Category Archives: EuroSun2008-10

Experimental testing

The schematic diagram of the laboratory installation and its photo are shown in Figs. 2 and 3. The warm and cold work vessels are made of cylinders and mounted one above another. The diameter and the height of the cylinders are 110 mm and 140 mm, respectively. Both cylinders are transparent and made of Plexiglas for visual observation. An electrical resistance heater with a capacity of 300 W was used as a source of heat. The heater had a temperature control for maintaining a few set-point temperatures. The control valve was made as a liquid seal. The heat exchanger (cooler) used for cooling the heat-carrier was located one metre below. The heat exchanger was made of a copper coil with a surface area of 0.15 m2 and placed into the cylinder vessel with a capacity of 7 litres filled with water. The vessel of the cooler was filled with water that was not running, but only accumulating heat leading to a temperature rise during the experiment.

The recorded data consisted of measurement of pressure inside the cold vessel and temperature measurements: at the top and the bottom of the inside warm vessel, at the descending pipe, at the

image165

up-flow pipe and at the vessel of cooler. Processes in the model are variable. It was decided to take measurements every 10 s since the duration of cycles was foreseen about 1 min and more.

Подпись:diagram of the passive heat transfer downward model:

4 — electrical heater, 7 — warm work vessel

5 — intermediate canal, 8 — cold work vessel,

6 — control valve (liquid seal), t — points of temperature

measurement

Experiments were done at a temperature of heat source of 33, 41, 49, 57 and 60°C.

Подпись: Q mc C (/desc.pipe image168

The amount of heat carried by liquid heat-carrier from warm vessel during one cycle was calculated by using average temperature difference, tdescpipe measured at lower pipe and average minimum value of temperature, tup_flow pipe measured at up-flow pipe:

Measurements on a New Developed Compact. Solar Combisystem in Practice

A. ТЬйг and S. Furbo2

1 AEE INTEC, Feldgasse 19, 8200 Gleisdorf, Austria
2 Department of Civil Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

* Corresponding Author, a. thuer@aee. at

Abstract

Based on elaborated knowledge in international research projects within IEA-SHC Task 26 and the ALTENER project “Solar Combisystems”, a new solar combisystem concept was developed. Therefore the focus was concentrated on minimizing the temperature in the system with the goal to reduce system heat losses and to increase the efficiency of the condensing natural gas boiler and the solar collector. After development and test of the first prototype in the laboratory, a demonstration system was built which replaced an old conventional natural gas heating system in a one-family house. Measurements in practice showed how this new natural gas — solar heating concept performs in comparison with the old one. A Solar gain of 370 kWh and energy savings of 536 kWh per m2 collector area were achieved for the new solar combisystem. Domestic hot water consumption reduced by 20 % and hot water circulation losses of only 7% of hot water consumption was achieved. The average space heating temperature difference could be increased by 50% leading to lowest possible return temperatures. In spite of more installed pumps and valves the electricity consumption of the heating system could be decreased slightly.

Keywords: solar combisystem, energy savings, measurements

1. Introduction

The project REBUS — „Competitive solar heating systems for residential buildings” was carried out from 2003 to 2006 with the goal to develop new concepts for solar combisystems in co­operation with industry partners [1]. Prototypes were first built and tested in the laboratory and further on installed in one family demonstration houses for in-situ long time testing and measurements. According to national boundary conditions PhD projects in Sweden and Latvia worked on solar combisystems in combination with pellet boiler and in Norway and Denmark solar combisystems in combination with natural gas boiler were investigated.

This paper summarizes the final results of the Danish PhD project where a new compact solar combisystem concept with direct integration of a condensing natural gas boiler was developed, tested as a laboratory prototype and finally installed as a second generation prototype in a demonstration one family house [2, 3, 4, 5].

2. Demonstration house

The demonstration one family house with 3 occupants is situated in a small city about 40 km north of Copenhagen/Denmark. The old heating system was a non-condensing natural gas boiler (construction year 1990) with 22 kW nominal power in combination with a 50 Liter domestic hot water tank. The heating system consists of old cast iron radiators with thermostatic valves. This old

1

Подпись: Fig. 1. Left: Demonstration house with collectors on the roof; Middle: installed solar combisystem in the basement; right: prefabricated technical unit before installation

heating system was monitored in the period August 2004 to April 2006. In spring 2006 the new solar combisystem was installed: collector area of 6.75 m2 and 360 litre solar tank in combination with a condensing natural gas boiler (Milton Smart Line HR24). Beside the installation of the new solar heating system also two additional rooms were prepared to be used in the basement: one living room and a bath room, both equipped with floor heating. Monitoring of this new system started in October 2006.

. Previous Studies

Two previous studies have been undertaken investigating ISAHP systems. The first study involved developing a model in TRNSYS [12], a transient simulation program, to investigate the feasibility of the system, performing both a performance and cost analysis. The second study involved building a prototype of the system, and performing a range of constant temperature input tests, comparing the results with the simulated results. A brief description of each previous study is provided below.

1.1. Numerical Analysis

The first study was conducted in the Solar Calorimetry Lab by Freeman [3] and simulated the performance of indirect solar assisted heat pump using TRNSYS. The program used mostly component models developed with the TRNSYS software, but models were created for the heat pump, the natural convection heat exchanger, and the heat pump controller. A detailed description of the TRNSYS model, and a theoretical analysis and derivation of the steady-state vapour compression heat pump model is given by Freeman [3], and is briefly summarized in a previous paper by the authors [13].

Results of these previous studies predicted higher seasonal solar fractions than conventional Solar Domestic Hot Water (SDHW) systems. It was concluded that the ISAHP gathered more energy from the environment during marginal weather conditions, as well as during the winter when compared to either an SDHW or air-to-water heat pump systems. The study also found that the life cycle cost of the ISAHP system showed up to 29% savings over the SDHW system for major cities across Canada.

Dimensioning for the best cost benefit ratio

The conventional dimensioning of a solar heating system for one — or multi-family houses is commonly based on a trade-off between the solar fraction and the level of utilisation [2], whereas the customer and user might rather be interested in elements like additional cost and primary energy savings. To meet these requirements a different dimensioning directive was introduced [1][3]. The simulation program TRNSYS was coupled with the optimisation program GenOpt for the dimensioning in response to this directive. A short description and the resulting optimal dimensions of the MaxLean system concept are given in the following and serve as basis for the subsequent sensitivity analysis.

3.1 Dimensioning directive

The objective function minimised by the optimisation algorithm is the cost/benefit ratio described in Eq. (1):

a*I0 + BMaxLean Bref

a annuity factor

I0 total investment costs of the solar thermal system

BMaxLean annual operation costs of the MaxLean system concept (including the heating circuit)

Bref annual operation costs of the conventional reference system

Eprim, sav primary energy savings

As long as a solar heating system is not economically rewarding, the primary energy saving is the preferential merit on which value is laid. The additional cost for these primary energy savings consists of the additional investment cost minus the difference in operation costs between the conventional and the solar assisted heating system. The investment costs of a solar heating system are calculated by Eq. (2), (cf. [2]):

A ( V 6536

I0 = 2559€ + *368€ + 3983€I I Eq 2

m2 t m3 I 4′

Acoll flat-plate collector area

Vstor storage device capacity

The annual primary energy savings are calculated from the difference between the energy consumption of the solar and non-solar heating system, as well as the embodied energy of the solar system. (Thus, the embodied energy of the non-solar heating system —i. e. a water heater store— is not accounted for). The gas and electricity consumption is converted to primary energy consumptions. To obtain annual values of the embodied energy the total amount is divided by the service lifetime.

State of the art and development in Europe

Solar thermal plants in the district heating scale were pioneered in Sweden the beginning of the 80’s. Since then a significant number of plants have been built around Europe, both with diurnal and seasonal storage. The technology was developed to a mature stage during the 90’s when a multitude of plants were built around Europe. Most of the plants today are found in Austria, Denmark and Sweden. Also in Germany there are a number of plants in operation. The difference in land prices between the northern and central Europe is probably the reason, that in Sweden and Denmark the collector arrays have been mostly built as ground mounted, whereas in central Europe they are normally mounted on roofs. The large number of solar assisted block heating plants in Austria is mainly thanks to generous subsidies, which have been around 40% of investment costs. Another fact that has made it feasible there is that traditionally the block heating networks were shut down for the summer to avoid the high heat losses. A new summertime heat supply business could be started when investing in solar and could be seen as extra income. The situation is different from the northern European view, where summertime district heating (even though with bad thermal efficiency) is taken for granted, and solar thermal is seen only as a means to save fuels. [1-12]

Twin-Store-System with heat pump (system A)

Подпись: Fig. 1: Twin-Store-System with heat pump (system A)

In the Twin-Storage-System (see figure 1) a combistore (nominal volume 600 l) is used for hot water preparation (by means of a stainless steel tube) and for space heating. It is charged by the collector via an immersed heat exchanger located in the lower part. The auxiliary part of the combistore is charged by the heat pump. Another pressureless buffer store (nominal volume 800 l) serves in addition to the borehole heat exchanger as a heat source for the heat pump. This store is charged by the solar collector only.

2.1. System with integrated heat pump (system B)

In this system (see figure 2) the heat pump exclusively uses a borehole heat exchanger as heat source. The heat pump is fitted at the combistore (nominal volume 750 l) and the condenser is located in the auxiliary part of the combistore. The return flow of the space heating loop is connected to the combistore using a special device for stratified discharging of the store. The heat gained by the collector is added to the combistore via an internal heat exchanger which is also equipped with a special stratification device. An external heat exchanger is used for hot water preparation.

Results and discussion of testing laboratory model

Temperature control of the heater has been set at 33°C in the first experiment. Temperature in the up-flow pipe was greater than in the tank-accumulator during the first hour. When these temperatures became equal, the rising of temperature in tank-accumulator stopped at the level of 25°C (see Fig. 4). The duration of the cycles became longer (see Fig. 5), and heat accumulation was discontinued (see Fig. 6). Greater duration of cycles occurs when water in the accumulator is heated up completely and the heater fills only thermal losses. The difference in temperature between the descending pipe and the up-flow pipe was about 7°C. Only 0.6 liters of water flowed through heat exchanger during one cycle independently of heat source temperature.

image169

The behavior of process was similar for other temperatures of the heater (see Fig. 7).

Time, h:mm

image170

Time, h:mm

Fig. 7. Operating temperature in tank-accumulator for heater temperature of 33, 41, 49, 57 and

60°C

2.

Подпись: Fig. 6. Operating capacity with respect to the time for a heater temperature of 33°C

Conclusion

Testing of laboratory model demonstrated stable operation and the heat flow depended on the capacity of the heater. This model of the circulating pump starts to act autonomously when the temperature difference between the descending pipe and the up-flow pipe exceeds 7 degrees. The proposed device can be used when heat has to transfer from a heat user source which is located below it, and can especially be used with a solar installation instead of an electrical circulating pump. The testing of a device which is integrated with a solar installation is currently underway.

References

[1] .Davidson J. H., Walker H. A., Lof G. O. G.. Experimental Study of a Self — Pumping Boiling Collector Solar Hot Water System. Journal of Solar Energy Engineering, 1989; 111 (3): 211 — 218.

[2] .Dobrianski Jury, Fieducik Jolanta Urz^dzenie do przekazywania ciepla w kierunku przeciwnym do konwekcji naturalnej. Patent PL 195490 B1 F24D 9/00 F03G 7/06

[3] .Dobriansky Y „Reverse thermosiphon’Undustrial heat engineering, Vol. 28, №6, 2006. — str. 44 — 48 (in Russian).

[4] .Dobriansky Yuriy. „Reverse thermosiphon”. IV International conference “Problems of Industrial Heat Engineering” Kiev, Ukraina 26 — 30 September, 2005. s 144.

[5] . Roberts C. C., Warrenville Jr. A Review of Heat Pipe Liquid Delivery Concepts/ Advances in heat pipe technology. London: Pergamon Press, Oxford, 1982: 693 — 702.

[6] . Walker H. A., Davidson J. H. Second — Law Analysis of a Two-Phase Self-Pumping Solar Water Heater. Journal of Solar Energy Engineering, 1992: 188 — 190.

[7] . Walker H. A., Davidson J. H.. Analysis and Simulation of a Two-Phase Self-Pumping Water Heater. Journal of Solar Energy Engineering, 1990 Vol. 112: 153 — 160.

The new solar combisystem concept

image236

The complete solar combisystem consists of two units, the “Technical Unit” and the “Solar Store Unit” (see Fig 1 and Fig. 2).

In the technical unit all components like boiler, pumps, mixing valve, switching valves, heat exchangers, hot water preparation unit, expansion vessels, etc. are pre-installed. The main difference of this concept compared to existing ones, is the kind of integration of the condensing natural gas boiler. Due to the fact that the boiler is powerful enough for direct domestic hot water preparation, it can be avoided to heat the standby volume of the solar tank up to high temperatures for hot water preparation (typically 70°C or more). Therefore, the standby volume is only used at the temperature level needed for space heating operation, which results in much lower average temperature of the complete system and leading to higher overall performance of the heating system thanks to reduced heat losses of the tank and the pipes as well (detailed simulation results are presented in [2]). If hot water demand occurs and the temperature in the top of the solar tank is not high enough, the gas boiler immediately starts running in hot water preparation mode at high temperature level for direct hot water preparation in combination with the flat plate heat exchanger (see Fig. 3).

image237

Fig. 3. Start/Stop frequency of the natural gas boiler during night for space heating; at about 06:30 and 07:42 domestic hot water preparation takes place (see also Fig. 2: Tc1-Tc20 in °C / DO1_Boil_S is the on/off

signal of the boiler).

 

Further advantage for the condensing natural gas boiler is the very low return temperature during domestic hot water preparation leading to higher condensation rate and higher efficiency. During periods where space heating load is less than the minimum power of the boiler which can be reached by modulation (5.7 kW) the use of the standby volume reduces significantly the start/stop frequency leading to less start/stop emissions and longer life time of the ignition unit in the boiler.

Due to this operation concept also the top of the solar tank (Tc1) never is heated to high temperatures by the boiler resulting in a higher heat storage capacity of the tank for the solar heating system.

Experimental Investigation

The preliminary experimental evaluation involved building and instrumenting an ISAHP system in a laboratory setting, based on the recommendations of component sizing given by Freeman [4].

The solar collector in Figure 1 was replaced with an auxiliary heater in order to perform controlled experiments. Quasi steady-state tests were run at range of constant input temperatures with all variables constant except for the natural convection flow rate. The natural convection flow rate varied throughout the duration of the test while the tank temperature increased. The preliminary results indicated that the original computer model over-predicted the actual COP of the system.

This discrepancy was determined to be due to an over-prediction of the heat exchanger effectiveness values for both the condenser and evaporator. After correcting the heat exchanger effectiveness values in the simulation, the results for power consumption and COP matched to within 3.0 % of each other for the 10oC test.

Resulting dimensions

Подпись:Подпись: primary energy savings [kWhprimary/a]The dimensioning method presented leads to a solar heating system with a comparably small collector area of 8.9 m2 and a storage device capacity of 0.67 m3; assuming a 60 kWh/m2 single family house located in Zurich with a hot water consumption of 3000 kWh/a. In Table 1 the optimal system configuration for these assumptions (called the base case) are summarised. To find the dimensioning parameters leading to the best cost/benefit ratio a number of simulation runs are necessary. The results of these runs — sorted by primary energy savings and

additional cost — are shown in Figure 2. Each dot in the chart represents a system with a different set of collector area and storage device capacity, leading to specific primary energy savings and additional cost. The quotient of these terms is the cost/benefit ratio which can be understood as the slope of a line through the origin meeting the respective point. The dimensions leading to the smallest gradient, which is also the tangent to a polynomial derived from all points, are the optimal dimensions (cf. [2]).