Malta.

The Housing Authority took the initiative to build the first Energy Saving Social Housing Project at Tal-Ftieh, Birkirkara, Malta. This was mainly comprised of two adjacent blocks with a total of 10 apartments and 1 showroom. A workgroup of experts were appointed to prepare a number of energy­saving measures for this project [3] . Figure 1 shows the external building envelope with some of the implemented energy-saving measures such as, louvered windows, double-glazing, shading and solar applications. Following the success of this project, the Housing Authority decided to adopt a policy of energy saving for all its future buildings. These include thermal insulation on roofs and walls, double­glazing, louvered windows, solar water heaters, and an underground rain-water reservoir [4] .

Подпись: Fig. 2. Evacuated-tube solar heaters installed on the roof of Tal-Ftieh Social Housing Energy-Saving Project.

specifications. This was followed by commissioning of the systems and a one-to-one meeting with the users, to explain the system for them and ask them to take readings from the electronic display in the morning and the afternoon. This data was then collected on a monthly basis and analysed accordingly.

Validation of the test sequence for the reference system with different characteristics

For each hydraulic scheme, different size of the collector area (from 5m2 to 25 m2) and heat store volume (from 400 litres to 1500 litres) were used. In order to know if the optimised test sequence is also suitable for other reference buildings (SFH30, SFH100) further simulations with varying solar combisystem sizes were carried out.

Storage volume (l)


area (m2)

Auxiliary consumption for heating and domestic hot water based on :



Annual simulation (kWh)

Test sequence and extrapolated to an annual value (kWh)














































Table 3. Comparison of the auxiliary consumption for an annual simulation, and for the prediction based on the result of the test sequence (The results in the table are related to the Zurich climate conditions and a single family house with a 60 kWh/m2.year space heating demand).

Подпись: Auxiliary energy used for the reference Solar Combisystem 5 000 7 000 9 000 11 000 13 000 15 000 17 000 19 000 21 000 23 000 25 000 Annual consumption (Kwh) 25 000

23 000

21 000 A


Fig. 3. Predicted consumption with the test sequence versus annual consumption for Zurich climate and the

reference Solar Combisystem.

Although the test sequence was optimised for the SFH60 building, the results from building type SFH100 and SFH30 are promising. Moreover the criteria chosen to select the days are working for different climates.

For Zurich climate it can be seen that predicted and annual simulation values are close to each other. Best results can be achieved in case of building type SFH100. On the other way for Stockholm climate best results are obtained for building type SFH60.


S. Furbo1* and A. Thdr2

1 Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs.

Lyngby, Denmark

2 AEE INTEC, Feldgasse 19, A-8200 Gleisdorf, Austria
* Corresponding Author, sf@byg. dtu. dk


In July 2006 a new developed solar heating/natural gas heating system was installed in an old one family house in Denmark. The new system is based on 6.75 m2 solar collectors and a condensing natural gas boiler. Before the installation of the solar heating system, the house was heated by a non condensing natural gas boiler.

The heat demand, the electricity consumption for the energy and heating system as well as the natural gas consumption were measured before and after installation of the solar heating system. Based on the measurements the yearly energy savings of the solar heating system are estimated to 3600 kWh for 2005 and 4000 kWh for 2007, corresponding to 530-590 kWh/m2 collector per year. The energy savings will vary from year to year. In years with a high heat demand and a high solar radiation, especially in the spring, the energy savings will be high. In years with a low heat demand and a low solar radiation, especially in the spring, the energy savings will be low. The yearly heat production of the solar collectors is in 2007 about 2/3 of the yearly energy savings.

Keywords: Solar combi systems, energy savings, measurements

1. Introduction

Only few investigations on energy savings for solar heating systems in practice have been carried out, [1]. This is remarkable since most solar heating systems are installed with the aim to save energy. The reason for the few investigations is that it is extremely difficult to measure/document the energy savings for solar heating systems in practice.

In order to determine the energy savings, seven energy quantities/efficiencies must be considered.

Before installation of the solar heating system:

• Utilization of energy for the energy system.

• Electricity consumption for the energy system.

After installation of the solar heating system:

• Net utilized solar energy of the solar heating system.

• Saved energy by turning off the auxiliary energy supply system during the summer.

• Utilization of energy for the auxiliary energy supply system.

• Electricity consumption for the auxiliary energy supply system.

• Electricity consumption for the solar heating system.

Further, to make the determination even more difficult, the above mentioned energy quantities and efficiencies and thereby also the energy savings are influenced by the heat demand and hot water

consumption, which will vary from year to year due to weather variations, variations of user habits and due to changes of the hydraulics of the energy system.

A Swedish investigation of solar heating systems, based on questionnaires filled in by home owners, showed unexpectedly high energy savings for solar heating systems in practice [2]. Also unexpectedly large variations of the energy savings were reported. Solar heating systems with collector areas between 4 m2 and 25 m2 were included in the investigations. The reported energy savings ranged from 0 kWh per m2 to 2750 kWh per m2. The average collector area of the investigated systems was 11 m2, and typical energy savings ranged from 650 kWh per m2 collector to 900 kWh per m2.

A theoretical investigation showed that the energy savings of solar heating systems are strongly influenced by the efficiency of the energy system prior to installation of the solar heating systems

[3] . Especially the efficiency during the summer period is of great importance.

An investigation of new natural gas boilers and oil fired burners installed in one family houses without solar heating systems showed unexpectedly low utilizations of natural gas and oil in the summer and in periods with low heat demands [4]. This is of great interest in countries, such as Denmark, where oil and natural gas are often used as primary energy sources in houses, where solar heating systems are installed. A condensing and a non condensing natural gas boiler as well as a non condensing oil fired burner were included in the investigations. In spite of high yearly utilizations of natural gas and oil between 80% and 95%, the utilization of natural gas/oil decreased to values between 50% and 80% for the boilers/burner in the summer months. In the 5 summer months May-September the energy loss defined as the oil/natural gas consumption minus the space heating demand minus the hot water consumption was about 1000 kWh for the oil fired burner and the non condensing natural gas boiler and about 500 kWh for the condensing natural gas boiler. These energy quantities can easily be saved by well performing solar heating systems.