From Figs.3 to 5 show the example simulation results for the simulation case of the collector area 30m2 and the storage tank volume 2.0m3 using the Case A DHW supply.

Fig.3 shows the simulation results of the solar DHW system from 18th to 20th in January as the example of typical days in winter. A collector efficiency exceeds 70% in the midday of 18th, a clear day. The water temperature defference at

 

Collector efficiency

 

•/""N

 

0

 

outlet

 

nlet

 

-1(top)

the inlet and outlet of the collector is 8.9 degrees C. The water temperature in the storage tank becomes 40 degrees C in the clear day. The temperature stratification is seen in storage tank when DHW is supplied in the afternoon, and the water temperature in the storage tank falls slowly. The maximum water temperature of the storage tank is 28 degrees C in January 19th, a cloudy day.

Fig.4 shows the simulation results of the

Fig.5 Hourly simulation results in summer days (collector area 30m2 solar DHW system from

and storage tank volume 2.0m3 with Case A DHW supply, A30V2.0). 22nd to 24th in May as

the example of typical

days in spring. The water temperature of the top in the storage tank rises to nearly 60 degrees C in the afternoon of the clear days in May 23rd and 24th. On May 22nd, a cloudy day, the maximum water temperature in the storage tank is 38 degrees C. The solar energy supplies a half of DHW heat load in the afternoon of the clear days.

Fig.5 shows the simulation results of the solar DHW system from 7th to 9th in August as the example of typical days in summer. As shown Table 2, smaller DHW supply rate was used in summer. As the water temperature in the storage tank exceeds 60 degrees C from 12:00 to 24:00 of August 8th and 9th, the DHW heat load is completely covered by the solar energy in this time.

One of the challenges of this project was to identify suitable areas on the roof to install the solar heating systems, in accordance with the Malta Environment and Planning Authority Guidelines [5] , while leaving sufficient space for drying clothes and for a photovoltaic system for the showroom. The task was further complicated by the existence of two lift rooms, which caused shading on large parts. In conclusion, it was deemed necessary to place 6 out of the 10 solar heaters on 1-metre elevated metal structures, to avoid shading by the perimeter walls, as shown in the background of Figure 2.

The solar heating systems were chosen to be of a 150-litre capacity each, as this would be sufficient for the washing needs of a family of four. Evacuated-tube systems were chosen due to the limited space, as they would occupy less area than an equivalent flat-plate solar system. Moreover, these systems were equipped with an electronic controller and display unit, which managed the volume of the feed-in cold water supply and the time at which replenishment is made. Also, it controlled the back-up electric booster element, in terms of thermostatic control and scheduling. The display showed the temperature and volume of water in the solar tank and also allowed for the control of the whole system from the comfort of one’s home. These features allowed a better control of hot water usage and gave a complete picture of the system’s conditions at one glance.

The project started by the preparation of detailed technical specifications for inclusion in the tender document. Adjudication was then made and the tender was awarded to the successful bidder. Close inspection during the installation stage was necessary to ensure full compliance to tender

[2] Be inherently safe and easy to fit for both professionals and those installing DIY domestic

solar panels for home use

• Be simple and tamper-proof, for example by having no interface buttons on the front of the

unit meaning that it cannot be programmed or deprogrammed without first unscrewing the unit.

• Have minimal need for component replacement by eliminating both batteries and mechanical

[3] D J. Naron, H Visser, (2002). Direct Characterisation Test Procedure for Solar Combisystems, Technical report, IEA Solar Heating & Cooling programme Task 26, http://www. iea-shc. org/task26

[4] P Vogelsanger, (2002). The Concise Cycle Test — An Indoor Test Method using a 12-day Test Cycle, Technical report, IEA Solar Heating & Cooling programme Task 26, http://www. iea-shc. org/task26

[5] H Druck, S Bachmann (2002). Performance testing of Solar Combisystems — Comparison of the CTSS with the ACDC procedure, Technical report, IEA Solar Heating & Cooling programme Task 26, http://www. iea-shc. org/task26

[6] M. Haller, R. Heimrath (2007). The reference heating system, the template solar system, Technical report, IEA Solar Heating & Cooling programme Task 32, http://www. iea-shc. org/task32

[7] S. A. Klein (2004). Manual of TRNSYS 16, a TRaNsient SYstem Simulation program, University of Wisconsin-Madison, USA

[8] Matlab Tutorial (2005), The Language of Technical Computing, The Math Works, Inc

[9] Intelligent Energy Europe

[10] Avis Technique 14+5/04-887, 2.00 Ecosol Collector by ESE.

[11] Night hot water storage is frequently used in France due to the lower electricity price (0.064 €/kWh) usually between 22h and 6h.

[12] The tank temperature is recorded at 10 levels equidistant along its height. Obvious, the temperature at the top of the tank is the highest on this graphic and the bottom temperature is the lowest.

[13] Introduction

The expected solar thermal market boom in Portugal, due to the implementation of the new building code, will pose new challenges to contractors, building owners and users, and some conflict situations will certainly arise from the novelty of adoption of these systems in very large scale over a short period of time. The lessons to take from these first times will be of paramount importance to all sectors and agents involved in the process.

EPUL, Lisbon public building promoter, has acted as a pioneer in the Portuguese real estate market for some years, asking INETI to make technical-economic feasibility studies for the installation of domestic solar hot water systems in new multi-owned buildings under their responsibility.

These studies consistently showed the feasibility and profitability of such investment, even at a time when the current escalade of oil prices was not foreseeable, and previously to the introduction of the new building code (RCCTE) as a result of the implementation of EPBD (Energy Performance of Building European Directive).

It is the case of TELHEIRAS XXI, an apartment building in Lisbon, whose study was done in October 2003. After project approval and construction, the building was concluded, and ready for occupation in early 2007, just after the enforcement of the new building code making the use of solar energy compulsory in all new buildings.

Y. Dobriansky*, M. Duda and D. Chludzinski

University of Warmia and Mazury in Olsztyn, Faculty of Technical Sciences,
ul. Oczapowskiego 11, 10-736 Olsztyn, Poland

Corresponding Author, dobr@uwm. edu. pl

Abstract

The purpose of the work is to develop a self-acting circulating pump which transfers heat downward and acts with help of local heat that has to be transferred. The pressure difference of the saturated vapour is used for moving warm liquid downwards. The principles of action and the possibility of developing such a device by using laboratory experimental methods are presented. The device operates cyclically, is self-controlled, has simple design, and has no mechanical moving elements except flaps of check valves. This pump can be used with solar installation instead of electrical circulating pump.

Keywords: passive heat transfer downwards, self-acting, circulation, pump, liquid, reverse thermosiphon.

1. Introduction

Devices which use the phenomenon of natural convection, transfer heat upward, act autonomously and are often used in practice. There has been great interest in the invention of a device of passive heat transfer downward and there have been some technical proposals [1], [5], [6], [7]. However, they are not used in practice due to certain shortcomings: complicated mechanical design, high cost or dangerous materials, short distance of heat transfer etc. Only one engineering solution is used in practice widely. This system is a mechanical liquid pump that is being powered by using external energy sources. The main drawback of this system is that it does not act autonomously.

Solar collectors as a rule are located above a tank-accumulator of warm water in solar installations. Electrical circulating pumps are used for moving warm heat-carrier downwards from collectors to a tank. The cost of an electrical circulation pump with appropriate control device amounts about 20% from total cost of solar installations for a family house. Usual solar installation depends on power supplies. Moreover, these elements fail the most often in comparison with other parts of a solar installation.

To determine the system operation with and without weather forecast and to compare our results with field information we use two parameters: annual solar cover coefficient т and annual coefficient of performance COP. The last one is classically computed for heat pumps and refrigeration machines, and we consider that it can be very useful for solar systems too. The two efficiency coefficients are define as follows:

т = — ^ 100 (13)

Ethw + Qloss

Where Qloss stands for the annual energy lost by the tank.

4.1 Comparison between cities

The performances of the above-described HWSH are computed in three different locations, having different solar radiation intensity: Paris (3.3 kWh/m2.day average value), Grenoble

(4.3 kWh/m2.day) and Nice (4.9 kWh/m2.day).

To estimate the potential of small scale Solar Combi+ systems, a market survey among AC systems retailers was conducted — the target group being the consumers, but the source to collect and analyze the necessary data the retailers, who could provide data not only on volume of sales but also on consumers attitude. Even if the research partners faced some difficulties in approaching retailers and collecting information, the filled questionnaires (elaborated by CRES) reveal some interesting aspects, which are presented here (the full report is available on the project website [8]).

Results are in line with the EU-survey of “consumer attitudes related to EU Energy policy” [9], which states that 80% of EU citizens say energy efficiency influences their decision when buying household appliances (nearly half of them very much) and that they are quite certain that within the next decade they have to change their every day behaviour, but also the technology used to make their living space comfortable. The Solar Combi+ survey in fact illustrates, that also in the selection of air conditioning systems in almost all countries in Europe, energy efficiency is a major topic (see Fig. 8). This has a clear consequence on the selected products. In fact in Italy most products sold are classified as energy class A while in Greece more class B and C than class A products are sold. (see Fig. 9).

While other criteria such as maintenance effort, noise, the trademark, aesthetics — and total cost! — are also important aspects when buying air conditioning systems (Fig. 10), a high percentage of consumers pay attention to energy label and efficiency data. 40% of them are even willing to pay more for a product if it is energy efficient (Fig. 11). This survey shows that Solar Combi+ systems can have a significant market if they are as reliable and convenient as conventional air conditioning systems even if they are somewhat more expensive.

5.

Conclusion

By the end of the project, validated standard system configurations for small scale combined heating and cooling will be at the disposal of solar thermal industry, producers of sorption chillers and system providers.

The participating companies will accordingly be able to offer their own package solutions. These pre-engineered systems will enable installers to offer the technology to customers and to promote it, thus triggering the pull market in a considerable way.

A detailed database which will be publicly available will result from the analysis of a broad range of technologies, applications and climate. It will provide in depth information on all important aspects of small scale combined solar heating & cooling for several key actors. The technical, economical and ecological evaluation of cases will be useful for involved industry and planers as well as for authorities.

For example can the project also contribute to tailored shaping of support programmes. The provided knowledge on economical and ecological performance of combined heating and cooling for different regions facilitates the establishment, where necessary and required at all, of long term support schemes. An avoidance of stop and go support would this way further smooth market entry

[1] .

The combined use of solar thermal for heating & cooling can accelerate the evolution of solar thermal from only domestic hot water supply to a significant contributor to heating demand by

increasing cost efficiency of the entire system, this way mitigating the threats of Europe’s growing heat and cooling demands[10] [11]. And the increased utilization of sorption chillers will mitigate summer electricity peak demand, a major threat in some European countries (e. g. Italy [12]).

Accelerating and smoothing the market entry of small scale Solar Combi+ systems, the project will contribute considerably to achieving important energy policy goals of the European Union; in particular relating to the share of renewable energies [13] and the security of energy supply in the EU [14].

Acknowledgements

The authors would like to thank the industry partners within the SolarCombi+ project (Climatewell, Rotartica, Sonnenklima, Solution, Sortech) for their engagement in the project and the contribution to the elaboration of the presented information.

The Solar Combi+ project receives funding from the European Commission within the EIE Program (Project number: EIE/07/158/SI2.466793). The sole responsibility for the content of this document lies with the authors.

References

[1] European Solar Thermal Industry Federation — ESTIF (2003): Sun in Action II — A Solar Thermal Strategy for Europe, Bruxelles

[2] Initiator group of ESTTP (2006) Solar Thermal Vision 2030, Brussels

[3] H. M. Henning (Ed.): Solar-Assisted Air-Condistioning in Buildings. A Handbook for Planners. Springer Verlag, Wien, 2004

[4] European Solar Thermal Industry Federation — ESTIF (2005): Solar thermal markets in Europe, Trends and statistics, Bruxelles

[9] Y. Vougiouklakis, E. Korma (2008), Report on market situation & trends about small scale chillers, Project Report D2.1 of the IEE Project — SolarCombi+, www. solarcombiplus. eu

[6] REFRIGE. COM Portal — HVAC & Refrigeration news, events, training, books, magazines and directory online http://www. refrige. com/february-2007/world-air-conditioner-market-contmued-to-expand-m- 2006/menu-id-2087.html

[7] CO. AER Associazione costruttori di apparecchiature ed impianti aeraulici http://www. coaer. it/indagine/_indagine_.htm#

[8] Y. Vougiouklakis, E. Korma (2008), Report on Market Potential & Relevant Consumers for Solar Combi+, Project Report D2.3 respectively of the IEE Project — SolarCombi+, www. solarcombiplus. eu

[9] Flash Eurobarometer series 206 (2007) Attitudes on issues related to EU Energy Policy — Analytical report

[10] ALTENER project “ECOHEATCOOL”, project Newsletter Issue 2

[11] F. Butera, The use of environmental energies for sustainable building in Mediterranean climates; Intelligent Building Middle East Bahrain, December 2005

[12] Gestore Rete Trasmissione Nazionale (GRTN): Audizione del Gestore della rete Presso le Commissioni Riunite Industria e Ambiente e Territorio del Senato della Repubblica, Rome, September 2003

[13] Energy for the Future: Renewable Sources of Energy — White Paper for a Community Strategy and Action Plan — COM(97)599 final (26/11/1997)

[14] Final report on the Green Paper "Towards a European strategy for the security of energy supply", COM(2002) 321 final, Brussels, 26.6.2002

R. Haberl1*, E. Frank1, P. Vogelsanger2

1 Institut fuer Solartechnik SPF, HSR University of Applied Sciences of Rapperswil,
Oberseestrasse 10, 8640 Rapperswil, Switzerland

2 Ingenieurburo Peter Vogelsanger, Nidelbadstrasse 94, 8038 Zurich, Switzerland
* Corresponding Author, robert. haberl@solarenergy. ch

Abstract

The effect of various parameters on the cost, the performance and the optimum dimensions of a combined solar heating system (for Domestic Hot Water and space heating) in Central Europe was studied. The parameters varied were: Collector cost and collector performance, storage cost and storage insulation as well as the rise in energy prices. For every set of parameters the optimum size of the collectors and the storage tank was identified with simulations. The cost/benefit ratio was used as the optimisation criterion.

As could be expected, favourable parameters result in economically more viable systems. The simulations showed that good (efficient or inexpensive) collectors also positively affect the system’s size and performance. Surprisingly, reducing the cost of the storage tank does not significantly affect the size and thermal performance of the system with the best cost/benefit ratio. Better insulation of the storage tank even leads to slightly smaller systems. While the price of the backup energy strongly influences the system’s economics, energy prices have hardly any effect on the optimum system size.

All simulations were based on an avant-garde system concept which features an unusual control strategy for space heating and a drain-back collector loop. This lean design could be appropriate to incorporate a non-pressurized tank and could be suitable for substantial cost reduction. However, its thermal performance characteristic is not very different from other, more usual solar combisystem concepts. The cost functions used in the base case simulations were derived from pricing information on products available on the market (i. e. pressurized tanks). The authors therefore suppose that the essential results of this work also apply to other combisystems in a similar climate.

Keywords: Solar heating system, cost/benefit ratio, additional cost, primary energy savings, dimensioning, optimisation.

1. Introduction

With today’s energy prices and without subsidies, the installation of a solar thermal system leads to comparably high payback periods. Thus, the cost effective saving of fossil fuels should take precedence in the comparison of installation costs against any conventional heating system. Better cost effective savings can be achieved with respective improvement of the solar thermal system components as well as paying attention to the combing of them into an ensemble. In this paper, a potential new concept called the MaxLean System is introduced, in the development of which a new dimensioning method has been devised that optimises the cost/benefit ratio instead of the annuity of the system. The principle of the method and the appliance on the MaxLean system

concept is presented. The same approach is then used for investigating the influence of various component parameters and cost function parameters on the dimensioning and the cost/benefit ratio of the system.

Fig.6 shows the year-round performance of the solar DHW system. In Fig.6 the collector efficiency, the DHW heat load, the solar contribution, the CO2 emission and energy cost are compared. The performance in Fig.6 is expresses for a housing unit. The CO2 emission was calculated from the gas calorific value of 45MJ/m3 and the CO2 emission coefficient of 2.21kg/m3. The energy cost was calculated using the fee structure of the city gas company in Tokyo. The average unit price of the gas is 149JPY/m3. The collector efficiency for the collector area of 20m2 with Case A DHW supply is from 53.0% to 63.4%. It tends to increase according to the storage tank volume is large. The collector efficiency decreases by 3.6%-7.2%, when the collector area increases by 10m2. The considerable difference in the collector efficiency is found when increasing from 0.5m3 to 1.5m3 of the storage tank volume, however there is only few difference when the storage tank volume is more than 2.0m3. The collector efficiency for the collector area of 30m2 and the storage tank volume of 1.0m3 is 53.0% with Case A DHW supply, 49.5% with Case B DHW supply, and 41.6% with Case C DHW supply, respectively. The CO2 emission for collector area of 30m2 and the

by 2.2% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 20m2 and Case A DHW supply. However, the solar contribution only increase by 6.7% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 50m2 and Case A DHW supply.

The comparison of the saved cost of the city gas for a housing unit was shown in Fig.8. The dark bars in Fig.8 show the suitable combinations of the collector area and the storage tank volume judged from the solar contribution. The saved energy cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply is estimated to be 19,700JPY/year for a housing unit. The comparison of the installation cost for 10 housing unit is shown in Fig.9. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply should be suppressed 2.0 million JPY when the pay back period of 10 years is assumed. The installation cost should be suppressed 3.0 million JPY when the pay back period of 15 years is assumed. The saved energy cost for the collector area of 20m2 and the storage tank volume of 0.5m3 with Case A DHW is estimated to be 80,000JPY/year, and the installation costs are 8.0 million JPY and 1.2 million JPY for pay back period of 10 and 15 years, respectively.

Fig.9 The installation cost for 10 housing units assuming the pay back periods of 10 and 15 years.

6. Conclusion

The central type of solar DHW supply system for 10 housing units was simulated to examine the relationships of the collector area and the storage tank volume with considering the DHW supply rate. The simulation results showed the followings.

1) The collector efficiency for the collector area of 30m2 varies from 38% to 59% depending on the storage tank volume and the DHW supply rate. The collector efficiency decrease by 3.6%-7.2%, when the collector area increases by 10m2.

2) The suitable storage tank volume is from 1.0m3 to 1.5m3, since the collector efficiency decreases extremely when the storage tank volume is 0.5m3. The efficiency for 2.0m3 is almost same as the case for 1.5m3.

3) The solar contribution for the collector area of 30m2 and the storage tank volume of 1.0m3 is 37.1% with Case A DHW supply, 52.5% with Case B DHW supply, and 74.6% with Case C DHW supply, respectively.

4) The boiler load for the collector area of 30m2 and the storage tank volume of 1.0m3 is 105.9GJ/year with Case A DHW supply, 48.6GJ/year with Case B DHW supply, and 10.8GJ/year with Case C DHW supply, respectively.

5) The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 2.0 million JPY with Case A DHW supply, 1.7 million JPY with Case B DHW supply and 1.0 million JPY with Case C when the pay back period of 10 years. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 3.0 million JPY with Case A DHW supply, 2.5 million JPY with Case B DHW supply and 1.5 million JPY with Case C when the pay back period of 15 years.

References

[1] T. Kusunoki and M. Udagawa, Planning of Solar Collector Arrangement for Solar DHW Heating Apartment House, Proceedings of JSES/JWEA Joint Conference 2006, pp. 125-128. (In Japanese)

[2] H. Roh and M. Udagawa, Study on Standardization of Solar DHW Heating System for Apartment Houses, Proceedings of JSES/JWEA Joint Conference 2005, pp.79-82. (In Japanese)

[3] M. Udagawa, H. Roh and M. Satoh, Design of Solar DHW System for Apartment Houses, Proceedings of ISES Solar World Congress 2005

[4] M. Udagawa, and M. Satoh, Energy Simulation of Residential Houses Using EESLISM, Proceedings of Building Simulation ‘99, pp.91-98. (In Japanese)

[5] Architectural Institute of Japan, Expended AMeDAS Weather Data, 2005. (In Japanese)

[6] S. Kaneko, M. Udagawa and T. Kusunoki, Design of Solar DHW Heating System for Small Apartment House, Proceedings of JSES/JWEA Joint Conference 2007, pp.213-216. (In Japanese)

The aims of the work are as follows:

— to develop a simple self-acting device that will transfer heat downward up to a depth above 10 m. The temperature difference between the warm and cold branches would be only a few degrees, and

— to test the performance of the device under laboratory conditions and when integrated with a solar installation.

The proposed device operates cyclically, is self-controlled, has a simple design and has no

mechanical moving elements except flaps of check valves.

The principles of action are shown in Fig. 1. The cycle of action include two stages [2], [3], [4]:

— heating the upper part of warm branch and pushing warm liquid downward. It will cause rising of level of liquid in the cold branch (see Fig. 1a),

—

opening the upper path in the flow circuit, equalising vapour pressure in both branches, pouring off the surplus of cold liquid through the intermediate canal to upper part of warm branch gravity (see Fig. 1b).

The upper parts of each branch have to be made wider. It will cause the pumping of a greater portion of liquid during each cycle. The directions of liquid flow in the main part of the circuit and intermediate canal are maintained by placing non-return valves. A special control valve has to be used for periodical opening and closing of the upper path of the flow circuit.

In a standard system the auxiliary electric heater run identically every day, no matter the season. During night, it brings the upper volume of the tank at the set-up temperature (here 70°C). This one is usually preset. Solar energy is recovered in the lower part of the tank, and, if the temperature rises above the set-up temperature in the upper volume, than the solar loop will heat up the entire tank. Due to a large variation of the solar radiation between winter and summer, the heat recuperation varies also. During winter the heat input of the solar loop is very poor and the tap hot water is produced mainly by the auxiliary heater. Contrary, during summer, the solar contribution to the tap hot water is important, even complete. Thus, the tank can contain an important thermal energy at a sufficiently high temperature to provide tap hot water without running the auxiliary electric heater.

Introducing a control device able to take into account the weather forecast for the next day, we try to diminish the electricity consumption during warm seasons keeping the customer’s comfort at a good level. For this, we are able to control the set-up temperature of the auxiliary heater depending on the solar radiation of the next day. We decided to introduce three temperature levels:

• If the maximum solar radiation of the next day is above 800 W/m2 the set-up temperature is Tsetup = 50°C, representing mostly the summer operation;

• If the maximum solar radiation of the next day is between 450 W/m2 and 800 W/m2 the set-up temperature is raised at Tsetup = 60°C, usually during intermediary seasons;

• And finally, for solar radiation lower than 450 W/m2 the set-up temperature is pushed up to the standard temperature applied during winter Tsetup = 70°C.

Next graphic presents the comparison between the standard system and the HWSH with weather forecast. The temperature variation at 10 levels[12] in the tank and the operation of the electric heater are shown for both cases. The tap hot water pouring, solar radiation and pump operation are alike for both cases.

One can observe that on April 1st the electrical heater change its behaviour due to the lower set-up temperature. This small electric economy is multiplied by an important number of days leading to a considerably annual economy. This one is presented in the next table computed for Paris and Grenoble.

One notice the annual electricity economy given by the weather forecast control of around 60 kWh in both cases, meaning 7% of the total electric energy consumption.

5. Conclusions and perspectives

The numerical model presented in this paper is able to simulate a HWSH with an advanced control system taking into account the weather forecast for the next day. Using a simple modification of the electrical heater algorithm we are able to cut down 7% of the annual electrical consumption and to increase with 3% the solar cover coefficient.

The electricity gain can be optimised by a fine setting of the two solar radiation limits used to determine the set-up temperature of the heater, depending on the system location and tap hot water pouring.

References

[1] F. A. Peuser, K-H. Remmers, M. Schnauss, (2003) Installations solaires thermiques. Conception et mis en oeuvre, Systemes Solaires, Paris.

[2] T. Prud’homme, D. Gillet, Energy and buildings, 33 (2001) 463-475.

[3] M. LeBreux, M. Lacroix, G. Lachiver, Energy and buildings, 38 (2006) 1149-1155.

A. Bridgeman* and S. J. Harrison

Solar Calorimetry Laboratory, Queen’s University, Department of Mechanical and Materials Engineering,

130 Stuart Street, Kingston, ON, K7L 3N6, Canada
Corresponding Author, Bridgeman@me. queensu. ca

Abstract

An indirect solar assisted heat pump (ISAHP) system for heating domestic hot water has shown promise as an alternative to conventional electric or natural gas water heaters. In a previously conducted theoretical study, it was concluded that an ISAHP could operate with a lower life-cycle cost than a conventional solar domestic hot water (SDHW) system. Therefore, to further investigate the feasibility of the proposed system, an experimental study was conducted on a prototype (ISAHP) system. To undertake the study, a fully instrumented heat pump water heater was assembled in a laboratory environment and connected to a simulated “solar heat” input. The “solar” input was provided by an electrically heated circulation loop that delivered temperature-controlled fluid to the heat pump evaporator. This allowed repeatable test sequences to be performed in the laboratory regardless of weather conditions. A simulated solar profile ranging from 750 — 1500 W was delivered to the heater throughout the test. The corresponding fluid temperature ranged from 15 — 37°C, and the results indicated coefficient of performance (COP) values ranging from 2.4 to 3.2. These results, while in close agreement, are approximately 12% lower than those predicted from previous theoretical values.

Keywords: Solar assisted heat pumps, Heat pumps, Domestic water heating, Canada

1. Introduction

In Canada, water heating is the second most energy intensive end use in the residential sector, accounting for 22% of the consumed energy [1]. Due to growing concern for depleting fuel supplies, higher fuel prices and greenhouse gas emissions, alternatives to the conventional water heating methods such as electric and natural gas water heaters are being investigated. Two systems currently receiving considerable attention worldwide are Solar Domestic Water Heaters (SDWHs) and heat pump systems that source energy from the ambient air, or geothermal energy.

While each of these systems may operate with lower energy consumption than a typical electric water heater, both systems have performance limitations. Air-source heat pump water heaters are attractive in temperate regions, but lack popularity in Canada due to the warm temperatures needed for their proper function [2]. Geothermal heat pumps demonstrate improved performance over air source heat pumps because the heat is drawn from the earth, which is much warmer than ambient in the winter. However, due to the ground loops necessary for these types of heat pumps to function, property alterations and high initial costs have made them less practical for existing homes [2]. Solar Domestic Water Heaters have been increasing in popularity in Canada, and can decrease the energy consumption of an electric water heater by up to 90% in the summer [3], but large temperature differences between the collector and ambient air during the winter months lower the collector efficiency significantly, limiting the seasonal performance.

A combined system, known as a Solar Assisted Heat Pump (SAHP) could be used to alleviate many of the disadvantages of either system operating independently. The advantage to the heat pump cycle, by coupling it with a solar thermal collector, is an increase in evaporator temperature over either air-source or ground-source heat pumps. This increase in temperature results in an improved heat pump coefficient of performance (COP). From the solar collector point of view, the use of the heat pump lowers the fluid temperature returning to the collector near or below ambient. This lower temperature increases the collector efficiency, and allows for substantial heat gains with low cost unglazed solar absorber panels, even under marginal conditions [4, 5, 6]. The combined system allows for efficient operation over a wider range of seasons and weather conditions, and for more hours throughout the day.

The concept of a SAHP dates back to 1955 when it was first proposed by Sporn and Ambrose [7]. Numerous studies took place in the 1980s and early 90s examining the feasibility of SAHP systems for either space or water heating. Most of these systems were classified as Direct Expansion Solar Assisted Heat Pumps (DX-SAHP), in which the refrigerant would flow through the solar collector directly, which doubled as the evaporator for the heat pump. Chaturvedi [5, 8] found that collector efficiencies between 40 — 70% were feasible with bare collectors operating under ambient condition in winter, and found heat pump COPs ranging from 2 — 3, which was 30 — 50% higher than air source heat pumps. In the mid 90s Morrison [6] stated that the majority of previous systems proposed had not achieved commercial success due to the complexity of the combination of heat pump and solar collector components, and high installation costs due to the need for plumbing, electrical and refrigeration connections during installation. He then proposed an integral design, in which the collector and heat pump unit were incorporated as an integral part of the storage tank, which must be installed outside. Morrison found only a slight decrease in performance compared to a typical system in Sydney, Australia, but predicted a substantial reduction in cost, and simplification of installation. Huang and Chyng [9, 10] have recently investigated similar integral DX-SAHP systems in Taiwan. They found COPs reaching up to 3.83 during a long term performance test, in which the system was run for 13,000 hours continuously.

Although the integral DX-SAHP overcame installation complexities of SAHP systems and achieved commercial success in some parts of the world, installing the water storage tank outdoors introduces another problem in the Canadian environment. The cold conditions in the winter months increase the heat loss from the tank decreasing the system’s performance. To avoid this problem, an Indirect Solar Assisted Heat Pump (ISAHP) is under investigation at the Queen’s Solar Laboratory in Ontario, Canada. A schematic of an ISAHP is shown in Figure 1. This system differs from a direct solar assisted heat pump in that the heat pump collects energy via a heat exchanger connected to the collector anti-freeze loop, rather than flowing through the collector itself. This eliminates the need for long refrigeration lines and costly refrigeration fittings on the collector, but allows for the heat pump unit and storage tank to be installed inside the residence. Another feature of this system is the external side-arm natural convection heat exchanger, which acts as the heat pump’s condenser. As the heat exchanger transfers energy from the refrigerant to the potable water, the water increases in temperature causing its density to decrease. This induces buoyancy driven natural convection, circulating the water through the heat exchanger and eliminating the need for a pump. Due to the relatively low flow of the natural convection loop, this configuration has the potential for increasing thermal stratification in the storage tank. The benefit of stratification is that it delivers cool fluid from the bottom of the tank to the heat pump condenser, while maintaining hot water at the top of the storage for distribution to the load. This improves the overall system performance [11].