Category Archives: EuroSun2008-10

Qualitative investigation of the development of combined solar and pellet heating systems in Sweden

M. Lundh1*, E. Wackelgard1 and A. Henning2

1 Department of Engineering Sciences, Uppsala University, The Angstrom Laboratory, P. O. Box 534, SE-751 21

Uppsala, Sweden

2 Solar Energy Research Center (SERC), Dalarna University, SE-781 88 Borlange, Sweden
Corresponding Author, Magdalena. Lundh@angstrom. uu. se


Solar collector systems require an auxiliary heat source in temperate climates, and one system solution increasingly used in Sweden is combined solar and wood pellet heating. The development of integrated systems adjusted to the combination is however slow. To investigate the process towards a concept of solar and pellet heating, ten semi-structured qualitative interviews were performed with representatives for the trades during autumn and winter 2007. The present development and experienced obstacles were discussed, and conditions necessary for further progress and to reach success were identified. Other aspects found in the interview material will be analysed and reported in future papers.

The main condition to reach increased market shares is extensive marketing and information to the public, for which larger assets are required. The governmental influence, both by taking stand and making long-term regulations, as well as the installer corps influencing the costumer’s choice of system were stressed. A recent change in attitude is however clear, and the combined systems are by all informants regarded close to reaching general market establishment.

Keywords: solar heating, pellet, combined systems, interviews

1. Introduction

Solar collectors can not supply all heat demanded in a building in temperate climates; an auxiliary heating system is always required. There are several possible solutions, where the most common Swedish combination is with electrical heaters. One system solution that has become more frequently used over the last years is however combined solar and wood pellet heating. In the beginning, separate standard solar heating and pellet heating systems were combined, and still few companies offer well — integrated combined solutions. The development of a concept of combined solar and pellet, on the other hand, means that the solar collectors and pellet heating units are well-suited to be operated as one system and that the combination of two energy suppliers does not cause the end-user or installer additional effort compared to a single energy supply system. It should rather be experienced as one system. Besides Sweden, combined solar and wood pellet systems are also marketed and sold in countries like Germany, Austria and United Kingdom.

Solar heating systems as well as pellet heating have been considerably developed since the 1970’s. Although the technologies themselves are today well-developed and the heating systems have reached cost-effectiveness, the development of combined systems is slow. There are even advantages, such as

increased system efficiency and sustainability that advocate combined systems rather than individual solar/electricity and pellet heating respectively. But how does a new system solution evolve and what obstacles towards extended market dissemination are experienced by the trades? To investigate the background of the modest market introduction rate of combined solar and pellet systems an interview study was performed.

1.1. Aim

The aim of this study is to investigate the development of the concept of combined solar and pellet heating systems for single-family houses. Obstacles as well as possibilities experienced within the Swedish trades are to be distinguished. The aim has not been to cover all members of the solar and pellet industries; representatives not included in this study may have disparate opinions or proposals. The Swedish solar and pellet industries are however relatively small and a large fraction of the well — established members have been interviewed or involved in the meetings attended. This study rather aims at lifting some of the main issues experienced as important to the trade people. The outcome of this study could for example be used to direct future research, political means and market focus to increased use of combined solar and pellet systems, and in the long run towards sustainability. There are most likely similarities to other areas within the renewable energy sector.

This part of the reporting focuses on results on experienced obstacles against further development and prerequisites to increase the market shares.

2. Method

The present state of combined solar and pellet heating systems was first investigated by a literature and internet survey. Trade papers and magazines were searched for advertisements about the systems and web pages of Swedish manufacturers, producers and retailers were visited to investigate how solar and pellet systems are described. The outcome of this pre-study constituted the basis for formulation of interview questions as well as the choice of informants.

To map the process towards a complete system solution or concept with combined solar and wood pellet heating, ten qualitative interviews were performed during autumn 2007. The informant group comprised ten persons involved in the process in different ways, such as manufacturers, retailers, installers and representatives for the solar and pellet trade organizations respectively. Some of the informants have more than one role in the trades, such as being both representative for a trade organisation and a company producing or selling heating systems.

The interviews were semi-structured, which means that topics and questions were formulated in advance, but the interviews more took the form of open discussions and the order of questions was not strictly followed. During the interviews follow-ups were made on new related themes. The informants were given a brief introduction to the background and aim of the project before the interview.

The informants were first asked to give a short background to themselves and their company or organisation. The discussion was then focussed on attitude to the heating systems as well as to the solar and pellet trades, obstacles and possibilities with combined systems, but also the present and past development, prerequisites for further development and knowledge. The interviews were recorded and partly transcribed. The material was then categorized and analyzed according to [1, 2] to find trends in opinions, ideas and problem identification.

Three different meetings were also attended to study the interaction between the actors and take part in the discussion of the investigated issue. Two of the meetings were between company representatives within the wood pellet industry as part of the initiative from the Swedish Energy Agency “Pellet heating — future heating” (“Pelletsvarme — framtidsvarme”) while the third meeting was a joint gathering between the interest organisations for solar thermal, solar power, wood pellet and wind power, where the Swedish Solar Energy Association was the main organiser. All meetings took place during autumn and winter 2007.

The interviewer has a background in technology and science, but is part of an interdisciplinary research program, where interview studies have also been performed in cooperation with social scientists earlier. During this research project the interviewer have had regular contact with a social anthropologist for methodological issues.

Product Design Specifications

The main product design specifications related to embodiment design are:

• Being a mobile system, it must assure safe movements and statically determined positions;

• Considering the main purpose of the installation, which is the experimental high accuracy use, an increased stiffness of the structure must be accomplished, in order to protect the most sensitive devices equipping the tracker;

• The material will be mainly construction steel; only some specific parts will be machined from alloy steel with proper treatment;

• The tracker is equipped with a PV panel with a surface 1.48×0.67 m2, approx. 1 m2;

• The tracker’s angles у and a must cover the ranges у = -90°.. .+90° and a = 9°.. .67°, set for the specific location of the system;

• Linear actuators must be used for both movements;

• The temperature field on which the system must work is -5 … 50°.

Theoretical comparison between solar combisystems based on bikini. tanks and tank-in-tank solar combisystems

E. Yazdanshenas1*, S. Furbo2 and C. Bales3

1’ 2 Department of Civil Engineering, Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby, Denmark

3 Dalarna University, SERC, S-781 88 Borlange, Sweden

Corresponding author, eya@byg. dtu. dk


Theoretical investigations have shown that solar combisystems based on bikini tanks for low energy houses perform better than solar domestic hot water systems based on mantle tanks. Tank-in-tank solar combisystems are also attractive from a thermal performance point of view. In this paper, theoretical comparisons between solar combisystems based on bikini tanks and tank-in-tank solar combisystems are presented.

The investigations are carried out for different designs and sizes of the two solar heating system types installed in different houses. The investigations show which types of solar combisystems are suitable for low energy houses, new houses built according to the building codes and old houses.

Keywords: Solar combisystems, bikini tank, tank-in-tank

1. Background

The solar heating market in most European countries and worldwide grows by 20-40% each year. In most European countries the percentage part of solar heating systems, which are solar combisystems is growing. Solar combisystems can cover both a part of the space heating demand and a part of the domestic hot water consumption. One of the most important studies on solar combisystems was done in the framework of Task 26 of IEA Solar Heating and Cooling programme between 1999 and 2002. Around 21 different solar combisystems were investigated numerically in detail in different IEA member countries. A design hand book for solar combisystems was published [1]. In order to gain high energy savings for solar combisystems, it is important to have:

• A small auxiliary volume in the heat storage.

• A low auxiliary set point temperature of the auxiliary volume in the heat storage.

• A low tank heat loss

• A high efficiency of the auxiliary heater

• A good thermal stratification in the heat storage tank

H. Drnck and E. Hahne [2] investigated four different solar combisystems in detail. They found that the most important parameters for a well performing combistore are low heat losses due to a good thermal insulation. They also suggested using small auxiliary volume and low set point temperature of the auxiliary boiler. Moreover, the connections for the auxiliary and the space heating loop should be in appropriate position.

E. Andersen and S. Furbo [3] investigated theoretically three solar combisystem designs in three different houses with different space heating systems. The solar combisystems are initially equipped
with heat exchanger spirals and direct inlets to the tank. A step-by-step investigation was performed demonstrating the influence on the thermal performance of using inlet stratification pipes at the different inlets. Based on TRNSYS simulations, they found increased thermal performance of solar heating systems by using stratifiers instead of internal heat exchanger and direct inlet to the tank. The best performing solar combisystem is based on a tank-in-tank storage with stratifiers both in the solar collector loop and the space heating loop.

E. Yazdanshenas and S. Furbo [4] investigated theoretically a new so called bikini solar combisystem. Three different houses with four different radiator systems were considered. The thermal performance of the bikini solar combisystem was compared with the thermal performance of a solar domestic hot water system based on a mantle tank. The thermal performance of the solar combisystem is higher than the thermal performance of a solar domestic hot water system based on a mantle tank. The investigation also showed that a bikini solar combisystem is promising for low energy houses.

The current paper deals with the comparison of two different solar combisystems: Tank-in-tank solar combisystems and solar combisystems based on bikini tanks. The aim of the paper is to study which of these two solar combisystem designs is suitable for different houses.

Model of a PV — farm with two-positional exposure

In the solar farm, PV-modules (“modules” below) are exposed by rows where modules are installed side by side in parallel. Usually there are several rows, one behind the other. Evidence of how the hindered row is shaded by the first one has been analyzed in the literature [2, 3]. We focused on the co-operation and mutual shading of two-positionally tracked neighbor modules in a

row. Most of the modules perform inside the farm and are surrounded by neighbors from both sides. Performance of such a module defined as an “inner” module was analyzed first. A module at the end of the row, which has a neighbor on one side only, is defined an “outer” module. Peculiarities of their performance will be described later.

In the theoretical analysis, a simplification has been made: the PV-module (or its column on the roof) is considered being infinitely long. It means that we will ignore edge effects on the top of the inner column. At these limitations the gain has its minimal value as the edge effects increase the illumination. Figure 1 shows the 2-D model of a row of modules deflected eastward in the morning with the deflection angle -%, which shows the view from the top along the axes around which the modules are rotated (deflected) twice per day. Around the noon (exact time is not critical) the modules are triggered into the westward position with the deflection angle +%. The processes in the afternoon are the mirror reflection of those in the morning and were not subjected to detailed analysis.


Fig. 1. 2-D model of a row of modules deflected eastward, top view.

Each module with a width WC has its axis parallel to the basis. The basis may be ground with the zero tilt angle p0=0, a roof with a free tilt angle 0<р0<л /2 or wall p0=n /2. To simplify the analysis we suggest that the tilted basis is looking south with the azimuth y0=0, but that is not obligatory. The inner modules are installed at the distance DR from each other, the outer module is installed on the distance DC from the edge of the base (roof, wall). An important parameter in the analysis below is the relative distance (density) dR= DR /WC. Illumination on the upper (outer) edge of the inner module appears when the sun has reached the position characterized by the clock angle юF. Then the sun is shining along the plane of a virtual envelope, parallel to the basis, it joins together all the upper edges of the farm. At the clock angle ®G, the whole area of the inner module is illuminated and after that ю >®G the module performs like a tracked stand-alone module. During a transient shading process, characterized by the angle ^=(®G ^F ), the shadow of the neighboring module will move across the module. Converted solar energy E from the direct component is proportional to the share of the illuminated area AC (ю). As AC (®)<AC, the tracked module inside the farm produces always less energy than the stand-alone tracked module. Consequently, the gain of a solar farm with the two-positional tracking will be somewhat lower, compared to that of the two-positionally tracked stand-alone module.

2. Approach

The task of the study is to calculate the hourly, daily, monthly or seasonal energy yield for a module exposed in the two-positional regime. This calculated (and experimentally measured)

energy yield will be compared with the energy yield of a fixed module, which is installed in optimal conditions. Improved efficiency, i. e. the profit (“gain”), is defined as the ratio of the energy produced in (two) deflected positions to the energy produced by the collector that is optimally exposed and fixed in this position. This is a south-faced collector with the tilt angle that warrants the most uniform energy yield during the season. For the latitude around 60° N, the tilt angle should be 45°<p <60°. We refer to the value of 45°.

Gain may be defined for an hour as follows: hourly gain = hourly energy yield of the deflected collector ETx divided by the hourly energy yield ET of the fixed collector, kWh/kWh.

Also, gain may be defined as the ratio of the corresponding irradiances kWm-2/kWm-2.

Gain = GTxj GT, (1)

where GT is the irradiance on the tilted module and GTx is the same in the deflected position.

Daily gain = energy yield in the two-position deflected collector per day divided by the energy

yield of the fixed collector per day, kWh/kWh.

Monthly and seasonal gains are defined analogously to the daily gain. The main goal is to maximize the seasonal energy yield, although the hourly gain is also of interest, considering co­operation of solar PV farms with the grid.

Gain is the function of several variables: geographical location due to the latitude Ф, season due to the changing declination 5, solar clock angle ю, initial tilt angle p0, initial azimuth y0, deflection angle x, and solar radiation that varies by site and time. In view of these circumstances, a general analytical solution would hardly be possible especially due to the radiation data presented as table functions. Therefore, calculations must be performed by help of computer simulation. Radiation data of the beam Gb and the diffuse Gd component have to be considered separately as they are absorbed by the module in the farm differently. In the analysis of a PV-farm performance, in addition, we have to consider the variable dR characterizing the density of the modules in the farm. Geographical (Ф ) and constructional data (p0, yo, x) can be considered as constant for each analysis, radiation data Gb and Gd are tabular functions, 5 and ю are continuous variables sampled for each step of the calculation. Auxiliary variables in the computation process are presented as functions of the independent time variable ю.

DC — Direct Characterisation

A test according to the DC procedure requires the set-up of the complete system in an indoor test facility and its operation for eight days according to a well defined test sequence [1-3]. It is to be used for small factory made systems as well as for small custom built systems with a collector area smaller than 15-20 m2 and a heat store volume of 1500-2000 litres.

The major performance indicator of the solar combiystem given by the DC test is the final energy used by the auxiliary heater, the same as in the CTSS method. For this case, it is essential that the solar combisystem is always tested in combination with the auxiliary heater. This feature is considered to be favourable, because many problems in system operation appear due to improper control strategy for the coupling of the solar and auxiliary parts of the system. In case of absence it is also possible to use a specific well defined laboratory heater.

The prediction for the annual final energy use is only possible when the test conditions during the sequence correspond more or less to the annual conditions. Hence, this test method is restricted to one specific climate zone in Europe and one specific building type with its characteristic insulation parameters, heat distribution system and domestic hot water profile.

Energy performance

image127 image128 image129 Подпись: (Equation 1)

From a simple energy balance for the thermally driven chiller, the specific primary energy consumption of the dry cooler and the cooling tower per unit of heat rejected, PEspec, cooling tower, including the electricity needed for the circulation pump of the cooling water cycle, can be expressed as follows:

In this equation Espec, coolin tower is the specific electricity demand of the cooling tower per unit of cooling energy (heat rejection) including the circulation pump of the cooling water cycle and it is expressed in kWhel/kWh

cooling, [13].

In addition to the above defined specific energy consumption, the total cooling energy produced by the chiller, the electricity consumption of the fans and the water consumption were calculated with TRNSYS and reported in Table 2. The simulations were conducted for an entire cooling season, (middle of June to middle of September).





Cool tower


Cool tower


Cool tower

Qsol inc [k^Wh-]







Qsol gain[k Wh]







Qeva [kWh]







Qheat reject [k~Wh]







COP [-]







Enele [kWh]







Evaw [m3]




Espec, heatrej








PEspec, heatrej








Table 2. Energy performance