Category Archives: EuroSun2008-10

Aims, scope and method

The aim of the project was to find the economic feasibility of using solar thermal energy as a complementing energy source in small scale (0.5-2 MWth) DH plants in north european conditions. Considered fuels saved are wood pellets or light heating oil. Also, the feasibility of moving the solar thermal plant was to be estimated, if possible.

Only short term thermal energy storage and ground mounted collector arrays were considered. Thermal load profiles for domestic hot water (DHW) and space heating (SH) on an hourly basis were created based on measured data from several sites. For the simulation studies the number of cases had to be limited; a “bad case” and a “good case” scenarios for solar were studied, assuming that most of other cases fall in between. Yearly energy balances and system sizings were obtained by simulations.

The feasibilities of the studied sizings were calculated with the Net Present Value (NPV) method. As sensitivity analysis different scenarios in fuel price increase and interest rates were considered.

Monitoring results

Подпись: Fig. 2. REBUS system schematic with sensor positions for the REBUS controller (blue with subscript c) and the monitoring system (red with subscript d).

In Figure 3 the heating energy supply for the building is presented from January 2006 to June 2007. The data before the installation of the REBUS system are calculated from the readings of the house owner from the main electrical meter and his records of wood use. The electricity for heating has been calculated based on measured household electricity in 2007. It can be seen that almost all bought electricity for heating was replaced by pellet and solar energy. About 200 kWh electricity were used by the electrical back up heater in the standby store in October 2006. The pellet stove was installed in the middle of October and not enough solar radiation was available, which explains the use of the electrical heater that month. Some electricity was also used in December 2007. Reason here was that the 80 litre standby volume heated by the pellet stove was too little to cover the peak heat demand so that the electrical heater in the standby store was backing up the heating.

Подпись: Fig. 3. Monthly heat supply for space heating and domestic hot water, *Electricity SH + DHW is the calculated electricity use for SH + DHW before the detailed monitoring.

This has also caused a high number of starts and stops with stove run times often not more than one hour. Consequently in the end of December 2006, the hydraulics have been modified so that also the upper third of the solar store, which is connected in serial with the standby store, is heated by the boiler.

This increases the standby volume to in total 200 litres. As a result of this modification it can be seen that the number of starts and stops have decreased drastically (Figure 4). The average run time increased to about 3 to 4 hours per start. Due to the longer run time of the stove also the proportion of the useful heat delivered from the stove to the water jacket has increased from 61% in December to 67% in January. This is still lower than the average 80% for the combustion range that is has been measured for the stove at the Austrian test institute BfL [1].


Подпись:Подпись:Подпись:Подпись: 2Подпись: 174Подпись: 291Подпись: okt-06 nov-06 dec-06 jan-07 feb-07 mar-07 apr-07 maj-07 jun-07Подпись: Fig. 4. Monthly heat delivery of the pellet stove and number of starts and stops.image0732500

In figure 5 the energy use for the monitored time period is presented. The monthly hot water demand varies between 240 and 400 kWh. The annual space heating demand is about 7400 kWh, which is relatively low for a building with a heated area of approximately 130 m2 and indicates a good thermal insulation.

During one year the solar collectors have delivered about 2529 kWh which is a reasonable value considering the small solar store volume, the non-optimal orientation of the solar collectors and the low heat demand. In terms of bought energy this gives a solar fraction of 19%.

Table 1: Annual heat supply and useful heat

Annual heat supply (kWh)

Annual useful heat




Space heating




Hot water









2. Discussion and Conclusion

The newly developed solar combisystem has been monitored for one year. For the most part the system is working as expected. All loops and components are properly controlled by the new controller software. Only some fine adjustments of some parameters were necessary. Most adjustments were necessary for the settings of the pellet stove controller to ensure a proper interaction with the main system. A modification of the hydraulic connections was done to increase

the buffer volume of the pellet stove. Using only the 80 litre volume of the standby store led to very short run times and many starts of the stove. After the change the run times of the stove increased and the number of starts and stops decreased drastically. Similar findings have been reported also in other studies (3; 5).

From the annual energy values in Table 1 the system efficiency can be calculated using the following equation:

_ annual useful heat

sys annualsupplied primary energy for heating

With a primary energy factor of 0.4 for electricity the total system efficiency is 0.83. Including also the parasitic electricity consumption of pumps, valves etc. decreases the system efficiency to 0.74. The parasitic electricity of 680 kWh accounts for almost 5 % or the total energy input to the system. It is obvious that the parasitic electricity consumption should be reduced e. g. by the use of more efficient pumps which stand for the main part of the parasitic consumption.

3. Acknowledgement

We are grateful to the Nordic Energy Research and the Dalarna University College for their financial support for this work within the REBUS project.

4. References

[1] BfL, "Prufbericht Pelletskaminofen EVO AQUA." BLT Aktzahl: 053/04, Bundesanstalt fur Landtechnik, Wieselburg, Austria. 2003.

[2] F. Fiedler, "Combined solar and pellet heating systems — Study of energy use and CO-emissions," PhD thesis, Malardalen University, Vasteras. 2006.

[3] F. Fiedler, C. Bales, and T. Persson, "Optimisation Method for Solar Heating Systems in Combination with Pellet Boilers/Stoves." International Journal of Green Energy, 4[3], 325 — 337. 2007.

[4] S. Furbo, A. Thur, C. Bales, F. Fiedler, J. Rekstad, M. Meir, D. Blumberga, C. Rochas, T. Schifter — Holm, and K. Lorenz, "Competitive Solar Heating Systems for Residential Buildings (REBUS)." Byg, DTU, Copenhagen, Denmark. 2006.

[5] A. Heinz, "Fortschrittliche Warmespeicher zur Erhohung von solarem Deckungsgrad und Kesselnutzungsgrad sowie Emissionsverringerung durch verringertes Takten." Technische Universitat Graz, Institut fur Warmetechnik, Graz, Austria. 2006.

[6] IEA, "International Energy Agency — Solar Heating and Cooling Program." IEA-Task 26. 2002.

The air-cooled heat exchanger

The air-cooled heat exchanger, also known as dry cooler, air-cooler or fin-fan cooler, essentially consists of a finned tube heat exchanger bundle arranged above a fan plenum chamber. An induced draft fan respectively draws air across the finned tube bundles. The cooling water flows through the finned tubes, and heat is transferred from the water to the ambient air. With air-cooled heat exchangers, it is not possible to cool the medium below the ambient dry bulb temperature. In this case the difference between the medium outlet temperature and the air inlet temperature is generally referred to as the terminal temperature difference (TTD) or the approach temperature and values in the range of 7-8°C are considered economically feasible [4].


Fig. 2. Air-cooled heat exchanger

The dry-cooler is an air-cooled compact, finned-tube heat exchanger. The type of surface selected is a finned circular tubes, surface 8.0-3/8T [8], the surface geometric characteristics are:

• Tube outside diameter = 0.0102 m

• Fin pitch = 315 per meter

• Flow passage hydraulic diameter = 0.00363 m

• Fin thickness = 0.00033

• Free-flow area/frontal area, ct = 0.534

• Heat transfer area/total volume, a = 587 m2/m3

• Fin area/total area = 0.913

• Number of tubes = 120

• Number of rows = 6

• Number of passes = 10

The heat transfer from a bank of circular finned tubes in counter flow with both fluids unmixed is considered here. The air and the fluid outlet conditions as well as the fans consumption are calculated with the EES code. The finned-tube rows are staggered in the direction of the fluid velocity.

The heat transfer characteristics of the finned-tube heat exchanger are:

• Heat exchanger effectiveness = 0.68

• NTU (Number of Transfer Units) = 1.893

• Water-side heat transfer coefficient = 12.75 (kW m-2 K-1)

• Air-side heat transfer coefficient = 0.14 (kW m-2 K-1), [9]

The dimensions of the dry-cooler were supplied by the manufacturer, 1.050 m x 2.725 m x 0.15 m for 40 kW nominal cooling capacity. Three fans are installed above the compact heat exchanger and the maximum power of each fan is 0.9 kW.

Future Utilisation Potential Factor (FUPF)

In order to translate the technical results of efficiency into an easy-to-understand term, which would not alarm the users but give a proactive impression of their system, a new term had to be defined as:

FUPF = n-~nmonth, for nrnonh >0 Equation (2)


Where, FUPF = Future Utilisation Potential Factor, which gives the ratio of hot water reserved for future use to current hot water consumption, for a specific period.

Птах = maximum attainable efficiency for the specific solar technology under local

operating conditions (found to be 81.7% for these systems under local climatic condition), and

nmonth = mean efficiency for a specific period (a month for this case).

Подпись: Jan Mar May Jul Sep Nov Fig. 5. Future Utilisation Potential Factor (FUPF) for Flats 3 and 6.

A comparison was made between the FUPF values of Flats 3 and 6, which had complete data sets of at least 1 year each. Figure 5 showed that even though both owners were satisfied with the performance of their systems, it was apparent that Flat No. 6 had virtually no extra hot water storage during most months, while Flat No. 3 had up to almost twice the amount of hot water used, in reserve. This is a direct result of proper management of hot water usage, which was facilitated by the electronic control unit and the interest of the users to follow the daily weather forecast. The FUPF values for summer have little significance, since Flat No. 3 had fully covered the evacuated-tubes, in order to avoid overheating, while Flat No. 6 did not cover the tubes fully. As a result, one could notice that for July, the FUPF for Flat No. 6 was higher than Flat No. 3. This also translated into alarmingly high temperatures in the solar tank, which could be a potential hazard for skin burns and for a reduction in the lifetime of plastic hot water pipes of the apartment..

Solar CombiSystems Promotion and Standardisation (COMBISOL project)


1 CEA, LITEN, INES 50 avenue du Lac Leman, 73377 Le Bourget du Lac, France.

2 SERC, Hogskolan Dalarna, 78188, Borlange, Sweden.

3 ITW, University of Stuttgart, Keplerstrasse, 70174, Stuttgart, Germany.

4 ADEME, 500 route des Lucioles, 06560, Sophia Antipolis, France.

5 AEE INTEC, Feldgasse 19, 8200 Gleisdorf, Austria.

6 INES Education, 50 avenue du Lac Leman, 73375Le Bourget du Lac, France.

7 Plan Energi, Jyllandsgade 1, 9520 Skoerping, Denmark.

* Corresponding Author, mickael. albaric@cea. fr


Solar combisystems (SCS) are solar heating installations providing space heating as well as domestic hot water in buildings. Within a global solar thermal energy strategy, SCS are a key element to decrease the fossil energy demand for heating in existing and new buildings. This project will help to reduce the use of fossil fuels and hence also the emission of greenhouse gases. During 3 years (December 2007 — December 2010), experts from research, testing institutes and industry will work in the aim to encourage an accelerated deployment of SCS market — hence a higher share of heat produced by solar energy — and promote an improved quality of the installed systems.

Keywords: Solar CombiSystems, Performance, Standardisation, Promotion

1. Introduction

Solar combisystems are solar heating installations providing space heating as well as domestic hot water in buildings. Within a global solar thermal energy strategy, SCS are key elements to decrease the fossil energy demand for heating needs in existing and new buildings. From that point of view, their further market deployment will contribute to achieve the objectives of the White Paper on Renewable Energy Sources, of the Green Paper on Energy Efficiency, and of the Directive on the Energy Performance of Buildings.

2. The context

The large number of existing hydraulic layouts, the differences in storage volume and the various degrees of prefabrication make the installation difficult for installers and increase the risk for mistakes during installations.

Standards are well defined for solar collectors (EN 12975) and for small domestic water heaters (EN 12976), but not for SCS. For them, some standards are available (ENV 12977) but not already validated and not able to deal with all systems available on the market.

In France, only for one-family houses, approximately 90 different systems are available from 38 manufacturers [7]. This leads to a large difficulty both for the installers and consumers to know


exactly which system is the best (with regard to cost and performances) adapted to a specific house, with specific equipments for auxiliary (oil, gas, electricity, wood, …) and specific heating devices (radiators, floor and wall heating or a combination of all).

Fig. 2. Heating floor storage and hydraulic storage.

Although the market share of SCS is increasing in most European countries, it is still quite small. CombiSol project, which is supported by Intelligent Energy Europe program, should encourage an accelerated SCS market deployment — and thus, a higher share of heat produced by solar energy.

This project will encourage quality improvements of the systems installed, by:

• promoting best practices for solar combisystems, both for new and existing buildings,

• promoting standardised systems and cost-effective solutions,

• recommending solar combisystem designs to manufacturers,

• training installers,

• developing specific dimensioning tools in order to facilitate the recommendation for solar combisystems based on the Energy Performance of Buildings Directive methodology,

• increasing consumers confidence with information on energy efficiency of solar combisystems, based on in-situ monitoring and test labs.

In order to achieve these goals, the consortium includes the main institutes in Europe dealing with this subject: SERC, AEE Intec, PlanEnergy, ITW, ADEME, INES Education and INES-CEA. It has a wide experience of solar combisystems since many years. Professional associations from France, Germany and Austria are also involved in this project, firstly as relay for the dissemination of project results to the relevant actors, and secondly, for providing valuable feed-back from manufacturers and installers.

Energy measurements

1.1. Monitoring systems

The space heating demand, the hot water consumption, the heat loss of the circulation pipe, the natural gas consumption and the electricity consumption of the natural gas boiler and the heating system were measured in periods before and after installation of the solar heating system. Further, the heat transferred from the natural gas boiler to the hot water tank was measured before installation of the solar heating system. Furthermore, the heat produced by the solar collectors, the heat produced by the boiler, and the electricity consumption of the solar heating system were measured in the period after installation of the solar heating system. The monitoring system is described in details in [5].

Based on the measurements it is possible to estimate the energy savings of the solar heating system.