Category Archives: EuroSun2008-10

The air-cooled heat exchanger

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

image124

Fig. 2. Air-cooled heat exchanger

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

• Tube outside diameter = 0.0102 m

• Fin pitch = 315 per meter

• Flow passage hydraulic diameter = 0.00363 m

• Fin thickness = 0.00033

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

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

• Fin area/total area = 0.913

• Number of tubes = 120

• Number of rows = 6

• Number of passes = 10

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

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

• Heat exchanger effectiveness = 0.68

• NTU (Number of Transfer Units) = 1.893

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

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

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

Future Utilisation Potential Factor (FUPF)

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

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

Tjmonth

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

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

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

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

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

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

Solar CombiSystems Promotion and Standardisation (COMBISOL project)

M. ALBARIC1*, C. BALES2, H. DRUECK3, B. GAGNEPAIN4, G. KUHNESS5, T. LETZ6,
B. METTE3, A. THUR5, J. E. NIELSEN7, P. PAPILLON1

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

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

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

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

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

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

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

* Corresponding Author, mickael. albaric@cea. fr

Abstract

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

Keywords: Solar CombiSystems, Performance, Standardisation, Promotion

1. Introduction

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

2. The context

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

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

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

image091

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

Fig. 2. Heating floor storage and hydraulic storage.

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

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

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

• promoting standardised systems and cost-effective solutions,

• recommending solar combisystem designs to manufacturers,

• training installers,

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

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

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

Energy measurements

1.1. Monitoring systems

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

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

Benefits of zero carbon solar thermal control to the wider solar thermal industry

Our aim in developing this controller is to offer the solar thermal industry the opportunity to zero carbonise all solar thermal systems. It is becoming accepted that zero carbon design is the gold standard in European housing design. The UK government consultation document3 published in December 2006 and entitled, Building A Greener Future: TowardsZero Carbon Development, discusses this in some detail and states that,“developing new homes to low and zerocarbon standards on a large scale, we can promote technologies and innovation which will help drive down emissions from the existing stock too. Our key goal is to achieve zero carbon new homes within a decade.”. Further, a recent life cycle analysis paper2 of three micro generators, states that “The UK domestic building sector, which contributes around 30% of final energy demand, and about 23% of greenhouse gas emissions, can play an important role in CO2 abatement. An uptake of low or zero carbon (LZC) distributed energy resources would help this sector to reduce fossil fuel energy use and CO2 emissions.”

3. Conclusions

Zero Carbon PV powered controllers and pumping for solar thermal systems should be seen as the new gold standard. These solar energy systems offer greater sustainability for the solar thermal industry avoiding the potential energy claw-back of up to 33% on conventional solar thermal. The opportunity for solar thermal manufactures and their customers to fully embrace zero carbon solar thermal technology has now become a simple reality.

Since the technology is now available to achieve this step change, it can be argued that all new solar thermal systems in Europe should be PV pumped by an agreed target date (such as 2012) and the industry’s current somewhat atomised approach on component efficiency should soon be

replaced by focussing on system sustainability. This should require zero carbon operation of solar thermal systems as mandatory in all domestic scale solar thermal installations.

References

[1] Martin C, Watson M. Side by side testing of eight solar water heating systems. ETSU S/P3/00275/REP/2, DTI/Pub URN 01/1292

[2] Allen, S. R., G. P. Hammond, H. Harajili, C. I. Jones, M. C. McManus, and A. B. Winnett, 2008. Integrated appraisal of micro-generators: methods and applications. Proc. Micro-Cogen 2008, Ottowa, Canada, 29 April — 1 May, Paper MG2008-SG-005, 8pp

[3] Building A Greener Future: TowardsZero Carbon Development 07HC04711, report available on download on 1st July 2007 at the following location

http://www. communities. gov. uk/publications/planningandbuilding/futuretowardszerocarbon

Study Performances of Thermosyphon with Heat Source. near the Top and Heat Sink at the Bottom

E. Yandri1* N. Miura1, T. Kawashima1, T. Fujisawa1, M. Yoshinaga2

1 Solar Energy Research Group, Department of Vehicle System Engineering, Faculty of Creative
Engineering, Kanagawa Institute of Technology, 243-0292 Atsugi, Japan

2Department of Architecture, Faculty of Science and Technology, Meijo University, 468-8502 Nagoya,

Japan!

Corresponding Author, yandri@ctr. kanagawa-it. ac. ip

Abstract

Solar energy can be converted into electricity with Photovoltaic cells and to heat with solar collectors. Especially for solar collectors, the heat collected can be utilized for both water heating and space heating applications. Solar collector researches for space and water heating has been developed and resulted many interesting designs, from simple thermosyphon systems with low maintenance to automatic operation systems which are depended so much with mechanical and electrical properties like pumps, valves, sensors, etc. Recently, a device which transfered heat from the hot reservoir near the top to the cold reservoir at the bottom was invented by Ipposhi et. al [6], called as the ITMI model. As same as the ITMI model was constructed and tested. We improved the ITMI model by proposing the IMT model. The first report was presented in SWC 2007 by comparing the performance of ITMI model and IMT model. The current experiments are completed with a digital flow mater of vapor in order to be able to calculate the heat energy transported. Some experimental parameters are varied in order to know the optimal operating condition for this IMT model. Heat input is varied for 100, 200, and 300W. Inclination angle between evaporator and top heat storage is varied for 0, 5, and 100. Level of heat store in the top heat storage are varied for 20, 110, and 200mm. Result shows that this IMT model can work better for all heat input (100, 200, 300W), and for all heat store in the top heat (20, 110, and 220mm) with inclination angle of 00, 50, 100. This model could be more interesting for water and space heating applications as more ecological approach.

1. Introduction

Solar energy can be converted into electricity by Photovoltaic cells, heat by solar collectors, and both electricity and heat by hybrid photovoltaic and thermal (PV/T) panels. The collected heat can be used for Space Heating and Solar Water Heating (SWH). A typical SWH system is a combination of solar collectors, an energy transfer system and a thermal storage system. SWHs are also characterized as open loop system (direct) which circulates potable water through the collectors and closed loop system (indirect) which uses antifreeze heat-transfer fluid such as polypropylene glycol to transfer heat from the collector to the potable water in the storage tank [2]. Depend on the way to circulate the working fluid, SWHs are divided into active system which uses

a pump to circulate the working fluid such as water or polypropylene glycol through the collectors and passive system which circulates the working fluid naturally by the effect of the gravitational force [2]. The passive system calls also thermosyphon which means the heat transport device that can transport a large amount of heat using body forces (gravitational and centrifugal forces). Thermosyphon has a great advantage because of no electrical energy and simple structure. That is why, the thermosyphon researches are not intended for SWH application only, but for many applications. Thermosyphon was studied as an alternative liquid cooling technique in which heat is transferred as heat of evaporation from evaporator to condenser with relatively small temperature difference [3]. Thermosyphon radiator used for domestic and office heating was studied and its performance has been tested with Freon 11, acetone, methanol and water as working fluid [1]. A model of the two-phase flow and heat transfer in the closed loop two phase thermosyphon (CLTPT) involving co-current natural circulation, which is focus for electronics cooling that exhibit complex two-phase flow patterns due to the closed loop geometry and small tube size [4]. The main reason to develop a thermosyphon with a heat source near the top and heat sink at the bottom is to solve weight problem when a thermosyphon installed on the roof [7].

Thermal performance evaluation of solar combisystems using a global approach

M. Albaric1*, J. Nowag and P. Papillon1

1 CEA, LITEN, INES 50 avenue du Lac Leman, 73377 Le Bourget du Lac, France.
* Corresponding Author, mickael. albaric@cea. fr

Abstract

Solar energy of today’s solar combisystems provides nearly 20 to 50 % of the total heat demand of a modern standard single family house, depending on climatic conditions. The European market offers a huge variety of solar combisystems and up to now no uniform standard exists that can prove the thermal performances of systems. The system design with its control algorithms plays a very important role and has to be considered carefully. It is obvious that the performance of solar combisystems can vary with different climate conditions and varying building typologies. The focus of this work lays on the choice of the weather data for a 12 test days which allows an annual prediction of the auxiliary energy used. They have to be chosen from a reference weather data for a specific climate zone. Solar irradiation and ambient temperatures should closely correspond to the curves of an annual weather cycle.

Keywords: Solar CombiSystem, performance evaluation, global approach, test lab

1. Introduction

Although the European solar thermal market for single and multifamily houses offers today many efficient and reliable products, there is still a high need in system optimization. It is particularly the case for solar combisystems, used for space heating and domestic hot water preparation, in which many individual designs with different control strategies entered the market during the last two decades. For a long time, it has been difficult to determine an accurate performance rating for those systems, and even more difficult to compare them, because there were no common definitions of terms or standard test procedures for that type of system. For the establishment of that fast increasing market and a sustainable further development, the existence of uniform standards, recognized by the whole solar industry, is very important.

To contribute to the national and European research needs, the French National Institute for Solar Energy ‘Institut National de l’Energie Solaire’ (INES) is setting up a new test facility in its laboratories. The objective is to develop a methodology for a 12-day performance test, which allows an annual prediction of the auxiliary energy used and therefore a statement on the energy savings during one year.

2. The context

Several methods have been developed that are briefly described below.

The open loop evaporative cooling tower

The open loop evaporative cooling tower consists of a shell containing packing/fill material with a large surface area. Nozzles arranged above the packing, spray and distribute heated cooling water from the condenser evenly onto the packing. The water trickles through the packing into a pond from which it is pumped back to the condenser. The water is cooled by air, drawn or blown through the packing by means of a fan. The air flow, which is either in counter-flow or cross-flow to the water flow, causes some of the water to evaporate. The evaporated water is continuously replenished by make-up water. Evaporation however also increases the concentration of TDS (Total Dissolved Solids) in the cooling water. Blow-down of the cooling water is therefore required which is replenished by additional make­up water to dilute and to maintain concentration levels within acceptable limits. It is possible to cool the water below the ambient dry bulb temperature using a cooling tower, as the wet-bulb temperature determines the degree of cooling. An approach of 4°C can still be achieved economically, which is the difference between the water outlet temperature and the ambient wet-bulb temperature [4].

The cooling tower was simulated with the TRNSYS Type 51 b, which models the performance of a multiple-cell counter-flow or cross-flow cooling tower and sump. To employ this Type the user has to enter two coefficients, used in the mass transfer correlation [10]. Although these coefficients are usually difficult to obtain, in the present case they were provided by a cooling tower manufacturer.

The physical parameters used in the cooling tower Type are:

• Maximum cell flow rate = 3800 m3/h

• Fan power at maximum flow rate = 0.33 kW

• Mass transfer constant, c = 2,28

image125

Подпись:Fig. 3. Open loop wet cooling

The selected TRNSYS Type is particular useful and simplifies the method of solution when compared to more conventional numerical procedures. As the Merkel method, this approach is based on the following simplifying assumptions:

• The Lewis factor relating heat and mass transfer is equal to 1;

• The air exiting the tower is saturated with water vapor and it is characterized only by its enthalpy;

• The reduction of water flow rate by evaporation is neglected in the energy balance.

Because of these assumptions, however, the Merkel method does not accurately represent the physics of the heat and mass transfer process in the cooling tower fill.

The method of Poppe, developed in the 1970s, does not make the simplifying assumptions of Merkel. Predictions from the Poppe formulation result in values of evaporated water flow rate that are in good agreement with full scale cooling tower test results. In addition, the Poppe method predicts the water content of the exiting air accurately [11]. An EES code exploiting the Poppe method for a wet cooling tower is being developed and will be employed in future works.

Other Test Results

4.1 Overnight temperature drop in December

Results showed that although Flat No. 3 — the most conservative case — had the highest average water temperatures, the overnight thermal energy losses from the solar tank were the least, as shown in Figure 6. Again, this is a proof of good management of hot water, since the users opted to partially fill

image019 Подпись: □ Overnight temp. drop □ Average water temp.

the tank with water during the winter months, to counter-balance the lower availability of solar energy. This action had also contributed towards lowering the overnight temperature drop, since there was less volume of water in the tank when compared to other flats. From this graph, one also concludes that it is important for the solar tank to reach a considerably high afternoon temperature in winter (say 45 or 50 °C), so that after loosing energy at night, the water temperature would still be warm enough in the morning, without resorting to back-up electric heating.

Fig. 6. Overnight temperature drop and average hot water temperatures in the solar tanks in December.

Project organisation

The project is divided into 5 major work packages covering all the aspects of technology dissemination like training, test methods development, in-situ monitoring, standardisation, implementation of public policy, and systems development.

2.1. Qualitative evaluation of combisystems

Until now solar combisystems sold on the market are only partly prefabricated systems, most of them are still consisting of quite a number of components which need to be put together by the installer on the construction site. Also, several controller settings of one or more controllers (e. g. solar controller, boiler controller, space heating controller) often need to be done in a correct way to start and run a solar heating system. Therefore there is a risk to make mistakes during installation or when doing the controller settings.

This work package should help to find the key hurdles which can cause less advantageous installed solar heating systems. The learned lessons of this work package should help to improve the designs of prefabricated solar heating systems and to improve the educational activities for professionals.

Based on a selection procedure with different boundary conditions a typical distribution of different system types and products was chosen in cooperation with solar industry. At least 50 systems will be evaluated.

In parallel to the qualitative evaluation of combisystems, the manufacturer datasheets and documentation available in the different countries are collected in order to have a first technical evaluation. This technical evaluation together with the qualitative evaluation will allow preparing the first recommendations regarding the design and the installation of solar combisystems.

A first draft for the evaluation procedure has been written and will be improved during the qualitative evaluation procedure. This evaluation procedure will be available on the CombiSol website.