Luca Tagliafico - Università di Genova - POMPE DI CALORE ELIO-ASSISTITE
LAST DEVELOPMENTS OF SOLAR ASSISTED HEAT PUMPS IN ITALYLuca A. Tagliafico, F. ValsuaniUniversity of GenoaDIME / TECDivision of Thermal engineering and EnvironmentalConditioningVia all’Opera Pia 15/a – 16145 Genoa ItalyTel. 010 3532880 – fax 010 311870 – e-mail email@example.comINTRODUCTIONConventional solar heating panels are a well-established technology, still sufferinghowever of low efficiencies and rather high investment costs when medium temperatureapplications (i.e. space heating purposes) are required. The main reason is that theworking temperature in the solar panel is strictky coupled to that of the water inside theboiler: this means that high efficiency can be reached only for relatively low workingtemperatures (not above 50°C) or for very high solar irradiation G values (above800W/m2). These conditions are for instance hardly reached in winter, when the majority ofyearly heating supply (electrical or gas) is required, and the mean coefficient of panelexploitment over the year is quite low.The use of solar assisted heat pump (SAHP) systems here presented is based on theconcept that the solar panel can work as the cold-side of an inverse cycle operated as aheat pump, with the hot side devoted to the heating system. The concept is not new [1, 2]and has the advantage of decoupling the boiler (that is the condenser-side) temperaturefrom the solar panel (that is the evaporator-side) temperature. However if conventional on-off technologies are used to drive the compressor of the plant, not so high coefficients ofperformance are reached (in the range 2.5-3.5, which is not particularly favourable ifcompared to actual air cooled heat pumps) and several difficulties are still present to adaptrefrigeration capacity to the solar heat rate, which is continuosly changing during the dayand with seasonal climate.Figure 1. Layout of a DX-SAHP (on the left) and operating prototype (on the right, University ofGenoa) DX-SAHP configuration (small device with 1m2 solar panel and 100W VCCcompressor). The panel can be just for hot water applications or can be also an hybrid panel(refrigerated PVT panel).
In recent years direct-expansion solar assisted heat pumps (DX-SAHPs) [3, 4, 5] havebeen deeply analysed, both from steady-state and dynamic point of view. Figure 1 showsthe layout and the prototype developed at the beginning of years 2000 at University ofGenoa.In DX-SAHPs several difficulties in the development of dedicated variable speedcompressors (VCC) and the need of large panel surfaces when turning towards mediumscale (single building instead of single apartment) applications made the indirect SAHPconfiguration, usually with a water-to-water heat pump (W-SAHP), more reliable andpromising.The simplest reference configuration for a thermal solar water heater is shown in Fig.2,where the main components are evidenced: the solar panel (be it simply a thermal or anhybrid PVT panel) at temperature Tp, the boiler and water storage at temperature TH, therefrigeration system (typically a water-water heat pump) and the user. An auxiliary waterheater (electrical or gas-fired, not shown in the figure) must be used to satisfay correctlythe “useful duty” needed for the user. Furthermore a by-pass can be introduced for furthersaving energy purposes, when solar and climate conditions are particularly favourable andallow the SAHP to work as a traditional solar panel.The panel efficiency ηp is the ratio between the captured mean thermal energy Qp (givento the working fluid) and the global radiation heat flux incident on the panel, GA (G solarinsulation, W/m2and A solar panel surface, m2):GAQη upp⋅= (1)As well known ηp is rougthly a linear decreasing function of the temperature differenceTp-Ta, and increases with solar insulation G. Introducing the non dimensional parameter δwe have:G)T(TKδδ0.790.86ηapp−⋅=⋅−=(2)where K, in the range 6I10W/(m2K), is a global thermal conductance between the solarpanel fluid and the ambient temperature.As already mentioned, while in traditional solar thermal panels Tp=TH, the working fluidcan give useful heat only for Tp>TH, in SAHPs the Tp value can be regulated at anydesiderated value by the refrigeration system, thus increasing significantly the panelefficiency (as shown by eq.2 when Tp becomes as low as or even lower than Ta).The new concept operations can be briefly described as follows: a proper refrigerationfluid is used to keep the solar panel temperature near to the ambient temperature, givingmaximum ηp and giving maximum regulation flexibility to the TH value, which can be easilyFigure 2. Sketch of the usual solar assisted heat pump (W-SAHP with a Water-water heat pump). Thepanel can be just for hot water and heating applications or can be also an hybrid panel(refrigerated PVT panel).
adapted to the user (Tu=TH) needs, thanks to the condenser exploitment.A similar innovation (Figure 1) has been tested for several years at University of Genoa(DIME), however heat pump performance coefficients of the order of only 2.6 have beenreached due to the very small size of the plant (1kWT). Despite that, the solution offersseveral very important advantages, such as the highest solar panel efficiency, longerworking times of the panel, which could be operated continuously from sun rise to sun set.Furthermore, much lower panel surfaces are necessary for given heating needs, thuscutting down the installation and surface costs, with benefits also for the visual impact ofthe system. A further advantage is the possibility to use this technology coupled to hybridpanels, enhancing also the photovoltaic conversion efficiency, thanks to the lower mean-working temperatures of the PV panel during the day.These advantages are paid for in terms of electrical energy consumption by thecompressor and assume we are able to regulate quite well the given compressor power,not only in terms of rotating speed, but also in terms of pressare drops in the expansionvalve. Only in recent years such characteristics have been achieved, making similarsolutions to be envisaged also in the small refrigeration plants.The development of large solar assisted heat pump systems is becoming more and moreinteresting for low temperature applications, such as swimming pool water heating. Aninteresting pilot plant was designed and built in Sestri Levante, with quite interestingexpected energy savings.Figure 3. shows the plant layout of the “bare” panels installed on the roof of the swimmingpool building, with more than 400m2solar panels deployment and a nominal power ofabout 150kWT.The basic design concept of figure 3 is the same as in figure 2, but all the control andgas-burner integration devices are shown, including by-pass regulation and auxiliary heatexchangers.The plant, which is operating from July 2012, is able to cover almost the 60% of thewinter (from November to March) energy needs of the swimming pool location, coveringthe 100% of the needs from April to October (just using the by-pass configuration, withalmost zero primary energy consumption). The mean solar panel efficiency is about 85%Figure 3. Sketch of the solar assisted heat pump (W-SAHP with a Water-water heat pump) installedand operating since July 2012 at Sestri Levante (Italy) for swimming pool water heating.
and mean heat pump efficiency COP=5.5 can be reached.Since these quite interesting results were obtained just in a quite favourable location, adetailed efficiency and energy saving analysis was performed for a lot of locations anddifferent climate conditions, well described by means of the concept of degree day andmonthly mean ambient temperature and radiation data.The results of this exhaustive research, with calculations performed over all of themunicipalities in Italy, are reported in figure 4. It is quite evident how the potential energysavings are well correlated to the degree days and offer, even in a mean annualcalculation based on the mean monthly ambient temperature, potential savings up to 50%in mild climates and well over 35% in much more cool regions. Indeed, keeping in mindthat in cold regions the heating user loads are quite high and that the installation cost ofbare panels is quite low, the expected economic return of W-SAHPs is quite favourable.Further advantages are expected when hybrid PVT panels are used coupled to W-SAHPs. Several applications are available now in northern Italy (more the 1000 smallplants) and a medium size plant is now under construction at University of Genoa (Italy) forthe electrical and thermal energy needs of a Sport building application. The PVT panellayout is shown in Figure 5 together with the PVT panel solution going to be used, and asummary of the expected performance and operating data is reported in Table 1.Figure 4. Energy saving index values (PES), referred to the days degrees (DD) of all the 110 ItalianMunicipalities (monthly averaged calculations and yearly-based values). Linear regression andcorrelation coefficient are evidenced. Only The cities of Trento and Varese show a strongdeparture from mean data.
Table 1 – Mean Energy data of the PVT W-SAHP plant at University of Genoa(Italy) – Yearly expected performance (140m2panel surface, 20 kWEpeak, 45kWT)Thermal Energy delivered 95772 kWhterFV electrical Energy produced 23990 kWhelHeat pump electrical Energy consumption (mean COP=6.6) 14530 kWhelNet electrical Energy balance 9460 kWhelGlobal primary Energy savings (tep/year) (0.067tep/m2) 9,34Annual thermal Energy income 7100 €/yAnnual electrical Energy income 2000 €/yTotal yearly money savings 9100€/yCost of the installation (integration burner already available) 250.000€Regione Liguria contribution 199.000 €REFERENCES1. Chaturvedi, D., Chen, T., Kheireddine, A., 1998. Thermal performance of a variablecapacity direct expansion solar-assisted heat pump. Energy Conversion andManagement, 30 (3), 181-191.2. Çomakly, Ö., Bayramoğlu, M., Kaygusuz, K., 1996. A Thermodynamic model of asolar assisted heat pump system with energy storage. Solar Energy, 56 (6), 485-492.3. Gorozabel Chata, F.B., Chaturvedi, S.K., Almogbel A., 2005. Analysis of a directexpansion solar assisted heat pump using different refrigerants. Energy ConversionFigure 5. Layout (on the left) of a hybrid PVT plant for electrical and thermal energy needs of theSports Building Carmine Romanzi at University of Genoa (Italy). Peak electrical power20kWE, peak thermal power 45kWT. The hybrid panel is shown on the right.
and Management, 46, 2614–2624.4. Qi Qi, Shiming Deng, 2009. Multivariable control of indoor air temperature andhumidity in a direct expansion (DX) air conditioning (A/C) system, Building andEnvironment 44 (8),1659-1667.5. Scarpa, F., Tagliafico, L.A., Tagliafico, G., 2011. Integrated Solar-Assisted HeatPumps for water heating coupled to gas burners; control criteria for dynamicoperation, Applied thermal Engineering 31, 59-68.6. M.A. Cucumo, U. Marinelli, G. Oliveti: Ingegneria solare – Principi ed applicazioni –Pitagora editrice 19947. D.M. 24 Aprile 2001 Individuazione degli obiettivi quantitativi nazionali di incrementodell’efficienza energetica negli usi finali – Supp. Ord. 125 G.U. 117, 22 Maggio 2001.8. B.J.Huang, C.P.Lee. Long-term performance of solar-assisted heat pump waterheater, Renewable Energy, vol. 29: 633-639, 2003.ACKNOWLEDGMENTSThe present work was developed in the framework of a project supported by RegioneLiguria), on the basis of an original idea of the authors and a proposal of technologicalinnovation promoted by Soc. Zenacalor Ing. Pio Benzi of Genoa, Italy.