M. Karagiorgas, M. Tsagouris, K. Galatis BONAIR Meletitiki, Greece. A. Botzios-Valaskakis CRES, Greece ABSTRACT

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1 491 Solar assisted heat pump in dual mode: direct and indirect space. Heating by the air collectors. Simulation results and evaluation with measurement results M. Karagiorgas, M. Tsagouris, K. Galatis BONAIR Meletitiki, Greece A. Botzios-Valaskakis CRES, Greece A ABSTRACT The heating system of the bioclimatic building of CRES comprises the entire heating plant including a solar assisted heat pump, the Solar Air Collectors (SAC as well as the heat distribution system (comprising a fan coil unit network. The solar air collector configuration as well as the fraction of the building-heating load covered by the heating plant are assessed in the article, for two modes of operation, the direct mode (hot air from the collectors is supplied directly to the heated space and the indirect mode (warm air from the SAC or its mixture with ambient air is not supplied directly to the heated space but into the evaporator of the heat pump. The technique of the indirect mode of heating aims at maximizing the efficiency of the SAC, at saving electrical power consumed by the compressor of the heat pump and, therefore, at optimising the coefficient of performance (COP of the heat pump due to the increased intake of thermal energy from the environment by means of the SAC. 1. INTRODUCTION In the bioclimatic building of the Centre of Renewable Sources of Energy (CRES, which is situated at Pikermi, Attiki, solar air collectors, whose surface is 25 m 2, supply hot air to the evaporator of an air to water heat pump (its standard thermal capacity rises up to 16,7 kw, pict.1. There is possibility of double operation of the air collectors: in the hybrid-active operation, the heated air is yield to an air to water heat pump and helps to its evaporator, in order to contribute to a higher heat pumping and, hence, increase the efficiency of the solar heat. This happens because the heated air is unable to cover directly the thermal losses of the spaces. in the bioclimatic-passive operation the heated air is supplied directly to the space, in order to cover the thermal losses, when the heated air is in the position to cover directly the thermal losses of the spaces. Picture 1: Solar air collectors, 25m2, placed south façade on the bioclimatic building of CRES coupled to a 16.7 kw air sourced heat pump. In order to apply the operation of the energy system in both modes as analyzed above, we have placed air dampers which control the air flow. For example, when the internal dampers D 1 are closed, the external dampers D 2 are opened and the system operates in indirect heating mode (hybrid operation, fig.1 while in different damper position (for both D1 and D2, the system operates in direct heating mode (passive operation, fig. 2. Figure 1. Hybrid operation_indirect heating of the bioclimatic building space. PALENC Vol 1.indd 491 3/9/2007 1:25:30 µµ

2 492 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Figure 2. Passive operation_ direct heating of the bioclimatic building space. collectors installed on the south vertical wall of the building. They are placed in two groups, their total surface is 25m 2 and their design flow rate is 2 x 1575 m 3 / h. The air is supplied, by two centrifugal fans, one by group, in the evaporator of the solar assisted heat pump, air to water, where it offers its heat to the evaporation phase of the thermodynamic (cooling cycle. During spring and the autumn, the solar system operates with direct heating mode in order to heat the area. During the direct heating mode we observe thermosiphonic air flow and its rate inside the collector fluctuates from 10 up to 900 m 3 /h. The heat exchange surface efficiency F R of the air collector is given by the equation: 2. THE HEATING SYSTEM COMPONENTS 2.1 The solar assisted heat pump Generally, the most important characteristic of a conventional air to water heat pump is that it pumps heat from the ambient air. The disadvantage of the air, as heat source, is its low temperature during winter, which decreases the coefficient of performance of the heat pump (COP while, simultaneously, the space heating needs increase. Also, another significant disadvantage of the air is its humidity that freezes in the fins of the evaporator coil, so that the air circulation speed decreases increasing the thermal resistance of the evaporator coil due to the ice built. These two disadvantages of the air, as heat source, become not significant for a solar assisted heat pump, air to water. For instance, in the bioclimatic building, due to its circulation behind the black paint absorber of the flat plate air collector, connected in row series, the fresh air is heated up to a significant temperature (i.e. from 5ºC up to 12ºC and then supplied to the heat pump evaporator (fig. 2. Finally, the heat pump evaporates at a higher evaporation temperature due to the increased ambient heat and supplies more efficient heat to the building by heating up the secondary water loop of the fan coil unit (FCU network. The COPh/p of the heat pump is given by the equation: COP = (T/Q el (T (1 (T = Useful thermal power of the heat pump Q el (T = Absorbed electriower of the heat pump T = the evaporator air inlet temperature. (i.e., 0 C 2.2 The air collectors During winter the solar system operates with indirect heating mode. The air circulates through the solar air (2 m: the air flow rate : specific heat of air flow A c : the aperture area of the collector U L : the heat losses factor of the collector F : the flow factor of the collector The collector s instant energy efficiency η is given by the equation: (3 : the useful collected heat (W F R : the heat exchange surface efficiency of collector I β : the intensity of solar radiation on the collector s surface (W/m 2 τ: the transmittance of the transparent cover of the collector α: the absorption efficiency of the collector absorber T i: the air entry temperature of collector ( C T α : the ambient air temperature ( C In the indirect heating mode of the air collectors, the air will be heated up to a final temperature,out able to produce useful collected heat for the heat pump s evaporator needs: = ρ V a, out - (4 I ρ : the air density : the air specific heat V a : the air volume flow rate = T i In the direct heating mode of the air collectors, the air temperature commence at T set and rises up to a final temperature,out, able to produce useful heat for the space needs: PALENC Vol 1.indd 492 3/9/2007 1:25:30 µµ

3 493 (T set = ρ V set, out - T set (4 D V set = the air thermosiphonic volume flow rate T set = the indoor setting temperature ( C 2.3 The building load For simlification reasons, a linear simpified model is taken into consideration : = G V (T set - + Q k ( 5 = losses rate of building (W G = simplifying coefficient of volumetric losses of the building (W/m 3 K V = the heated volume of building (m 3 = the ambient temperature ( C Q k = internal thermal gains in the building (W 2.4 Combine both heat pump s and collector s useful heat To cover the building s heating load, we combine both the operation of the solar air collector as well as the one of the heat pump and we make addition of the two relevant useful heat amounts. For the indirect mode of operation: =,out (6 I Fr the direct mode of operation: = + (T set (6 D 3. EFFICIENCY CRITERIA ESTABLISHED WITHIN THE SIMULATION MODEL TSAGAIR The system of equations (1 to (6, called as well Tsagair model, comprises two basic meteorological variables T α and I β. It calculates a number of parameters, efficiency related, such as the (Τ α or,out, the Q el (Τ α or Q el (Τ α,out, the and the (T α, Q κ or, which can give us the possibility to calculate efficiency criteria, for instance the three suggested below: ((βαβταittuf 1. COP = /Q el, the coefficient of performance of the heat pump during the conventional heating of the building (the heat pump operates single. 2. COP I tot = /Q el, the coefficient of performance of the whole system (heat pump + SAC during the indirect heating of the building. 3. COP D tot = ( + /Q el, the coefficient of performance of the whole system (heat pump + SAC during the direct heating of the building The couple of meteorological parameters where collectors produce useful heat can indicate the operation mode in which the system should be placed (indirect or direct heating. For the same couple, I β and for the whole building heating system, we may calculate the two respective coefficient of performance, respectively COP I tot and COPD tot. 4. SIMULATION RESULTS ACCORDING TO THE MODEL TSAGAIR Using the simulation model (1, (2, (3, (4, (5 and (6, we made the energy s most Figure 4. Characteristic daily operation loop and transition curve from indirect to direct heating operation of the air collectors: the curve has COP I tot = COP D tot optimal use of the solar air collectors in the building of CRES for each hour of the typical winter day, the latest placed on 15 th January. A number of assumptions are taken into consideration: 1. the radiation for each hour of this day was calculated on the basis of the Hottel model. 2. the ambient temperatures for each hour were calculated on the basis of the Kouremenos Antonopoulos model. Following these, in the figure 4 are presented: 1. the transition curve from indirect to direct operation mode as the characteristic solution of the equations for the external constrain: COP D tot = COPI tot 2. the lower limit curve for the space, under which < 3. the upper limit curve for the space, under which > 4. the daily characteristic loop for the 15 th January which consists of 12 points (these are the hours when we can have positive solar radiation I β. 5. EVALUATION OF SIMULATION RESULTS USING REMENT RESULTS 5.1 Measurements on the solar assisted heat pump, air sourced For the passive operation, the daily values of the heat pump s useful heat, the absorbed electriower Q elh /P and the average daily coefficient of performance of the heat pump COP D are given in table 1. Table 1. Quantitative results for the heat pump, when the collectors are in passive operation (the operates with ambient air (4/2/03 (5/3/03 Τ a (Wh/d Q el /d COP D For the hybrid operation, the daily value of the heat pump s useful heat, the absorbed power Q el and PALENC Vol 1.indd 493 3/9/2007 1:25:30 µµ

4 494 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and the average daily coefficient of performance of the heat pump COP I are given in table 2. Table 2. Quantitative results for the heat pump, when the collectors are in hybrid operation (the operates with solar air (21/2/03 (11/3/03 Τa (Wh/d Q el /d COP I Comparison between the simulation model calculations and the on site measurements Comparison on daily result basis For the passive operation, the daily value s of the heat pump s useful heat, the absorbed electriower Q elh /P as well as the positive variation between calculations and measurements Δ CΟΡ are given in table 3. Table 3. Variations between measurements and calculations for the heat pump, when the collectors are in passive operation (the operates with ambient air (4/2/03 (5/3/03 (Wh Q el COP D Δ COP (% , ,93 3,68 7,5% ,0 7857,97 2,95 11,7% For the hybrid operation, the daily value of the heat pump s useful heat, the absorbed power Q el, the average daily coefficient of performance of the heat pump COP I as well as the positive variation between calculations and measurements Δ CΟΡ are given in table 4. Table 4. Variations between measurements and calculations for the heat pump, when the collectors are in hybrid operation (the operates with solar air (21/2/03 (Wh Q el COP I , , Δ CΟΡ 16,8% (11/3/ ,0 5029,0 5,32 7,7% We observe that the simplified simulation model Tsagair for the assessment of the solar assisted heat pump in the building of CRES presented an acceptable level of variation between the calculations and the measurement results. This variation can reach up to 16, 8% Comparison on seasonal result basis To achieve model evaluation based on comparison of measurements and calculations with criterion the completion of various integrated sums of energy (within the heating period is not reliable, since total seasonal measurement results have never been made in the building. Nevertheless, with scope to experiment the SAC in various operation modes, we have made measurements, at constant parameters, for short time intervals covering for instance two weeks. Moreover, it has become feasible to compare the results of simulation using the program TRANSYS and the ones of the simplified model. The comparison relates to the direct heating (only and for the heating period. The relevant comparison results of calculations as well as the resulting variatin from the TRANSYS are shown in the table 5. Table 5. Calculations of parameters and variations between two models of simulation for the entire heating period (direct heating in the bioclimatic building of CRES on 15th January 2003 METER FOR COMPARISON Seasonal solar radiation Seasonal production of solar air Total load of two offices (with thermal Calculations with TRANSYS 15 Calculations with TSAGAIR Variation ,81% kwh/m 2 kwh/m ,90% kwh/m 2 kwh/m kwh 1935 kwh + 3,04% Solar Fraction 86% 94,4% + 9,58% 6. CONCLUSIONS The technique of the indirect mode of heating has optimised (maximized the efficiency of the Solar Air Collectors (SAC and yield to electrical power savings consumed at the compressor level of the heat pump since we have optimised the coefficient of performance PALENC Vol 1.indd 494 3/9/2007 1:25:30 µµ

5 495 (COP of the heat pump due to the increased intake of thermal energy from the preheated air inside the solar air collectors (see table 2, COP D =4.94 against COPI H/ =3.33 in the table 1 for two days with about same ambient temperatures. P There exist a transition curve which relates the ambient temperature and the solar radiation on the collector s surface. If the solar radiation on the collector s surface is higher than this transition value, then a direct heating mode is reliable and more efficient. Against code TRANSYS 15 and when calculating seasonal amounts of useful energy, the model Tsagair presents variations up to about +11% To conclude, the use of TSAGAIR model is considered reliable. ACKNOWLEDGEMENTS The measurements that were held in the bioclimatic building of CRES and the calculations on the system of equations were object of the research programme PAVE 063 of the Sole SA company with scientific responsible Dr M. Karagiorgas and title Design and Development of Air solar collectors for building applications» REFERENCES ASHRAE, Handbook of Fundamentals, American Society of Heating, Refrigeration and Air-conditioning Engineers. Balaras, C.A., Kallos G., Stathi A., Kritikou S., 1989, On the Relationship of Beam Transmittance on Clearness Index for Athens, Greece, Int J. Solar Energy, Vol. 7, p.171. Bololia M., 2002, Evaluation of Solar Collectors of Air in the Bioclimatic Building, Diploma Thesis, Acetem-Selete. Drosou V., Most optimal Planning of Flat Solar Collector of Air for Use in Dryers of Agricultural Products, Thesis work, Athens TEI, 1994 Gueymard C., 1998, An Isotropic Solar Irradiance Model for Tilted Surfaces and its Comparison with Selected Engineering Algorithms, Solar Energy Vol. 40, pp.175. IΕΑ. Report on Solar Air Systems-A Design Handbook Karagiorgas Μ.,1992, Heat pumps: Seasonal factor of behaviour and its relation with the economic aspects, 4 National Congress on the Renewable Energy Sources, Proceeding Vol. B, Xanthi. Kotsianas-D.Huntas, Collectors of Solar Energy, Theory - Manufacturing of water heating systems, F. 1989, 2 nd Edition. Kouremenos D. K. Antonopoulos, Temperature characteristics of 35 Greek towns, EMP, SOLE Α. Ε, Specifications of solar air collectors AIRSOL-300, 2003 PALENC Vol 1.indd 495 3/9/2007 1:25:31 µµ