The TRNSYS Type 107 models a single-effect hot-water driven chiller using a lookup approach from a performance data file to predict the part load performance. With the release of TRNSYS 16 this Type was made a standard TRNSYS component. Although originally adapted to the operation and control of large scale absorption chillers (e. g. QE, rated > ~200 kW), this type and — as often expected — also the available example performance data file can be used for small capacity chillers also, since its data are normalised. Nevertheless some difficulties occurred when applying Type 107 in combination with the example performance file to the EAW Wegracal SE 15 chiller. Some of them are also mentioned in the manual of Type 107.

The example performance data cannot be used for the intended purpose since it does not suit to the operation conditions — neither to the manufacturers’ information nor to the conditions at Ebner Solartechnik. For example, the range of hot water inlet temperature (tGi) in the provided example performance file allows a range between 109°C and 116°C only; the manufacturers brochure depicts a range between 80°C and 90°C and, finally, the measured values at the solar cooling plant are often lower than 70°C.

If the chilled water inlet temperature (tEi) is too high or the available cooling capacity is too low to reach the chilled water set point (tEo, set), the part load conditions in Type 107 are calculated under the assumption of tEo, set nevertheless. Therefore the simulated cooling capacity is too low compared to what is happening in reality, since the real tEo will be higher as tEo, set.

As a consequence two steps of improvement have been carried out: A new Type — labelled as Type 307 here — was created on the basis of Type 107 and a new performance data file based on laboratory measurements has been prepared. Both will be explained in the next section.

The first task of the pool 1 is to define the monitoring for all the solar cooling processes. Indeed it is important to have the measurement enabling the comparison whatever the running conditions of the installation or whatever the climate. The pool 1 worked out schemes for the measurements of the desiccant and the absorption technologies. In order to evaluate the exergy exchanged during all the processes and to provide a comparison of the three technologies, a first reflection has been carried out to define what should be consider has the reference temperature. The choice has been made first to use a fixed temperature instead of a variable one. Finally the pool 1 proposes to rely on the lowest outside air temperature as the reference temperature [3]. This pool proposes an evaluation of the weather conditions as well. The experimental facilities being located in different climates (tempered in France and tropical in La Reunion), an exergetic analysis of the weather conditions and more specifically of the solar radiation has been carried out to define which technology is suitable for a given climate [4], [5].

4.1. Pole 2

The desiccation process is the key issue of the pole 2; therefore a great effort is made for the modelization of all the components allowing the drying of humid air. This pool has already carried out the validation of a TRNSYS model for the desiccant exchanger (TYPE 882) and another new model, describing the behaviour of a desiccant wheel has been created. It is now possible to propose a simulation of the whole desiccant installation coupled with a building.

4.2. Pole 3

The first results for the pool 3 focus on the simulation of the absorption facilities with EnergyPlus and Simspark. The modelling under EnergyPlus is made following three steps: the first one is to simulate the building without considering the cooling installation in order to validate the physical description of the building. In a second step, the solar cooling installation is modelled considering the solar loop, the storages, the cooling tower and the absorption machine. The last step is to propose a coupling between all this components. However some problems relating to the simulation of the absorption machine connected to the solar collectors still remain. The solution would be to move to the SIMSPARK simulation environment. Two partners of the project have undertaken this work and it should be completed very soon. The first experimental results obtained on the RAFSOL installation show that some modifications can be proposed to reduce the energy consumption of the auxiliaries and consequently improvement of the COP can be achieved.

4.3. Pole 4

The simulation of the thermo chemical installation requires the description of nine main components. Three 3 elements remain to be integrated into the global model before considering the control strategy of the plant. This modelling describes the thermochemical unsteady process thanks to the Gibbs equation systems. As the model includes entropy and exergy calculation for each

component, it is possible to carry out second law analysis and to optimize the process. The experimental facility is used to validate the model and to identify the phenomenological parameters. Moreover, the analysis of the experimental sequences during summer 2007 leads to several improvements of the instrumentation and the control command.

4. Conclusion

This study of the various solar cooling technologies is needed to better understand and control the functioning of these complex processes in order to ensure satisfactory comfort conditions in buildings despite the instability of the energy resource. By providing tools to help design and optimization of the installation, it will be possible to significantly reduce implementation costs and remove a barrier to the widespread dissemination of this technology.

References

[1] M. Clausse, Y. Perigaud, F. Meunier, F. Boudehenn, H. Demasles, (2007). Experimental characterisation of a novel adsorber heat exchanger for dessicant cooling applications. Proceeding of the 22nd IIR International Congress of Refrigeration, august 21th/26th , Beijing, China.

[2] H. Demasles, F. Boudehenn, M. Clausse, (2007). Numerical and experimental studies of a novel adsorber heat exchanger for desiccant solar air conditioning. Proceeding of the 2nd International Conference Solar Air-Conditioning, October 18th/19th, Tarragona, Spain.

[3] M. Pons, (2008). Bases for second law analyses of solar-powered systems, Part 2: the external temperature. , ECOS-2008 21st Int. Conf. on Efficiency, Cost, Optimization, Simulation & Environmental Impact of Energy Systems, Krakow, POLAND, June 24-27 2008, 2008.

[4] M. Pons, (2008). Bases for second law analyses of solar-powered systems, Part 1: the exergy of solar radiation. , ECOS-2008 21st Int. Conf. on Efficiency, Cost, Optimization, Simulation & Environmental Impact of Energy Systems, Krakow, POLAND, June 24-27 2008, 2008.

[5] M. Pons, (2008). Exergie en environnement reel pour la climatisation solaire. Congres Frangais de Thermique, SFT 2008, Toulouse, 3-6 juin 2008.

The comparisons of simulated and measured outlet temperatures of the solar collectors and temperature inside the hot tank are shown in Fig. 7. We chose to compare the values from 9 AM to 5 PM when the outlet temperature of solar collectors exceeds hot tank temperature. In this case, the mean absolute error is 10.6% on the outlet temperature collectors and 2.8% on the hot tank. Our solar collector model based on efficiency curves slightly overstated the performance of solar collectors, while the evolution of the hot tank temperature determined by our simulation seems fairly close to our experimental platform value.

4.1. Analysis of the outlet temperatures of generator, condenser and cold tank

The comparisons of simulated and measured outlet temperatures of the generator, the condenser and temperature inside the cold tank are shown in Fig. 8. We chose to compare the values during the functioning period of the absorption chiller (from 11:10 AM to 4:20 PM). The mean absolute error on the outlet temperature of the generator is about 5% while the mean absolute error of the condenser is about 6%. These errors are quite high but in general, our model follows quite well the actual evolution of these temperatures. In contrast, the mean absolute error on the cold tank temperature is about 40%. This value is very high, explained by the fact that our model of absorption chiller is very simplified.

4.2. Analysis of the generator, the evaporator and the cooling tower powers

Looking at the temporal evolution of the powers, visible on the fig. 9 and 10, and calculating mean absolute error on the generator (16.6%), the evaporator (30.2%) and the cooling tower (10, 6%) we reach the same conclusion as seen before. Therefore it is imperative to establish a more accurate absorption chiller model particularly for start-up (peak powers on the actual values of the generator and the cooling tower) and stop (real refrigerated production enduring 10 minutes after shutdown).

The Armenian-American Wellness (mammography) Center (AAWC) in city of Yerevan situated in the latitude 40011 (N) and longitude 44024 (E) at approximately 900m above see level. Armenia is a highland and has continental climate with hot dry summer and average annual global irradiance of about 1700kWh/m2 [1].

The PV roof of the building is south-east oriented and inclined at 280 to the horizontal plane. The 10kW PV array consists of 144 modules divided into 9 groups with 16 serial connected modules in each group. The module is made of silicon Uni-Solar laminate on the 0,7mm thick stainless steel sheet with 332cm length and 45cm width. Each module has rated power 68W, voltage (Vmp)

16,5V, current (Imp) 4,1A, open circuit voltage 23,1V, short circuit current 5,1A, and weight 12kg. The PV roof has 220m2 total area including 124m2 of photosensitive area (Fig.1). The rated power output of PV array is 9,8kW.

The PV array is connected through three grid-tied FRONIUS IG 30 inverters and reversible electric meter (with remote data acquisition) to the 3-phase national grid. Each inverter has three inputs with voltage range of 150-400V and one output of 230V, 50Hz. PV modules in upper horizontal row may have higher temperature than modules in lower row and their temperature dependent output values may differ. Therefore, inverter’s inputs are connected to the vertical columns of PV module groups to ensure similar phase power supply (Fig.2). The PV station has solar radiation,

PV module and ambient air temperature sensors for monitoring.

As a grid-connected PV power station it is the first one in CIS countries. According to local regulations any grid connected power plant should receive a license from the Public Services Regulatory Commission. The PV station has been completed and tested with the grid a few months ago but due to numerous bureaucratic obstacles it is still not commissioned and no monitoring data are available at the moment. PV station’s commissioning is expected in one-two months.

Temperature is measured using PT100 platinum resistance temperature detectors to DIN 43760 class A with an accuracy of ± 0.15°C (T in Figure 1). Humidity is measured using capacitive humidity sensors, complete with sensor mounted transmitter electronics (RH in Figure 1). Two different models are

being used with a low-temperature accuracy (0-40 °С) of ± 1.5%RH and a high-temperature accuracy (40-90 °C) of ± 1.3-3%RH. Velocity is measured by a hot wire anemometer with an accuracy of ±

0.032 m/s per °С difference to 25 °С (V in Figure 1). The pressure loss over the wheel is calculated using differential pressure sensors with an accuracy of ± 12.5 Pa (DP in Figure 1).

2.1 Material data

The desiccant wheel under investigation in this work is coated with a Ferro-alumino-phosphate zeolite with an iron content of 5 mol%, named “Functional Adsorbent Material Zeolite 01” by the manufacturer and traded as FAM-Z01. The wheel investigated has a diameter of 300mm and a depth of 200mm. Material isotherms for the FAM material are shown in Figure 3. More details on the material itself are given by Kakiuchi [6].

It can be seen in Figure 3 that the adsorption capacity of FAM-Z01 is low in the low relative pressure range but increases drastically at higher relative pressures. A large amount of water can be adsorbed in a rather narrow range of relative pressures. Increasing equilibrium temperatures cause this pressure range to extend further. Figure 3 also shows the desorption equilibrium isotherm for 60 and 75°C regeneration temperature. It can be seen that there is only a small difference in the amount of water ad — /desorbed for ad — and desorption, respectively.

The basic scheme for this configuration is shown in the next figure:

This configuration can be assimilated to an installation in which the demand is much higher than the one that can be supplied by the solar cooling installation, therefore in the moment there is energy available, the system will be in conditions to use it. It has been taken an inlet temperature to the evaporator of the chiller, constant at 14 °С.

The main result within the third level is given by the fractional solar heating & cooling savings (f sav, shc) which is defined by the (Equation 14. It presents a comparison of the primary energy need of the installed SHC system with a reference system, which is defined based on the real needs for heating and cooling.

4. Acknowledgments

The authors would like to thank all partners from IEA SHC Task 38, Task 26 and Task 32 who contributed to the elaboration of this procedure. The authors from Eurac would further like to thank to the Stiftung Sudtiroler Sparkasse for the financial support.

References

[1] W. Sparber, A. Napolitano, P. Melograno, 2007, “Overview on world wide installed solar cooling systems”, 2nd International Conference Solar Air-Conditioning, Tarragona.

[2] H. M. Henning, 2005, ”Solar Air-Conditioning R&D in the framework of the international Energy Agency (IEA)”, 1st International Conference Solar Air-Conditioning, Bad Staffelstein.

[3] W. Sparber et al, 2008, “Modeling of a Solar Combi Plus System — Framework and Hydraulic Scheme Proposals”, Ninth International Symposium Gleisdorf Solar, 3rd — 5th September 2008, Gleisdorf, Austria

[4] Directive 2006/32/EC of the European Parliament and of the Council of 5 April 2006 on energy end-use efficiency and energy services and repealing Council Directive 93/76/EEC, L114/76, Annex II, footnote

3

[5] http://www. iea-shc. org/task38/index. html

[6] H. M. Henning (ed), “Solar-assisted air conditioning and buildings, a handbook for planners”, IEA SHC Task 25, Springer Wien New York

[7] http://www. iea-shc. org/task26/index. html; http://www. iea-shc. org/task32/index. html; http://www. iea- shc. org/task25/index. html

[8] T. Letz, 2002, "Validation and background information the FSC procedure", A technical report of subtask A, IEA-SHC Task 26, http://www. iea-shc. org/outputs/task26/A_Letz_FSC_method. pdf

[9] W. Weiss (ed), 2003, “Solar Heating Systems for Houses — A Design Handbook for Solar Combisystems ” IEA-SHC Task 26, James & James Ltd London, pp 125-154

[10] R. Heimrath, M. Haller, May 2007, “The Reference Heating System, the Template Solar System”, IEA SHC — Task 32 Project Report, Institute of Thermal Engineering University of Technology Graz Austria, pp 28-30 ; available from the task 38 download area.

2.1. Thermodynamic design

The proposed prototype is a so-called single effect cycle that uses ammonia as refrigerant and water as absorbent. This working pair also does not bear a crystallization risk as may occur in lithium bromide cycles, and no freezing at sub-zero conditions as occurs to all cycles using water as refrigerant (lithium bromide, silica gel and zeolite chillers). Another advantage is the continuous operation, without charge-discharge cycles (that occur for example in solid sorption or triple phase absorption) with consequent performance fluctuations and cycling losses. The ammonia cycle works at relative high pressure and therefore it does not need purging of non-condensable gases (vacuum maintenance). Assembly is also less critical and costly than for water-based cycles, since it does not need to be vacuum-proof. Ammonia has no significant ozone depletion or global warming potential.

The single effect is the basic working cycle in the absorption technology. It incorporates four main heat exchangers, a solution heat exchanger, a pre-cooler, a rectifier, a solution pump and two throttle valves. The main design task is centred on the four main pieces of equipment within which the entire process takes place, except for the refrigerant and solution transfers between the different pressure levels of the chiller and for the heat exchange in the solution heat exchanger. The basic, single effect cycle is shown in Figure 1. The main components are the evaporator, condenser, absorber, generator, rectifier, pre-cooler, and solution heat exchanger. A recirculation loop connects absorber and generator. In this loop, the working solution is re-circulated. The refrigerant vapour, i. e. ammonia, is boiled off in the generator by heating the working solution (driving heat from the sun). The solution becomes poor in refrigerant content. The refrigerant vapour flows to the condenser where it is condensed releasing waste heat and is then throttled to the evaporator. There, low-temperature heat (from the building) is dissipated to evaporate the condensate refrigerant. Finally, the low-pressure refrigerant vapour enters the absorber and is absorbed into the

poor (diluted) solution throttled from the generator. This process releases additional waste heat. The resulting concentrated solution is pumped back to the generator to close the solution loop. Normally, condenser and absorber produce heat at the same temperature, which is released into the ambient air; in a dual-use heat pump/chiller, the waste heat could be used either for domestic hot water (DHW) production or for space heating in winter. Given that the energy balance over the complete cycle must be nil, the total waste heat released is equal to the total heat required, i. e. the sum of the driving heat (solar) and of the low-temperature heat (cooling). The ratio between cooling and driving heat, i. e. the benefit vs. the effort, is the coefficient of performance (COP) of the chiller. Furthermore, precooler and solution heat exchanger help reducing thermodynamic losses in the cycle, thus enhancing the COP.

Ammonia as refrigerant reaches at the required operational temperatures a quite high vapour pressure (between 400 and 2000 kPa), which enables for a very compact construction of the heat exchangers. Historically, shell and tubes or tube-in-tube heat exchangers are utilized in the small capacity range. Recently, also plate heat exchangers have been tested in laboratory prototypes. On the refrigerant side, a pipe with throttle valve connects the condenser with the evaporator, like in the well-known recompression chillers. A throttle valve is also needed in the solution circuit.

Finally, an electrically driven solution pump is used to circulate the working solution. Its power consumption can reach values from 3 % up to 10 % of the cooling capacity. It is therefore mandatory for a solar-based chiller to find a very efficient and reliable pump for a satisfactory and energy-saving operation. An image of the first prototype built at Ao Sol is depicted in Figure 2. Abundant empty space was foreseen in the inner volume in order to carry out typical laboratory work such as changes and repairs.

1Tchanche Fankam Bertrand*, 1George Papadakis, 1Gregory Lambrinos and 2Antonios

Frangoudakis

Department of Natural Resources and Agricultural Engineering,

1 Laboratory of Agricultural Engineering,

2 Laboratory of Agricultural Constructions,

Agricultural University of Athens, 75 Iera Odos Street 11855 Athens, Greece
Tel.: +30 (210) 529 4046; Fax: +30 (210) 529 4036
* Corresponding Author, tfb@aua. gr

Abstract

The working fluid is one of the most important components of a Rankine cycle power system. It influences the performance and thereby the economics of the system. Therefore, special care should be taken when selecting the working fluid. In this paper a list of criteria that should fulfill a fluid to be considered as good for a Rankine cycle is given. These criteria include good heat transfer properties, non-toxicity, non-flammability, high efficiency, availability and low cost. Finally, a general methodology for working fluid selection made of three steps: (1) data collection, (2) data analysis and (3) decision is proposed.

Keywords: criteria, working fluids, methodology

1. Introduction

Steam has shown its ability to serve as working fluid in high temperature power plants. In low temperature (< 200 °С) or low-output power plants (< 10 kW), the use of water is not economically feasible [1-2]. Therefore, other fluids should be sought. Interest was found for refrigerants and some other fluids. These fluids are: Halons, hydrocarbons (HCs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and natural fluids like carbon dioxide, ammonia, air, etc. Unfortunately, some of the above categories of fluids were phased-out or are to be banned soon by the international regulations [3]; pushing manufacturers of substances and researchers to look for new environmentally friendly fluids. So, new categories of fluids like zeotropes, azeotropes and other multicomponent fluids were born [4]. Of various working fluids existing that could be used in Rankine power cycles, the problem that comes out when designing a cycle is the choice or the selection of the best suited fluid for a particular application. The choice of the working fluid is very important as it determines the efficiency and the economics of the power plant. If in literature many criteria are proposed for the working medium selection, very few works point how the best fluid can be picked up from a group of several potential candidates. In the present paper, we list these criteria and propose a general methodology for working fluids selection in organic Rankine cycles.

2. The criteria

Before going through a selection process of an item in a precise group of similar items, the criteria should be well established. This applies also to the selection of working fluids for Rankine cycles. For this particular case of organic fluids, many works have been reported in literature. Badr et al. [1], Stine and Geyer [2], Lee et al. [5], and Maizza and Maizza [6] are some authors who
investigated the criteria that should fulfill an ideal working fluid. In this section, these criteria are reported.

The basic process of cold generation is the compression of a refrigerant fluid, which causes evaporation of the liquid at low temperatures and pressures and condensation of the vapour at higher temperatures and pressures. Instead of mechanical compression like in electrical air — conditioners, thermal driven chillers use thermal energy for the compression of the fluid.

The basic principle of the thermal compression is the ab — or adsorption of the refrigerant in a liquid or solid material. Whereas absorption chillers use the liquid lithium-bromide in adsorption chillers solid adsorbents like silica gel or zeolites are used. In both machines the refrigerant is water, which results in the technical task that the machine has to be operated at very low pressures in a vacuum tight containment.

In our chiller we use silica gel as adsorbent. Silica gel is a porous glass with a high capacity of adsorbing water vapour. For that reason it is used as desiccant in various applications.

The working process of the adsorption chiller is described below (compare figure 1).

Step 1: Desorption — Drying of the adsorbent

The adsorbent is dried by heat input. Water vapour is set free, flows in the condenser and is liquefied there under heat emission. When the material is dry, the heat input in the adsorber is stopped and the upper check valve closes.

Step 2: Adsorption — water vapour is adsorbed at the surface of the adsorbent

*

After a cool down phase the reverse reaction and the evaporation of the liquid condensate starts. The lower check valve to the evaporator opens and the dry adsorbent aspirates water vapour. In the evaporator, water evaporates and generates cold, which can be used for air-conditioning. During the adsorption process heat is rejected which has to be dissipated.

Step 3: Return of condensate

In a final step the liquid condensate is returned to the evaporator and the circuit is closed.

In order to achieve a continuous cold production two adsorbers work in combination, i. e. one adsorber desorbs while the other adsorber generates cold by adsorbing in the meantime.

water vapour liquid process water check valves condensate return driving heat heat rejection cold generation

2. Construction Principles

SorTech AG has developed and patented two construction principles, which are important for achieving a compact and lightweight chiller design: