Category Archives: EuroSun2008-7

Radiant ceiling plant control strategy

The created control strategy permits acting directly on the flow rate by means of an ON/OFF signal, which operates directly on the pumps in order to avoid overheating conditions in winter and excessive cooling in summer. For the internal air temperature control a band of variability of 19^21°C was assumed in winter and of 25^27°C during summer.

The adopted control strategy also intervenes on the supply temperature of the radiant ceilings. It is in fact necessary to supply such terminals in a way that the surface temperatures are not too high in winter, in order not to generate conditions of thermal discomfort, nor too low in summer in order to avoid water vapour surface condensation phenomena [9].

During winter the control system determines the required inlet temperature as the sum of a first stationary term corresponding to the regime and of a second variable term [6,10]. The inlet temperature of the radiant ceiling (tiC) in stationary conditions it is the function of the external air temperature (tOA), seen as the sole constraint on the system during winter. This bond has good linear approximisation with the incline of the line which unequivocally depends upon the dispersive properties of the building shell elements [1,6]. The stationary term was determined with a prelimiary simulation campaign conducted in the absence of solar radiation and the relation obtained is the following:

tIC (steady) = -0.4726 • tOA + 29.452 (1)

Exactly because the effect of solar radiation is not taken into consideration, it is necessary to join the ON/OFF control on the feed pump to interrupt the flow rate in order to avoid overheating of the environments. The value calculated with the Equation (1) permits the regulation, in an efficient manner, of the internal air temperature when this is around 20°C, but if it moved away from such a value, for example upon morning starting of the plant, such a temperature is not sufficient in that it requires some time to work properly. In such a case, to accelerate the response, the inlet temperature value determined by the Equation (1) is correct with a second transitory term which is made to depend upon the internal air temperature (tIA) and external air [6,10]. The relations obtained are the following:

tCC (transient) = K1 • (20 — tIA) (2)

with Ki linear function of the external air temperature:

к = 0.0473 • tOA +1.019 (3)

The function of the K1 parameter is that to produce an increase in the inlet temperature of the plant decreasing with external air temperature. The transitory term gradually decreases as the internal air temperature increases, until it cancels itself for tIA=20°C. The inlet temperature of the radiant ceiling in winter is determined by the relation:

tIC =(- 0.4726- tOA + 29.452)+ [(0.0473- tOA +1.019)0 — tIA)] (4)

In summer, regulation of the inlet temperature is obtained by means of a control system which determines, hour by hour, the air dew-point temperature (tdp) within the environment. The inlet temperature is then set equal to the dew-point temperature, since the thermal resistances between the water circulating in the pipes and the ceiling surface ensures a ceiling surface temperature that is always greater than that of the dew-point [11]. A defect of this control strategy consists of the necessity of maintaining the value of relative humidity within the environment at acceptable levels. In fact, too high relative humidity leads to high dew-point temperatures, and therefore to surface ceiling temperatures that are insufficient for the complete removal of sensible loads. Preliminary simulations have demonstrated that the external air exchange, set at equal to 1.2 Volh-1, is sufficient in order to maintain, during the plant functioning times, relative humidity between 40% and 60%. Furthermore, the thermic resistance between the fluid circulating in the pipes and the ceiling surface leads to a AT« 4°C, therefore in the most critical moments this safety margin is exploited to further lower the inlet temperature and to guarantee the complete removal of sensible loads. The relation obtained for the inlet temperature of the radiant ceiling which ensures the variability of the internal air temperature between 25°C and 27 °C during summer is the following:

tIC(summer) = tdp — 2 • (tIA — 25) (5)

Fourth step : quantitative design of the system and energy performance estimation

This step is aimed at defining the technical features of the solar cooling systems thanks to the information of the project. To reach this goal, several tools are to be assembled together for a coherent and optimal process of sizing : a load generator permitting to create load files from a basic description of the targeted building and a simplified sizing tool giving the simulated performances of a proposed system in accordance with the building conditions and the climate. The load generator developed by Fraunhofer ISE in the framework of the PolySMART© [3] E. U project could be used for example thanks to some specific developments. Beside, an internal tool from TECSOL or the SACE tool [4] could permit to make a quantitative design of the solar cooling systems : definition of the main components, energy balance, primary energy savings, presentation of the energy results in a synthetic sheet.

Mathematical description of Type 307

The principle way of calculation for Type 307 is the same as for Type 107. It is based, on the one hand side, on ordinary calculation equations and, on the other hand, on external performance data that supply the fraction of nominal cooling capacity (fNC) and the fraction of design energy input (fDEI) for the chiller at current operating conditions.

Подпись: QE Подпись: QE, rated Подпись: NC Подпись: (1a) image423 Подпись: (1b)

The cooling capacity (QE) and the heating capacity (QG) are calculated by equation (1a) and (1b).

image425 Подпись: (2)

The rated capacity (QE, rated = 15 kW) and the rated thermal COP (COPrated = 0,71) are fixed parameters in the parameter list of the type for the EAW chiller. The heat (QAC), which has to be removed from absorber and condenser to the environment, is calculated as the sum of QE and QG and the thermal COP as their ratio. Finally, the outlet temperatures of each circuit are calculated analogue to equation (2):

In contrast to Type 107 the coefficients fNC and fDEi in equation (1a) and (1b) for Type 307 depend on the hot and cooling water inlet temperatures as well as the chilled water inlet temperature (tEi), instead of the outlet set point temperature tEo, set. Since additionally the load dependency is set to a constant value of one, this disables the implemented control of Type 107. Finally the auxiliary power (Qaux) is excluded from the COP as well as from the QAC calculation, since the parasitic energies (solution pumps, fluid stream pumps, controls, etc.) are normally electrical capacities which should not be mixed with thermal capacities or their heat ratio.

IEA Annex 34: Thermally Driven Heat Pumps (TDHP). for Heating and Cooling

Juan Rodriguez Santiago1*, Peter Schossig2, Patrizia Melograno1, Wolfram Sparber1

institute of Renewable Energy, European Academy of Bolzano, Viale Druso 1, 39100 Bolzano, Italy
2Fraunhofer Institute for Solar Energy, Freiburg, Germany
*Corresponding Author, jrodriguez@eurac. edu

Abstract

In this article is going to be presented the IEA HPP project Annex34. This three year project has been initiated in October 2007 under the umbrella of the IEA Heat Pump Programme as a continuation of Annex 24 ’’Absorption Machines for Heating and Cooling in Future Energy Systems”, with the goal of reducing the environmental impact of heating and cooling by the use of thermally driven heat pumps (TDHP) [1].

The project is led by Fraunhofer Institute for Solar Energy (ISE) and divided in five different subtasks to work, as the main objective, on the identification and quantification of the economic, environmental and energy performance of integrated TDHP’s in cooling and heating systems for a range of climates, countries and applications.

Keywords: Heat pumps, sorption, energy savings, efficiency

1. Introduction

In the last years, and due to the increase of the oil price, there has been noticed a new interest in technologies, whose development had nearly been stopped not so long ago. Machines mainly based on sorption processes, usually working as water chillers, are being rediscovered and taking notoriety in a scenario that nowadays tries to save the maximum primary energy amount. Because of this, the interest in joining solar and geothermal energy, waste process heats — energy flows that could be defined as “free” — and this kind of technology that can be driven by these flows is growing.

The ability of these technologies to produce both cold and hot water within certain temperature levels is a very interesting solution, but no widely spread, that solves in some of the studied cases, the problem of having two different technologies seasonally used in the same facility (boiler/chiller). In this way, a considerable amount of the investment costs are going to be saved, that is nowadays one of the main important barriers found. Therefore the new concept may be more competitive compared with classically installed solutions.

The best example for those cases is the domestic market of heating and cooling, where an important number of new companies are appearing in the last years providing solutions with capacities under 20kW of chilling power and nearly 30kW of low temperature heating power.

Analysis of the air temperature inside the classroom

The mean absolute error on the air temperature into the classroom is 11.8%. This value is quite high, because our building model is fairly simple, but it’s quite acceptable in the case of a pre­sizing study conducted by a technical research department.

5. Conclusion and prospects

Although using simplified models, this first study provides coherent results and makes possible to evaluate the comfort conditions into the building with an accuracy of 12%, being able to precede a usable tool by technical research departments for feasibility or pre-sizing study. With the sights of the errors on certain components, it appears that we can improve the overall accuracy of modelling by integrating more accurate models for the field of solar collectors, absorption chiller and the building. The absorption chiller and solar collector models are in progress. Moreover stratified tanks models (hot and cold) have been completed and will be integrated into the general model. Regarding the building, we chose, at first time, a very simplified model because the coupling between the software Energyplus (modelling of the building) and the simulation tool Spark is under development. If this coupling could not be done, a model closer to reality must be considered under Spark, particularly to take into account the inertia and the exposure of the building but also the ventilation.

Nomenclature

m

Mass flow [kg. s]

in

Inlet

Cp

Calorific capacity [J. kg_1.K_1]

out

Outlet

T

Temperature [°C]

captor

Solar collector

M

Mass [kg]

gen

Generator

Ф

Heat flows [W]

cond

Condenser

S

Area [m2]

evap

Evaporator

U

Internal energy [J]

outside

Outside

n

Efficiency [-]

hot tank

Hot tank

cold_tank

Cold tank

Index

air

Air into the classroom

wa ter

Water into the solar collector

simul

Simulation

Water into the tank

mes

Measure

Air into the building

References

[1] Lucas F., (2006). Presentation du projet ORASOL : Optimisation de precedes de rafraichissement solaire.

[2] Castaing-Lasvignottes J. (2001). Aspects thermodynamiques et technico-economiques des systemes a absorption liquide.

[3] Tittelein P., (2008). These : De l’interet d’utiliser un environnement de simulation performant, SimSpark, dans l’etude du comportement des batiments a basse consommation d’energie, Universite de Savoie, Polytech’ Savoie, Le Bourget-du-Lac.

[4] Schuco, (2007). Manufacturer documentation about solar collector SchucoSol U5 DG.

[5] Schuco, (2007). Manufacturer documentation about absorption chiller LB 30.

Absorption chiller

AAWC building is a newly reconstructed five-storey building with approximately 3300m2 total area. Space heating is based on fan-coils system and 300kWgas boiler. The estimated cooling load of the building is about 150kW. In summer time the ambient temperature in Yerevan often reaches 400C. For cooling needs single effect hot water lithium bromide absorption chiller YORK Model YIA-HW-1A1-50-C-S-C has been installed in the basement of the building near the gas boiler. The chiller is shown on Figure 3.

The absorption chiller has nominal cooling capacity of 420kW and is connected to the VXT 70 cooling tower with 400kW capacity placed on the roof top of the building 25m above the boiler house. The cooling tower can be seen on the roof of the building on the right side of the PV array (Fig.1).

In cooling season the gas boiler provides hot water to the absorption chiller installed inside the same boiler house. Chilled water enters the fan-coils system of the building. The gas boiler provides hot water for chiller’s generator loop at 90°C/70°C supply/return temperature. For similar applications the hot water could be supplied by appropriate solar water heating system with gas boiler back-up. The water temperature in absorber/condenser-cooling tower loop is 290C/ 330C, and chilled water supply/return temperature is 80C/120C.

The power need of the chiller is about 24kW including chiller’s solution, refrigerant, purge pumps and cooling tower fan and water pump. A conventional compression chiller with

Подпись: Figure 3 Lithium bromide absorption chiller at the AAWC
similar cooling capacity would require almost 10 times more electric power and, consequently, too large size cabling from building to transformer sub-station. .

Absorption chiller is an attractive energy efficiency solution, particularly when gas or solar heat source is available.

The absorption chiller has been tested with building’s fan-coils system and commissioned in June2008. The building is still under renovation and at the moment there is no need for absorption chiller which is kept in standby mode.

2. Conclusion

The 10kW grid connected BIPV station and 420kW lithium bromide absorption chiller are installed, tested, and ready for operation and will be monitored during operation.

Their performance monitoring data, particularly related to the power consumption and hot water supplied to absorption chiller, can be used when considering combinations of solar PV and/or water heating system with similar absorption chiller. The performance of the liquid absorption chiller can be compared in the same climatic conditions with the performance of the solar driven desiccant cooling system operating in Yerevan since February 2002 [2].

The combination of the grid connected PV power station and lithium bromide absorption chiller on the same place provides also the opportunity for determination of produced PV power’s

contribution to chiller’s power consumption in different cooling conditions. The results of the study can be useful for countries with similar climate conditions.

References

[1] http://www. renewableenergyarmenia. am

[2] EU INCO-COPERNICUS Programme. Design and Installation of Solar Driven Desiccant Cooling Demonstration System, Contract #ICOP-DEMO-4034/98. INETI (PT), AUA (AM), FhG-ISE (D), CONTACT-A (AM), INTERSOLARCENTER (RU), Final Report, 2002

Experimental results and discussion

Two different kinds of experiments have been performed with regard to the regeneration conditions. In the first series of experiments constant regeneration humidity has been used. This allows the comparison of the FAM wheel data with other desiccant wheels. However, it prevents predictions for the wheel performance in a desiccant-evaporative system where ambient air is heated up for regeneration purposes. Obviously, depending on the ambient air conditions, the regeneration air conditions will vary, too. Therefore a second series of experiments has been conducted with the
regeneration humidity matching the ambient (supply) humidity. Constant parameter in both series of experiments included desiccant wheel speed of 20 1/hr, supply/regeneration air flow of 400 m3/hr, a counter flow arrangement and a resulting air velocity at the wheel face of 3.25 m/s. The wheel face area has been split equally between supply and regeneration stream.

Cold storage configuration

The basic scheme in this case is shown in the following figure:

image254

Fig.4. Configuration with cold storage

In this case, to have cold storage allows having an intermediate case between the two previous, so that they can be stored excess production and face bigger loads of instantaneous production of the chiller.

The cold storage has been determined to have an autonomy of one hour. And so that, from the flow of the evaporator (that coincides with the one of cold load) it’s determined the volume necessary.

3. Execution of the simulations

As it was mentioned before, for each one of those three schemes they have been tested different control strategies: three for the solar camp (C1, C2 y C3) and another three for the chiller (C4, C5 У C6).

With regard to the solar part:

C1: Constant flow control on the solar part and constant flow on the absorption.

In the case of the solar pump, it has been used the output temperature of the collectors as variable to control, as well as the reference variable is the one at the bottom of the tank. It was used hysteresis, and the values adopted were: upper for the start 6 °C, and for the stop 1 °C. To start up the absorption the storage temperature and the room temperature of the zone to be conditioned are taken into account.

C2: Control with variable flow on the solar part starting by differential temperature and constant flow on the absorption.

The system is similar to the one on the previous case, being the start up in function of a differential temperature but using flow regulation for the solar pump. In this way can be compared the improvement using constant flow on the solar flow.

C3: Control with variable flow on the solar part starting by critical radiation and constant flow on the absorption

In this case the start up of the solar pump is made in function of the critical radiation and the flow is regulated to maximize the output temperature of the collectors.

Regarding to the absorption chiller: For regulation on the following schemes, it was chosen for the solar part the C3 strategy and some changes have been applied to the control of the chiller in order to compare their effect using a constant system for the solar control. For the control of the chiller, once there were the necessary conditions for its start up, was established the control of the corresponding variable, in order to obtain a power produced by the evaporator, equal to the one demanded by the system. Thus facing the real demand in the system taking into account the higher delay produced by the effect of the exterior conditions on the load.

C4: Control with variable flow on the solar part starting by critical radiation and control by temperature on the generator for the absorption

In this case there is a three way valve on the suction of the generator pump, that allows controlling the temperature on the input, and with it the output power for the evaporator.

C5: Control with variable flow on the solar part starting by critical radiation and control with variable flow on the generator using a diversion valve.

Other possibility to regulate the power of the generator is the variation of the flow on the chiller. In this case all the water of the solar part is driven, and a three-way valve is used to let pass more or less water through the generator in function of the demand.

C6: Control with variable flow on the solar part starting by critical radiation and control with variable flow on the generator by means of a pump.

In this case, the flow regulation on the generator has been performed by means of pump with variable flow.

4. Results

Three tables are shown next with the comparative of the different data obtained:

As can be seen, two of the configurations are better than the rest: on the configuration of the solar part the one with cold storage, and with regard to the performance of the performance of the absorption chiller, the one with infinite storage. The fact of not having to fulfill the condition of demand of the load, allows a better use of the available energy. On the other hand, the cold storage allows having a bigger impact over the temperature of the solar installation, what increase the solar collection, but in return, penalizes the chiller COP.

Table 1. Results for the simulations with direct use

C1

C2

C3

C4

C5

C6

Energy produced on the collectors [kWh]

7.400

8.650

8.640

8.590

8.650

9.100

Performance of the collectors [p. u.]

0,35

0,32

0,41

0,40

0,41

0,43

Energy given to the generator [kWh]

5.770

6.710

6.670

6.700

6.650

7.660

Energy produced on the evaporator [kWh]

2.575

3.050

3.040

3.180

3.030

3.880

Energy given to the load [kWh]

2.575

3.050

3.040

3.180

3.030

3.880

Medium COP of the chiller [p. u.]

0,45

0,45

0,46

0,47

0,46

0,51

Load Fraction [p. u.]

0,50

0,59

0,59

0,61

0,58

0,75

Solar COP [p. u.]

0,16

0,14

0,19

0,19

0,19

0,22

Table 2. Results for the simulations with total use

C1

C2

C3

C4

C5

C6

Energy produced on the collectors [kWh]

9.100

8.650

8.800

8.800

8.740

8.740

Performance of the collectors [p. u.]

0,43

0,41

0,41

0,41

0,41

0,41

Energy given to the generator [kWh]

8.100

7.850

7.950

7.650

7,650

7.950

Energy produced on the evaporator [kWh]

3.550

4.600

4.600

4.450

4.520

4.770

Energy given to the load [kWh]

3.550

4.600

4.600

4.450

4.520

4.770

Medium COP of the chiller [p. u.]

0,44

0,59

0,58

0,58

0,59

0,60

Load Fraction [p. u.]

0,69

0,89

0,89

0,86

0,87

0,92

Solar COP [p. u.]

0,19

0,24

0,24

0,24

0,24

0,25

Table 3. Results for the simulations with finite storage

C1

C2

C3

C4

C5

C6

Energy produced on the collectors [kWh]

8.850

8.850

9.050

9.800

9.000

9.500

Performance of the collectors [p. u.]

0,42

0,42

0,43

0,46

0,42

0,45

Energy given to the generator [kWh]

7.210

7.300

7.350

8.270

7.280

8.250

5.

Подпись: 3.150 3.250 3.250 3.270 3.250 3.710 3.015 3.150 3.160 3.180 3.150 3.590 0,44 0,45 0,44 0,40 0,45 0,45 0,59 0,61 0,61 0,61 0,61 0,69 0,18 0,19 0,19 0,18 0,19 0,20

Conclusions

Different control actions on the solar part have repercussion on the chiller performance and vice versa, different control systems for the chiller, change the working conditions of the solar installation.

From the previous results, we can deduce that the configuration with total use is the one that better performance shows, as it has more hours of use than the rest of configurations.

As well, it can be seen how the most adequate control for the solar installation is the one with variable flow, combined whit start based on critical radiation. The control for the chiller that better results offers is the one based on variable flow by means of the pump. The cold storage, reduces the performance of the installation, but from the point of view of design, allows the use of a smaller absorption chiller.

References

[1] National Renewable Energy Laboratory. User’s Manual for TMY2s (Typical Meteorological Years), NREL/SP-463-7668, and TMY2s, Typical Meteorological Years Derived from the 1961-1990 National Solar Radiation Data Base, June 1995, CDROM. Golden: NREL, 1995

[2] TRNSYS 16 Documentation.. A transient Simulation Program. Solar Energy Laboratory, University of Wisconsin, Madison, 2006.

[3] SACE: Solar Air Conditioning in Europe. Final Report, EU Project NNE5-2001-25, 2003

[4] ESESA (1996), Manual de instalacion: Grupos refrigerantes por absorcion de agua caliente WFC-10.

[5] M. Kovarik, F. Lesse, Optimal control of flow in low temperature solar heat collectors, Solar Energy 18 (1976) 431-435

[6] C. B. Winn, D. E. Hull III, Optimal controllers of the second kind, Solar Energy, 23 (1979) 529-234.

[7] P. Dorato, Optimal temperature control of solar energy systems, Solar Energy, 30 n 2 (1983) 147-153

[8] D. Zambrano, E. F. Camacho, Application of MPC with multiple objective of a solar refrigeration plant., proceeding the 2002 IEEE International Conference on Control Applications, 2002.

[9] M. A. Corchero, M. G. Ortega, R. R. Rubio. .Aplicacion del control robusto H® a una planta solar. XXV Jornadas de Automatica de Ciudad Real. 2004

[10] J. C. Blinn. (1979) Simulation of solar absorption air conditioning. Ms. D. Thesis University, of Wisconsin-Madison.

[11] R. Lazzarin, Steady and transient behaviour of LiBr absorption chillers of low capacity, Revue Internationale du Froid, 3, n 4, (1980), 213-218.

[12] J. A. Duffie, W. A. Beckman, (1991). Solar engineering of thermal processes 2nd edition, John Wiley & Sons, New York.

Control Strategies for Solar Thermal Cooling System in Office Building in Almeria, Spain

J. Bote Garcia, A. Gal, D. Tavan*

Energy Efficiency & Comfort Group, R&D Division, Acciona Infraestructuras
Calle Valportillo II, 8, 28108 Alcobendas, Spain

* Corresponding Author, dtavan@acciona. es

Abstract

This article gives an insight on working solar-driven cooling systems that were integrate in an office building used by researchers at the Plataforma Solar de Almeria. Implemented systems combined both passive and active solar cooling techniques in order to minimize the energy consumption of HVAC systems while maintaining an adequate thermal comfort inside the building. The control strategy is briefly described whose goal is to maximize the use of renewable sources over conventional ones whenever they are available. This is the key to achieving a reduction by 80 to 90% of overall energy consumption with regard to typical energy needs for office buildings.

Keywords: Solar driven cooling, Absorption Heat-pump, Night cooling, Radiative cooling, Supervision control and data acquisition

1. Introduction

With the ARFRISOL (Bioclimatic Architecture and Solar Cooling) project, the Spanish Ministry of Innovation and Science is promoting energy efficiency in office buildings. As part of the project objectives, five office buildings are to be built or rehabilitated in different climatic zone of Spain in order to experiment various energy saving and renewable energy techniques and demonstrate that the energy consumption of office buildings can be cut by 80-90%, while the remaining conventional energy consumption can be supplied by active solar systems (e. g. solar thermal collectors for heating and cooling and photovoltaic panels for electricity). Acciona Infraestructuras is responsible for the construction of one of these high energy performance buildings situated in the Solar Platform of Almeria (PSA) in the south of Spain. The purpose of this paper is to explain the solar cooling techniques used in this building and the control strategies that have been designed to achieve high energy performance.

Technical specifications of Ao Sol’s solar chiller

The novel chiller unit is developed for the year-round thermal energy supply of residential buildings under Mediterranean conditions. Envisaged are detached houses with a heated area from 150 to 250 m2 or more. This building segment was chosen for the high relevance of air­conditioning in buildings with high living comfort.

The present development is based on previous work accomplished at the University of Lisbon, Instituto Superior Tecnico, by Prof. Mendes [5], who developed several water-cooled ammonia/water absorption chillers. The thermodynamic cycle of the present solar absorption chiller has been consequently re-calculated and fine-tuned for direct air-cooled operation. The absorption unit is designed to work using solar thermal energy from CPC collectors or waste heat from a cogeneration engine fuelled by biomass. For the use linked to solar collectors a back-up gas burner is foreseen. The device is designed to produce 8 kW of chilled water in a range of temperatures between 5 °C and 18 °C. The chilled water is normally used in buildings either for air-conditioning via fan coil elements or directly for space cooling via radiant ceilings. The chiller developed can be operated for both concepts. Moreover, using ammonia as the refrigerant, ice production or even deep refrigeration could be achieved via changes in the control strategy. The solar chiller is driven by hot water at temperatures at around 95 °C, stored in a solar buffer. The device directly dissipates the own waste heat produced at around 40 °C without any need for an external wet, hybrid or dry cooling tower.

The detailed technical distinctive features of the AO SOL solar chiller are summarized below:

The chiller is air-cooled. Overall sizes of the chiller and operation cost are minimized. No water is needed; this ensures market compatibility even in the extremely hot and dry continental regions of the peninsula’s interior. The absence of a cooling tower decreases considerably the overall dimensions compared to concurrent products.

Plate heat exchangers are used wherever possible in the thermodynamic cycle and take advantage of the high efficiency typical of this technology (up to more than 95 %!) coupled with very compact dimensions and low specific weight (kg/kW).

The remote monitoring system acting via mobile phone technology ensures complete safety. A timely alarm transmission with the possibility of intervening remotely on the chiller control adds to the on-site safety features and it minimizes the service needs.

2.2. Control strategy

The concept envisaged for the chiller is completely self-sufficient. Automatic procedures for start­up, load changes, shutdown, and safety issues have been implemented. The control acts on two 3­way valves, which stabilize the water temperatures in the hot and chilled water loops. Further, it triggers the adjustment of fan and solution pump speed, and the actuation of the refrigerant throttle valve. The chiller is controlled through the adjustment of temperatures in the hot and chilled water loops. The adjustment is activated by means of PID controllers. The machine reaches stable operation within 15 minutes. The automatic shutdown procedure is immediate; the machine reacts quickly and without swings. A new start without any manual intervention in between is secured.

The chiller is the core component of a harmonised chain of components representing a turn-key solution: an absorption cooling machine driven by hot water and producing chilled water for fan coil or radiant ceiling use, and Ao Sol’s unique solar CPC MAXI collector, which will deliver on the order of 25% more energy than a very good flat plate collector of today [6].

The ideally combined system will provide hot water all year around, heating in winter and cooling for half of the year. The control unit developed combines and actuates all components of the solar

heating, cooling and DHW system, from the solar collector, to solar buffer and backup, up to the chiller and the space to be conditioned. Figure 3 shows the flow scheme.

3. Experimental results

image455

The prototype has been taken successfully into operation. The novel machine showed a satisfactory behaviour in the start-up phase and was from the very beginning stable and predictable in its reactions. The generator capacity ranged roughly between 7 and 14 kW, whereas the evaporator has been regularly producing cooling between some 3 and a peak power of 7.8 kW. The COP, defined as the ratio between evaporator and generator power, showed a maximum of 0.53. The results of a test rig at nominal conditions are shown in Fig. 4 and 5. The unit was run with 96 °C hot water and produced chilled water at 12.5 °C. The cooling air left the machine at around 36 °C. Under these conditions more than 7 kW of chilled water can be already steadily produced with a COP above 0.5.

Fluctuations due to oscillation in air temperature can be clearly noticed in the evolvement of thermal capacities and relative COP. Nevertheless, these fluctuations did not show any trend and did not compromise the reliability of the operations.

Fig. 4. Experimental working conditions for hot water, cooling air and chilled water.

The experimental tests already gave an impression of the capacity of the prototype. So, it can be said that most components seem to have the right size or even to be oversized, as e. g. the air heat exchangers and the refiner. The air fan — run in part load — provided the necessary cooling for condenser and absorber, even if the laboratory room is very narrow and a certain air backflow could not be excluded

However the prototype has shown some limitations, which are now fully identified and can be easily corrected in the next prototype, to be built ant tested until the end of 2008. It is the expectation of the authors that the nominal values will be easily reached then, namely the design

COP of 0.6

image456

Fig. 5. Generator and chilled water capacity and reached COP.