Category Archives: EuroSun2008-10

Chemical storage of the sun energy

The chemical conversion of the solar radiation to the useful, easily transportable energy carrier belongs to large scale storage modes and is based mainly on two processes. The first one is the photosynthesis of biomass from carbon dioxide (CO2) and water. The reaction is supposed to be CO2 neutral, which means that the speed of releasing the CO2 into the atmosphere during energy retrieval from the biomass equals the CO2 consumption rate during the photosynthesis of the biomass. Unfortunately, this assumption is not fulfilled in the present time.

The second process, able to convert insolation to useful energy carrier, is the reduction of CO2 to methanol. Following scheme shows principles of the process:

image092+ 3 H2O ^3 H2 +1У2О2 ; 3H2 + CO2 = CH3OH + H2O. (1)

Neither water electrolysis nor methanol synthesis from CO2 are unknown processes [1]. Solar-thermal-electric systems (STES) are commercially available [2] and can produce enough electric current for large scale water electrolysis. CO2, instead of dumping it to Earth cavities after removing it from stack gases of power plants, can be easily transported in the liquid form to hydrogen production plants. Methanol, extremely versatile energy carrier, is a liquid, easily transportable. Water is partially recirculated in the process. Methanol plants can be therefore situated in rare populated areas, rich in solar radiation. And the process is 100% CO2 neutral.

Market and consumers’ role

The costumers bringing pressure on the building companies by demanding certain heating systems was identified as very useful in gaining market shares. The costumer has probably reached further than the building trade in the awareness of those systems; costumers ask for systems that the installer or

plumber is not willing or able to deliver. And due to that demand, companies work on system development at present. If you can not offer what the costumer wants, you will loose him. But, as one informant expressed it: “the costumer is so easily influenced, when the plumber says that he should have a heat pump, he believes in that”. The costumers must learn to assert themselves. By making the installer aware of other companies offering a certain type of systems, the costumer will force the installer to offer the same system solution. And that in turn affects the current assets of the supplier positively, causing growing companies, which in turn increases the interest from the installer, who can serve the costumer. This interplay sets the development going. But without demand and pressure from the market and costumers, the installers will not focus on those systems, but will continue with what they are already good in; they will “take the easy way out”. “If [the installer] has heard once [that someone wants a solar and pellet system] he says that it does not pay, but if he hears it ten times the same day […], he realises […] that he also has to offer it”.

Satisfied costumers are the best way to increase the selling rate, since they tend to tell neighbours and friends about their experiences. The opposite is however also valid, and has been a problem so far. Particularly some of the early pellet systems have been experienced as causing lots of trouble and non­satisfied costumers. And success will not be reached until the costumers’ requests are fulfilled.

MONITORING RESULTS OF A NEW DEVELOPED. COMBINED PELLET AND SOLAR HEATING SYSTEM

F. Fiedler1*, C. Bales1 and J. Vestlund1

1 Solar Energy Research Center SERC, Hogskolan Dalama, S-78188 Borlange, Sweden
* Corresponding Author, ffi@du. se

Abstract

In this study the monitoring results of prototype installation of a recently developed solar combisystem have been evaluated. The system, that uses a water jacketed pellet stove as auxiliary heater, was installed in a single family house in Borlange/Sweden. In order to allow an evaluation under realistic conditions the system has been monitored for a time period of one year.

From the measurements of the system it could be seen that it is important that the pellet stove has a sufficient buffer store volume to minimize cycling. The measurements showed also that the stove gives a lower share of the produced heat to the water loop than measured under stationary conditions. The solar system works as expected and covers the heat demand during the summer and a part of the heat demand during spring and autumn. Potential for optimization exists for the parasitic electricity demand. The system consumes 680 kWh per year for pumps, valves and controllers which is more than 4% of the total primary heating energy demand.

1. Introduction

Solar heating systems that provide domestic hot water and space heating, so called solar combisystems, have become more and more popular in the Middle and Northern European countries. Solar combisystems usually require an auxiliary heat source to be able to provide enough heat even in the seasons with low solar irradiation. In Sweden, electrical heaters and wood boilers are typically used as auxiliary heaters in solar combisystems. In recent years wood pellet boilers and stoves have also become a good alternative. The design of solar combisystems has been studied intensively in the IEA-SHC Task 26 “Solar combisystems”. A number of systems have then been, based on system simulations, optimised and improved. The results from Task 26 including a variety of technical reports and design tools are available for the public [6].

Within the Nordic research project REBUS a new combined solar and pellet heating system for the Nordic market has been designed, built and tested [4]. During this project typical existing system solutions for this combination have been investigated by the help of measurements and computer simulations [2]. It has been shown that these system solutions, due to their design and size, are often not suitable for typical Swedish houses without heating room. Also the thermal performance, such as heat losses and solar savings, offered potential for improvements. These finding have been included for the design of the new system. The first prototype of the REBUS system was tested intensively in the lab. The second improved prototype was installed in a single family house and has been monitored for a time period of one year.

Comparison between tank-in-tank and bikini solar combisystems

In order to compare bikini and tank-in-tank solar combisystems, the best tank-in-tank model system from Fig. 5 is compared to a bikini tank system. Thus, Model 7 is the tank which performs best among the tank-in-tank models with different space heating loads and the Task 26 DHW profile. Both bikini and tank-in-tank system have the same H/D ratio of 4. Fig. 8 shows the net utilized solar energy of the solar combisystem installed in the old house, the house with medium space heating load and the low energy house for collector areas of 3, 5 and 8 m2. The smaller the collector area, the better the bikini solar combisystems performs compared to tank-in-tank solar combisystems. For a collector area of 3 m2, the net utilized solar energy increases for the tank-in-tank solar combisystem and decreases for the bikini solar combisystem as the space heating load increases. For bikini solar combisystem, the set point temperature for the auxiliary heater is increased from 55°C for the low energy house to 60°C and 75°C for the houses with medium space heating demand and high space heating demand. For the tank — in-tank system, the auxiliary set point temperature remains constant at 59°C. It can be seen that bikini solar combisystems have higher thermal performance than tank-in-tank solar combisystem as long as the collector area is small. When the collector area increases, the thermal advantage of bikini tank is decreased especially for houses with high space heating demand. The results are explained by the different required set point temperature for the auxiliary energy supply system and the better thermal stratification in the bikini tank compared to the thermal stratification in the tank-in-tank system.

image122

Fig. 8. Net utilized solar energy versus space heating demand for Task 32 DHW profile 4. Conclusion

Solar combisystems based on bikini tanks and tank-in-tank solar combisystems have been studied theoretically. Bikini tank systems require, if installed in low energy buildings, low auxiliary volume set point temperatures resulting in high thermal performances. High auxiliary volume set point temperatures are required for bikini tank systems installed in houses with a high space heating demand. This results in a relatively low thermal performance. Bikini tank systems are therefore suitable for low energy buildings, while tank-in-tank combisystems are more suitable for normal and old houses. Tank- in-tank stores with domestic hot water tanks with large auxiliary volumes are recommended for normal variable domestic hot water consumption patterns in order to achieve a high thermal performance.

References

[1] W. Weiss, (2003). Solar Heating Systems for Houses — A Design Handbook for Solar Combisystems, James&James, London, http://www. shop. earthscan. co. uk/

[2] H. Druck, E. Hahne, EuroSun98 — The second ISES-Europe Solar Congress — September 14 — 17, (1998) Portoroz, Slovenia.

[3] E. Andersen, S. Furbo, Theoretical Comparison of Solar Water/Space-Heating Combi Systems and Stratification Design Options. ASME, Journal of Solar Energy Engineering, 129 (2007) 438-448.

[4] E. Yazdanshenas, S. Furbo. Solar combi system based on a mantle tank. Proceedings of ISES Solar World Congress 2007, Beijing, China, ( 2007).

[5] TRNSYS 16, User Manual, University of Wisconsin, Solar Energy Laboratory, 2005.

[6] S. Knudsen, (2002). Heat Transfer in a ‘Tank-in-Tank’ Combi Store, Department of Civil Engineering, Technical University of Denmark, Report No. R-025.

[7] H. Druck, (2006). MULTIPORT Store — Model, Type 340 for TRNSYS. Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart.

[8] U. Jordan, K. Vajen, (2001). Realistic Domestic Hot-Water Profiles in Different Time Scales, FB. Physik,

FG. Solar, Universitat Marburg, D-35032 Marburg.

Performance Analysis of Water-in-Glass Evacuated-Tube. Solar Heating Systems in Malta

C. Yousif1*, C. Fernandez Vazquez2 and V. Buhagiar[1]

1 Institute for Energy Technology, University of Malta, Triq il-Barrakki, Marsaxlokk, MXK 1531, Malta

2 E. T.S. Ingenieros Industriales, Universidad de Valladolid, Paseo del Cauce, s/n 47005 Valladolid, Spain

3 Faculty of Architecture & Civil Engineering, University of Malta, Tal-Qroqq, Msida, MSD 2080, Malta

Corresponding Author, charles. yousif@um. edu. mt

Abstract

This paper describes the results obtained from analysing the performance of a number of water- in-glass direct solar heating systems, which were installed on the first Energy Saving Social Housing Project in Malta. The efficiency of the systems varied between 30 and 80%, depending on the season and the hot water usage profile of the users. It was concluded that the ‘Efficiency term could be confusing to the users, as low efficiency is usually associated with a pessimistic outcome. A new parameter had to be introduced, which could provide a better explanation of the performance of the systems. This was termed the ‘Future Utilisation Potential Factor, FUPF, which is the fraction of hot water reserve for each month to the current hot water consumption. For the first time, answers to frequently asked questions were found for Malta, especially with regards to the real savings that could be expected from a solar system, the typical hot water consumption of different users, the amount of energy losses to be expected overnight and the number of days that a back-up electric heating element would be expected to switch on. These results could be instrumental to recommend solar systems, which would better fit the needs of the clients, not only based on the number of persons, but also on their hot water usage profile and any future increase in their hot water demand.

Keywords: Solar heating, evacuated-tube, water-in-glass, domestic.

1. Introduction

The domestic sector in Malta widely utilises electric boilers to heat water, which typically accounts for 15 to 30% of the household’s electricity demand. With the introduction of fuel surcharge on electricity and water bills (90% in 2008), the cost of heating water has risen dramatically. Solar heating in Malta has been introduced in the 1980’s; however it was only recently that the market started to become more active. The National Census of 2005 showed that only 5,010 solar heaters were installed in homes, covering a mere 3.6% of Maltese households [1] . Following the Government’s decision to give capital grants of up to 25% in 2006, the uptake of solar heating increased to some extent and is now estimated to have reached 10,000 to 12,000 units. Some of the major reasons for the relatively slow uptake, in spite of capital grants, were described in another publication [2] . These could be summarised as lack of legislation, poor access to information and education on solar heating and negative experiences of some users that result in skepticism in a small island scenario.

Evaluation of the test sequences

As the test sequences have been elaborated with one reference SCS. The evaluations of these two test sequences were done for different configuration of solar combisystem:

— the reference SCS of the IEA Solar Heating & Cooling programme Task 32 with different size of collectors and volume of storage,

— a commercial SCS storing heat within a tank,

— a commercial SCS using the thermal mass of the building as heat storage,

— different buildings with various heating loads of 30 kWh/m2 (referred as SFH30) and 100 kWh/m2 (SFH100) in Zurich conditions.

Future investigations

The measurements showed that the used lifeline has poor thermal characteristics. The heat loss of the hot pipe is not much lower than the heat loss of the hot 10/8 mm pipe, while the cold pipe in the lifeline has a lower heat loss than the cold 10/8 mm pipe. The decreased pipe heat loss will however only slightly increase the thermal performance of the solar heating system.

Therefore, in July 2008 the 16 m lifeline was replaced by 33 m 6.35/4.85 mm copper pipes with 9 mm INSUL-TUBE insulation. At the same time the positions of the pumps were changed from the cold side of the solar collector loop to the hot side of the loop. That is: The solar collector fluid is now passing the pumps just before the fluid enters the mantles. In this way the pump power which heats up the fluid is utilized in a better way than if the fluid is heated before it enters the solar collectors. Further, the SOLAR 15-40 pump was replaced by a normal circulation pump, type UPS 15-40, since this is the circulation pump used in most Danish marketed solar heating systems today.

The results of the measurements for the period July 30 — August 12, 2008 are shown in table 3. The test period was a rainy period resulting in relative low solar fractions.

Table 3. Measured energy quantities in the test period July 30 — August 12, 2008.

Measured energy

Solar heating system with 6.35/4.85 mm copper pipes and SOLAR 15-65

Solar heating system with normal solar collector loop and UPS 15-40

Solar radiation on solar collector

370 kWh

370 kWh

Tapped energy

64.4 kWh

64.4 kWh

Auxiliary energy to top of tank from electrical heating element

19.1 kWh

21.3 kWh

Solar heat transferred to hot water tank

49.9 kWh

48.9 kWh

Net utilized solar energy

45.4 kWh

43.0 kWh

Solar fraction

70 %

67%

Operation time of pump

111 h

104 h

Pump energy

4.2 kWh

2.3 kWh

The thermal performance of the system with the 6.35/4.85 mm copper pipes and the SOLAR 15-65 pump is 6% higher than the thermal performance of the solar heating system with the normal solar collector loop and the UPS 15-40 pump for the test period of 2 weeks. The extra pump energy for the SOLAR 15-65 pump is now lower than the extra net utilized solar energy for the system with the SOLAR 15-65 pump. Consequently, the use of the SOLAR 15-65 pump together with the small separate pipes in the solar collector loop is now justified from a thermal performance point of view. However, the results might be different after a long test period. The measurements will therefore be continued for a long period.

4. Conclusion

Side-by-side laboratory tests for two small low flow SDHW systems have shown that a solar collector loop based on a marketed lifeline with a SOLAR 15-65 pump has no thermal advantage compared to a solar collector loop based on normal pipes and a normal solar circulation pump. The reason is the poor thermal characteristics of the lifeline.

The laboratory measurements will be continued with an improved solar collector loop for the SDHW system with the SOLAR 15-65 pump. The lifeline was in July 2008 replaced with small well insulated separate copper pipes. Further measurements will elucidate if the thermal advantage by using such small pipes is large enough to compensate for the extra pump energy required by the SOLAR 15-65 pump.

References

[1] S. Furbo (1990). Small Low Flow DHW Solar Heating Systems — Status. Thermal Insulation Laboratory, Technical University of Denmark, report no. 90-13.

[2] S. Furbo (1993). Tests of the components of the Solar Boiler system from Thermo Dynamics Ltd.

Thermal Insulation Laboratory, Technical University of Denmark, report no. 93-20.

The Zero Carbon Solar Thermal Solar Controller Barry Johnston,1[2] and Simon Sharp2

1Solar Twin Ltd., 50 Watergate Street, Chester, Cheshire, CHI 2LA, United Kingdom, Tel: +44 1244

403404

2 Solar Twin, Ageito, 4940-681 Rubiaes, Paredes de Coura, Portugal, Tel: +351 913 759 291
* Corresponding Author, barry@solartwin. com

Abstract

The zero carbon solar thermal solar controller is an innovation from Solar Twin enabling solar water heating to get rid of its Achilles’ heel: the significant CO2 waste associated with using mains electric powered pumps for fluid circulation in solar panels. This invention is a photovoltaic (PV) powered solar controller which switches a variable speed brushless DC pump on when the water in the panel is hot enough to collect. This innovation could boost carbon savings in solar water heating systems by typically around 20%. These systems offer greater sustainability for the solar thermal industry avoiding the potential energy claw-back on conventional solar thermal. Solar water heating can now go green.

Keywords: Solar water heating, solar controller, zero carbon, solartwin.

1. Introduction

Solartwin is an innovative solar thermal system designed for solar water heating, utilising a number of unique design features, PV pumping, flexible silicone rubber pipework, a solar powered solar controller and extensive use of polymers. A major concern of the renewable energy industry as a whole relates to carbon footprinting of the manufacture, installation and operation of renewable energy technologies. Focussing on the operational carbon factors (carbon clawback) and the need to minimise it or even to removing it altogether has been a major challenge to the solar thermal industry. Regarding solar thermal systems, a UK government funded study1 of eight domestic scale solar water heating systems confirmed what had long been suspected: that the environmental benefits of solar can be substantially improved by eliminating mains electricity. In this report, flat plate solar water heating systems negated an average of 17% of their potential global warming benefits (i. e. CO2 savings) by using mains electricity. For evacuated tubes, their loss averaged even higher, typically 23%. In other words, running a mains powered solar for ten years, its electricity consumption can negate its CO2 saving by around 2 years. However, while much of Europe uses mains pumped solar thermal systems, even at present, three relatively well known forms of solar thermal systems have zero carbon clawback. These are: systems where the stored volume of water forms part of the collector itself, thus negating any need for pumping, and two further systems where the collector is separated from water store. In the first of these, where the solar panel is positioned below the store, thermosystem pumping, due to hotter water being less dense, can occur. In the second, the panel may be above the water store and pumping is carried out wholly by photovoltaics. It is this final option which is considered in this paper. A recent life cycle analysis paper of three microgeneration technologies2 wind, PV and solar thermal, found that zero carbon solar hot water paid back the energy used in production of the system sixteen times over an assumed 25 year life. Therefore as innovators in the renewable energy sector, we felt the adoption of zero carbon solar hot water was an issue which needed to be tackled, not specifically for the Solar twin system since that was already PV pumped and zero carbon in operation, but for the industry as a whole. With improvements in operational carbon footprint in mind, we have developed a differential temperature controller which can be used for controlling a variety of photovoltaic (or low voltage DC) solar heating systems, including the Solartwin system.

Table 1. Solar Thermal system electric consumption comparison. A gas-electric carbon weighting figure of

2.5 was used in the paper. (Data taken from UK government funded study1 of eight solar water heating systems http://www. berr. gov. uk/files/file16826.pdf)

2. Development

Our primary aim in the development of this innovation was to zero carbonise the majority of standard domestic solar thermal systems. We considered the various elements to make this a possibility and assessed that the following criteria must apply to our zero carbon solar thermal solar controller. It should:

• Be PV powered only (though with an option for low power DC)

• Be flexible enough to work with a wide range of PV’s and DC pumps

• Have a variety of easily chosen standard software programs onboard to accommodate a

range of solar thermal systems and allow further engineer based customisation within the standard operational programs

We were also certain that the controller must be consumer rather than engineer orientated which meant that it must: •

image038

relays. (Most rechargeable batteries are only capable of 200 — 1000 charge / discharge cycles, while relays end to degrade at physical contact points.)

Other elements vital to the design and optimum operation of the controller were felt to be:

• That it should use very little power in operation, since PV power is costly

• Use an integral microprocessor

• Offer simple diagnostic display of situations where the controller is not working correctly

(sensor disconnection, for example)

• That the controller should be delivered pre-programmed

In developing the system we felt it important to design a schedule of development ensuring that we met both the needs of the consumer and the relevant regulatory bodies. Our initial perspective once we had identified a gap in the solar thermal market was to place a value on this gap and then identify regulatory issues for the relevant world markets. The next preparatory stage was to define boundaries making note of where boundary extensions were optimal.

Beginning product development, we defined several solutions to the issue of carbon claw-back in the solar thermal industry technically, both core and extensions. We further considered boundary issues and then on the basis of cost-benefits decided where they should be:

• Included in version 1

• Allowed for as a simple iteration of version 1

• Would only be enterable as a remanufacture of version 1

On resolution of these issues we progressed to the development of hardware and software and to performing laboratory and in the field tests and then to iterate further product refinements.

Finally, we performed regulatory validation, finalised product documentation and brought version 1 to market in a predefined way, having previously completed market and competitor research.

Physical storage method of the sun energy

The thermal capacity of the physical heat reservoirs is based on thermodynamic properties of the storage material: specific heat and/or the latent heat.

1.2.1. Sensible heat storage.

While most of common materials, used for heat storage, have specific heat cp = 0,8 — 1,6 kJ/kg/°C, (0,23-0,44 Wh/kg/°C), cp of water substantially differs with cp = 4,19 kJ/kg/°C (1,16 Wh/kg/°C). Heat content of any reservoir utilizing sensible heat can be calculated as follows:

AHs = cp*At*m (2)

where m = mass of the storage volume and At = temperature difference between loaded and “empty” heat storage space. Thus when the sensible heat only is used for energy storage, 1m3 water 90°C can theoretically release max.104,7 kWh of useful energy after cooling it to 0°C.

1.2.2. Latent heat storage.

Latent heat of materials (phase transition heat) is usually much higher than specific heat. Thus the condensation of water vapour releases 2520 kJ/kg (700 Wh/kg) at10°C and freezing of water releases 334 kJ/kg (93 Wh/kg). Phase change materials (PCM) as energy storage medium are richly represented in search databases. European patent database Esp@cenet [3] returns more than
6000 citations of the term “phase change material” and Internet database Google returns over 7 millions citations. Systematic treatment of PCM for heat storage can be found in [4,5].

Water as PCM is used scarcely in spite of the large heat content and unlimited access. The reason may be the volume changes during the phase transition and low temperature of the fusion heat. Water expands about 2,15% (linearly) when converting to ice and about 1330 times at 10°C after evaporation at atmospheric pressure. The huge volume change makes it inpractical to use liquid to gas transition of water as heat storage medium in closed systems (1 kg water vapour stored at 100°C in 10 l requires a container dimensioned for the pressure at least 200 bar), but this phase transition occurs daily in the atmosphere. Water vapour content in the saturated air is about 0,4% at 0°C, 0,8% at 10°C and 1,5% at 20°C (w/w) or 1 kg H2O in approx. 130 m3 of the humid air.

The average temperature measured in Stockholm area year 2007 (59°27’N, 17°45’E) was 8,2°C and the average relative humidity was 82,2%. Average retrievable energy in each 100 m3 air is thus at least 2800 kJ (786 Wh), if the heat retrieval system can be cooled to temperature below 0°C. The sensible heat of the air contributes then with 36%, condensation of the water vapour with 56% and the fusion heat of the condensed water with 8 % of the total energy content. While the fusion heat of water does not contribute substantially to the heat retrieval from the moist air, the situation becomes very different, when the heat is retrieved from the soil. 1 m3 soil, saturated to 50% with the water, 15°C warm, can release 60 kWh when cooled down to -0°C. A small pit, 5x10x1 m, filled with humid soil, can thus supply enough heat for a single house during 1-3 winter months, if the water in the soil is allowed to freeze.

A domestic heating system, covering energy needs of a family house under a whole year, based on usage of solar energy, should utilize both direct insolation (mainly for summer hot water need and loading of the seasonal heat magazine), heat saved in humid air (during nights, autumn and spring months) and heat stored in the ground magazine during the winter months. Such a system has to contain heat pump and open solar heat collector connected in such a way, that the cold, expanded fluid in the heat pump circuit can cool down the heat absorbing surface of the SHC. The solar heat collector obtains thus double function: it collects the insolation and in the absence of the solar radiation it collects the heat from the air.

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.

Acknowledgement

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.

References

[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.