([SHULPHQWDO�DQG�WKHRUHWLFDO�LQYHVWLJDWLRQ�RI�VRODU�GULYHQ�DEVRUSWLRQ�FKLOOHUV� )� $ ������������� ,�$VVRFLDWH�3URIHVVRU��8QLYHUVLW\�RI�3HUXJLD��,WDO\��IDVGUXED#XQLSJ�LW�)� = �� ��� �� ��)XOO�3URIHVVRU��7HFKQLVFKH�8QLYHUVLWlW�%HUOLQ��*HUPDQ\��IHOL[�]LHJOHU#WX�EHUOLQ�GH -� $ �� ���� ��5HVHDUFK�IHOORZ��7HFKQLVFKH�8QLYHUVLWlW�%HUOLQ��*HUPDQ\��DOEHUV#LHPE�GH�*� % ��������� ���� ,�5HVHDUFK�IHOORZ��8QLYHUVLW\�RI�3HUXJLD��,WDO\��EDOGLQHOOL�XQLSJ#FLULDI�LW�$� 35(6&,877,��5HVHDUFK�IHOORZ��8QLYHUVLW\�RI�3HUXJLD��,WDO\��SUHVFLXWWL�XQLSJ#FLULDI�LW�6� 3(7(56(1��5HVHDUFK�IHOORZ��7HFKQLVFKH�8QLYHUVLWlW�%HUOLQ��*HUPDQ\��DOEHUV#LHPE�GH� $%675$&7� Within the Vigoni Programme for cooperation between Italian and German universities, the University of Perugia and the Technische Universität Berlin have been carrying out in the last years a joint research programme on two similar solar driven absorption plants, with evacuated tube solar collectors and Water-Lithium Bromide thermosyphon absorption refrigerators of the same manufacturer, but of different cooling capacity. The performance – especially in part-load - of these systems is not as satisfactory as it should be theoretically, so the research focused on the phenomenon of the overflow of the refrigerant, in order to improve this situation and to forward the technology. A detailed analysis of the thermodynamic process was conducted, employing comparable and shared measurement chains; the results suggested a control strategy, which balances the machine in such a way that the evaporator is neither overflowing nor running dry. ���,1752'8&7,21��

The summer air conditioning demand is growing continuously, not only in the tertiary sector but also in residential applications; the correspondent request of electric power involves frequent crisis of the electrical net, that must cover higher and higher load peaks. Such peaks are mainly satisfied recurring to fossil fuel thermoelectrical plants, with consequent increase of greenhouse effect.

Solar driven absorption chillers produce cooling with negligible requirement of electrical energy and can work using low temperature heat such as waste heat (otherwise lost in the environment, causing a temperature raise), or heat produced by renewable energies, such as solar energy. Therefore, they are discussed frequently for energy-environmental issues and can constitute a valid alternative to compression refrigerating machines, especially for the countries of the Mediterranean area, which are characterized by a growing demand of electrical energy for summer cooling.

In this context, within the Programme Vigoni for cooperation between Italian and German universities, a research work has been carried out by the University of Perugia, Industrial Engineering Department, and the Technische Universität Berlin, Institut für Energietechnik, on the theoretical analysis and the operational experience of solar absorption cooling plants.

The laboratories of the two universities have access to two similar solar driven absorption plants, with Water-Lithium Bromide thermosyphon absorption refrigerators of the same manufacturer, but of different cooling capacity. As a matter of fact, the performance – especially in part-load - of these systems is not as satisfactory as it should be theoretically.

In order to improve this situation and to forward the technology, an analysis of the thermodynamic process was conducted, employing comparable and shared measurement chains. In both plants measurement facilities allow to record in real time all the main operating parameters of internal and external circuits (temperatures, pressures and flow rates).The results of various measurement campaigns are presented, compared, and discussed. The experiments are supported by theoretical analysis. Particular attention is given to two special features of the installed systems: to the thermosyphon desorber and to the evaporator which suffers from overflow depending on the working conditions. The final aim is finding the best working conditions of the plants by analysis and optimization of the external and internal parameters. The results should allow to improve the performance and, consequently, help in disseminating this environment-friendly technology.

Experimental and theoretical investigation of solar driven absorption chillers

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In Berlin two solar assisted cooling systems (SAC-Systems) are installed at governmental buildings. The first system is located at the Federal Ministry for Traffic, Buildings and Housing (BMVBW) and the second one is located at the Press- and Information Office (BPA). Since all the reported measurements are taken from BPA, especially this system will be explained more in detail. The Press and Information Office is a large building complex in the town centre of Berlin, which consists of four parts: Historic building, Conference centre, Administrative building and a new building part.

Figure 1 – New building part with double glazing facade (1a, left) and cooling concept (1b, right) at the Press and Information Office in Berlin.

Figure 1 shows the new building part of the Press and Information Office, which has been erected in front of

the firewall of the adjacent historic building. A small part of the historic building can be seen on the right hand side of Fig. 1a. On the lower left hand side the entrance to the conference centre is located. The administrative building, which is without any air conditioning system, is not shown in Figure 1. The conference centre and the computer centre in the historic building are supplied by a 500 kW absorption chiller with its own chilled water distribution net.

Due to the double-glazing facade an energy demand for cooling can be avoided in the standard office area of the new building part by three functions:

• the movable glass blades are used for external sun shading thereby reducing the external loads; • an automatic control system reduces the internal loads caused by artificial lightning. • the remaining cooling loads are stored in concrete ceiling and the heavy walls of the adjacent old

building. These loads can be removed by natural ventilation during night time (even under thunder storm conditions).

One task of the Press and Information Office is to evaluate the news from TV and radio broadcasts from all over the world during 24 hours. For this purpose special office rooms are needed with a high technical standard. Only for these special offices and the meeting rooms an air-conditioning system is needed, because of the high internal loads from technical equipment and persons, respectively. Although the building is facing to east, cooling load and sun irradiation occur more or less simultaneous due to the possibility of night ventilation and the storage capacity of massive walls and ceilings. Thus it was decided to complement this innovative cooling concept by a solar assisted cooling system (SAC-System) with two absorption chillers from the manufacturer Yazaki, Type WFC-10. In Figure 2 the system layout and the position of temperature and flow meter probes is shown

Experimental and theoretical investigation of solar driven absorption chillers

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����7KH�3HUXJLD�3URMHFW� At the Laboratory of the Department of Industrial Engineering at the University of Perugia, a Yazaki chiller

(Type WFC-5) has been installed in order to investigate its performances under different service conditions. (Asdrubali, 2003). The chiller is a single-stage Water-Lithium Bromide absorption machine and it is fed by an electric boiler with a thermal input of 30 kW. The machine has a nominal chilling power of 17 kW which is employed to cool the Laboratories; the system is equipped with an evaporative cooling tower to refrigerate the condenser and absorber (Asdrubali,2003).

Figure 3 – Layout of the solar assisted plant in Perugia.

The preliminary tests showed that the chiller is able to work with relatively low temperatures of the supplying water, thus making possible to feed it with solar collectors(Asdrubali,2005). The research focused therefore on the simulation, design and realization of a solar plant to feed the absorption machine. The solar plant will be added to the chiller absorption plant inserting a preheating tank, linked to the electrical boiler, for

Experimental and theoretical investigation of solar driven absorption chillers

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solar energy storage. 15 flat collectors (vacuum tubes) placed on the laboratory roof will supply the heat for the chiller system, guaranteeing the thermal demand of machine in hottest summer days. Figure 3 plots a layout of the plant. The varied parameter in the simulation is the volume of preheating tank, since it is not allowed to change the boiler volume. The length of the pipes connecting the heat exchanger to the solar collectors is also determined by the characteristics of the building where the plant is placed. In the simulation code the assumed length of the pipes is equal to 30 m. It was found that other parameters such as thickness of the insulating material or pipes diameter do not influence significantly the system performances.

Modern solar flat collectors can have good efficiencies for high values of the temperature difference between collector and the environment. Vacuum pipes collectors allow to reduce conduction and convection heat losses increasing the efficiency of the collector. So, two different collector typologies have been analyzed by the simulation model; in particular, four models have been compared. Figure 4 shows the efficiency curves of the collectors, for a value of solar incident radiation equal to 500 W/m2, versus the difference between the average temperature of collector tav and the air temperature ta. Collectors number 3 and 4 are vacuum pipe collectors, while collectors 1 and 2 are high efficiency flat plate collectors.

Figure 4 – Collector efficiency vs. difference between the average temperature of the collector and the air temperature (tav-ta).

The fluid temperature inside the collector must be at approximately ten degrees over the working temperature

of the machine to succeed in feeding it (all the losses from the collector to the plant have to be taken into account). Since the average air temperature in summertime in Perugia is about 20 °C, only efficiency values, corresponding to a difference tav-ta between 65 and 85°C shall be considered. In this temperature range the collector that shows the highest efficiency, is collector number 4. Therefore, these collectors (H1V 12 vacuum tubes) were introduced into the simulation model, varying their number and consequently their surface. The results showed that a surface of 30 m2 is necessary to cover about 1/3 of the mean monthly thermal request over the whole cooling period. Others fundamental parameters which influence the flat solar collector performances are constituted by the tilt angle and the orientation.

The calculation code allows to introduce the climatic data of the place and to determine the optimum of such parameters. The values of direct and diffused solar radiation, the average temperature and the average wind speed in Perugia, introduced into the simulation model, are shown in table 1.

Table 1. Values of direct and diffused solar radiation, average temperature and average wind speed in Perugia. H�I<JLKNMPO�QRKTSUI�V�K ��W V�KXSUI7YZH�[RJLK\H�[R] K � [R^<K � O W K`_�a�b K ��ced K � Ofa�Khg�O�I�V

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The visual impact of collector installation has been evaluated to respect the building aspect. Figure 5 shows

as the Laboratory roof will appear when the collector will be placed with a tilt angle equal to 20°. Finally, the solar plant has been designed to allow the heating in the winter connecting the heat exchanger

directly to fan coils. In fact the solar plant will allow to heat the Laboratory with 6000 thermal kWh from month of November to March. The plant start-up is foreseen during the summer of the year 2005.

Experimental and theoretical investigation of solar driven absorption chillers

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In Figure 6 typical measured data for the SAC-System at BPA are shown during the course of a sunny summer day in 2005 with a maximum solar irradiation (qgh) of 650 W/m². On this day the system was operated as a solar autonomous system, which means that no district heating is used for back-up purposes.

The collector pump is switched on (approximately at 7:30 a.m.), if the global horizontal irradiation is higher than 250 W/m². At 10:00 a.m. the outlet temperature of the collector field (tKh) is higher then the mean storage temperature (tPS,m) and the valve in the collector circuit (VEK, Figure 2) is opened in order to heat up the storage. Since the storage is at a high temperature level from the previous day, the minimum driving temperature for the absorption chiller (which is set to 78°C) is reached near 11:00 a.m. and the solar operation of chiller AKA2 is started. This can be seen from the heat input to the generator QG2.

Since the heating capacity of the collector field is higher than the heat used in the generator of AKA2 the storage temperature is increased. Thus at a certain storage temperature (set to 82°C) the second chiller is switched on. In order to maintain clarity, only averaged values for QG1 and QG2 over 15 minutes are shown, since the high temperature difference during the chiller's start-up period may cause heating capacities higher the 100 kW.

Figure 6 – Typical day of solar autonomous operation of the SAC system at BPA, Berlin.

It is also seen from Figure 6 that the collector outlet temperature tKh is not constant during the operation

period of chiller AKA1 and AKA2, but varies between 80° and 88°C. Thereby the part load performance of the chillers is influenced by the collector efficiency. The internal operational behaviour of both chillers (WFC-10 and WFC-5) is complicated due to the fact that a thermosyphon generator is used for pumping the solution. In addition a refrigerant circulation pump does not exist. Therefore, there is a domain of operation where the evaporator of the absorption chillers is overflowing and another domain where the evaporator runs partly dry.

Experimental and theoretical investigation of solar driven absorption chillers

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The overflow of refrigerant from the evaporator into the absorber sump occurs when the absorber capacity is too low to absorb all of the refrigerant or – in other words – when the mass flow rate of refrigerant (which is desorbed in the thermosyphon generator) is too large to be evaporated by the cooling capacity. On the other hand, the cooling capacity is limited by the amount of refrigerant which is desorbed by solar heat in the thermosyphon generator. If the cooling capacity or absorber capacity is high enough to evaporate or absorb all of the refrigerant, the evaporator runs partly dry. In contrast to a conventional absorption chiller neither the absolute nor the specific solution flow rate is constant in the WFC-chillers, since the mass flow rate of pumped solution is simultaneously determined by the mass flow rate of desorbed refrigerant, which is in turn given by the operating conditions for the thermosyphon generator. This is illustrated by measurements which have been carried out at the WFC-5 absorption chiller in the Perugia’s labs. An ultrasonic sensor was used in order to determine the flow velocity of the poor solution pumped by the thermosyphon generator.

Figure 7 – Evolution of temperatures, thermosyphon desorber capacity and flow velocity of chiller WFC-5, Perugia. Since the condenser temperature TC, and the inlet and outlet temperatures of the solution heat exchanger (TLia

and TLoa) have been measured in parallel, the density of the concentrated solution can be calculated. The ultrasonic sensor was also used to determine the inner cross section of the riser tube connecting solution heat exchanger and absorber. Thus the mass flow rate of the poor solution is known from continuity equation.

In the WFC-5 chiller the cooling water paths of absorber and condenser are connected in parallel and a valve is installed in the pipe to the condenser to adjust the share of external flow rates MA and MC. In figure 7 it is shown that by closing this valve partly at about 12:35 o’clock, the internal condenser temperature TC is increased, although the external inlet temperature tCi to the condenser is kept constant. As in an absorption chiller with mechanical solution pump the increase of TC leads to a decreased driving temperature at the condenser and, as a consequence, the generator capacity QG decreases as well. But in addition, due to the reduced refrigerant desorption, the solution flow rate is lowered also, which is seen from the decreased velocity wa.

In general this is a welcome reaction of the thermosyphon generator since the specific flow rate does not increase as in the case of an uncontrolled mechanical solution pump. On the other hand care has to be taken that the decreased solution flow rate is still high enough to maintain good wetting in the absorber.

After the valve was brought back to the starting position, TC decreases nearly to the same value as in the beginning. But generator capacity and solution velocity are slightly higher due to the increase in hot water temperature tGi from 83°C in the beginning to 86°C at the end of the test. This effect is caused by inertia in the hot water supply. Thus, the main result of this section is that solution flow rate and desorber capacity of a thermosyphon generator can be controlled by hot and cooling water temperature, respectively. This knowledge can by used to find a control strategy, which balances the three heat exchangers in the solution circuit: absorber, thermosyphon generator and solution heat exchanger in such a way that the evaporator is neither overflowing nor running dry.

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Since the cooling capacity of a given absorption chiller is determined by all three external temperatures, a certain load (which is characterised by the set values of QE and tEo) can be supplied by several combinations of

Experimental and theoretical investigation of solar driven absorption chillers

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hot and cooling water temperatures. The difference between these temperatures is called the external temperature thrust (+tGACi = tGi – tACi). In addition it has been shown that the refrigerant and solution flow rates of WFC-5 and WFC-10 are determined by the hot and cooling water temperature [3]. Thus the efficiency is highly influenced by the temperature thrust. In Figure 8 this is highlighted for an exemplary load condition for the WFC-10 (e.g. QE=35 kW, tEo=8°C), which can be supplied by hot water of 95°C and cooling water of 31°C. But the same cooling capacity and chilled water temperature is available at 88°C hot water temperature, if the cooling water temperature is lowered by only 1.5 K.

Figure 8 – Part load behaviour and new control strategy for WFC-10; characteristics according to manufacturer. Nevertheless this behaviour is also known for conventional absorption chillers with mechanical solution

pumps. A lower driving temperature at the generator can be compensated by higher driving temperatures at the absorber and/or condenser giving the same cooling capacity.

But for the WFC-10 even the flow rates in the solution circuit (established by the operating conditions for the thermosysphon generator, tGi and TC=f(tACi)) are better adjusted to the load condition in the second case. This leads to lower losses in the solution heat exchanger and/or decreases the overflow of refrigerant. Thus the COP of WFC-5 and WFC-10 chillers can by considerably increased, by a simultaneous control of hot and cooling water temperature. In addition the simultaneous control is especially advantageous for the start-up period of SAC-systems in the morning. When the collector outlet temperature is not high enough to cover the full load, low cooling water temperatures can be used without a high electricity demand for the cooling tower. This is illustrated in fig. 9 where the set value for the chilled water outlet temperature is 14°C. According to the momentary load condition (QE) and the available solar driving temperature (tGi §�tKh in Fig. 9 and 6) the cooling water temperature tACi is controlled in a way to find the maximum value, which is enough to cover the load.

Figure 9 – Solar operation of a WFC-10 in Berlin (BPA) with variable hot and cooling water temperatures. During the start-up period, where the driving temperature is relatively low (tGi § 82°C) but the load is high

(QE) the cooling water temperature is decreased to approximately 24°C. Thereby a temperature thrust (§�tGACi) of nearly 60 K is adjusted, which leads to a high desorber capacity of the thermosyphon generator. Consequently the cooling capacity is high, since no overflow occurs under these conditions (relatively high chilled water temperature). During the day the solar driving temperature is increased up to 88°C and the cooling capacity is

Experimental and theoretical investigation of solar driven absorption chillers

waving around 30 kW. Now the cooling water temperature is increased, taking the varying load conditions and the momentary hot water temperature into account. Simultaneously the chilled water temperature is kept constant at the set value of 14°C. Despite of relatively high cooling water temperatures (up to 31°C) the COP is normally above 0.7. In addition the energy and water demand for the cooling tower is reduced. Thus the efficiency of the whole SAC-system is increased.

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Solar driven absorption chillers represent an interesting technology to reduce the need of electricity for summer cooling and therefore to reduce greenhouse gases emissions. Among these systems, thermosyphon absorption refrigerators are especially designed for solar plants, but in some cases their performances is not as satisfactory as expected.

Two similar solar driven absorption plants, located in Perugia (Italy) and Berlin, have been studied by the University of Perugia, Department of Industrial Engineering, and the Technische Universität Berlin, Institut für Energietechnik, within a research programme funded by the Programme Vigoni for cooperation between Italian and German universities. The two plants have Water-Lithium Bromide thermosyphon absorption refrigerators of the same manufacturer, but of different cooling capacity; the comparison of the results is possible thanks to comparable and shared measurement chains. In both plants measurement facilities allow to record in real time all the main operating parameters of internal and external circuits.

The experimental and theoretical analysis focused on part-load conditions of the machine; the results show that solution flow rate and desorber capacity of a thermosyphon generator can be controlled by hot and cooling water temperature, thus suggesting a control strategy aimed at optimizing the performance of the machine itself. 120(1&/$785(��

�������5()(5(1&(6� Albers J., Ziegler F., Asdrubali F., ,QYHVWLJDWLRQ� LQWR� WKH� LQIOXHQFH� RI� WKH� FRROLQJ� ZDWHU� WHPSHUDWXUH� RQ� WKH�RSHUDWLQJ�FRQGLWLRQV�RI�WKH�WKHUPRV\SKRQ�JHQHUDWRUV; Proc. of International Sorption Heat Pump Conference, Denver, USA, 20-22 June 2005.

Asdrubali F., Grignaffini S.,�([SHULPHQWDO�HYDOXDWLRQ�RI�WKH�SHUIRUPDQFHV�RI�D�+ z 2�/L%U�DEVRUSWLRQ�UHIULJHUDWRU�XQGHU�GLIIHUHQW�VHUYLFH�FRQGLWLRQV��International Journal of Refrigeration, 2005, 28 (4), pp. 489-497.

Asdrubali F., $Q� H[SHULPHQWDO� SODQW� WR� HYDOXDWH� WKH� SHUIRUPDQFHV� RI� DQ� DEVRUSWLRQ� UHIULJHUDWRU, Proc. of International Congress on Refrigeration, Washington, D.C, 17-22 August 2003.

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A Area, m² H Solar irradiation, W/m² M Mass flow rate, kg/s Q Heat flow, kW t External temperature, °C T Internal temperature, °C w Velocity, m/s

2WKHU�VXEVFULSWV��A Absorber a Poor solution av average C Condenser dif diffuse solar irradiation dir direct solar irradiation

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