Solar cooling between thermal and photovoltaic: An energy and economic comparative study in the Mediterranean conditions

Energy xxx (2014) 1e12

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Solar cooling between thermal and photovoltaic: An energy and economic comparative study in the Mediterranean conditions M. Noro*, R.M. Lazzarin Department of Management and Engineering, University of Padova, Stradella S. Nicola, 3 36100 Vicenza, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2013 Received in revised form 28 May 2014 Accepted 9 June 2014 Available online xxx

This paper considers different cooling systems and investigates the most promising alternatives when solar energy is to be used to supply the cooling demand. All the systems are evaluated during a summer cooling season by the energetic and economic point of view by dynamic simulation for two different climates. For Milan (Cfb climate) the highest OSE (overall system efficiency) is reached by LiBr (lithiumbromide) double effect absorption chiller driven by parabolic through collector (0.53). In terms of the collecting surface area, the best systems for Milan feature 0.08 m2 MJ
1 per day both for electric system (mono-crystalline photovoltaic coupled to water cooled chiller) and thermal system (PTC (parabolic trough collectors) coupled to double effect water-LiBr absorption chiller). Southern latitudes like Trapani (Csa climate) allow a quite better performance for thermal solar cooling solutions. The NPV (net present worths) of electric solar cooling solutions are favorable with respect to the traditional solution and the DPV (discounted payback periods) are all lower than the period of economic analysis above all for water cooled chillers. Finally a sensitivity analysis of the specific investment cost (V MJ
1 per day) is carried out regarding the investment cost of collectors, the solar ratio and the interest rate. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic Solar collector Solar cooling Sorption systems

1. Introduction Generally speaking, cooling demand of a building depends on the internal gains (type of activity and number of people, equipment, illumination, etc) but particularly on outside hot condition and on the solar radiation intensity; for this reason solar cooling, that is the use of solar energy in summer to satisfy cooling loads of the buildings, can be considered as a logical solution. There were many projects for development or demonstration of solar cooling technologies since the 1970s, after the 1973 energy crisis [1e10]. A variety of solar cooling technologies was developed and many of them are available in the market [11,12]. Most review articles in the past were limited to solar thermal, especially sorption cooling technologies [6,13e16]. More recently other Authors presented reviews with a broader overview including solar electric [17e29]. Fig. 1 depicts the main alternative routes from solar energy into cooling (and heating) effect using thermodynamic cycles. The main options are solar “thermal” or “electric” (photovoltaic). Since the first decades after the energy crisis and till very recently the PV (photovoltaic) option was excluded for the high cost of the * Corresponding author. Tel.: þ39 0 444 998704; fax: þ39 0 444 998884. E-mail addresses: [email protected] (M. Noro), [email protected] (R.M. Lazzarin).

modules. So solar thermal collectors have been widely developed in the last decades in order to improve efficiency and durability and decrease cost. Lazzarin [24] reports a concise history of the thermal collectors development, for both concentrating (PDC (parabolic dish collector), PTC (parabolic trough collector)) and no-concentrating (ETC (evacuated tube collector), FPC (flat plate collector)) technologies. The former (left hand side in Fig. 1) allow the use of thermodynamic cycles (mainly Stirling and Rankine Engines) to drive a traditional vapor compression chiller (thermo-mechanical cooling), because of the high temperature of thermal vector produced (order of hundreds of Celsius degrees) that allows a reasonable engine efficiency. The no-concentrating technologies (right hand side in Fig. 1) allow the use of the sorption cooling only. Very widespread are absorption chillers (single and double effect); recently, also adsorption chillers are at hand [30e33] because some main drawbacks (the poor heat exchange between solid adsorbent and the cooling or heating fluids and the intrinsic intermittence of the system) have been partly solved. Other sorption technologies are available and allow good energy performance (open cycle systems like liquid and solid rotating desiccant) [14,34e43] but they treat directly the air introduced into the rooms: therefore it is difficult to equip an existing building with such systems and, even if they allow good energy performance, they are not dealt with in this comparison.

http://dx.doi.org/10.1016/j.energy.2014.06.035 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

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Nomenclature

Symbols a coefficient on the equation of thermal efficiency for solar thermal collectors A area (m2) AM air mass C cost (V, V m
2, V W
1 p ) CdTe cadmium telluride photovoltaics CIGS copper indium gallium (di)selenidephotovoltaics CIGSS copper indium gallium sulphur (di) selenidephotovoltaics CIS copper indium diselenide photovoltaics DPB discounted payback period (y) E energy produced by the solar thermal collectors or the PV modules or the chillers (J, kWh) EER energy efficiency ratio ETC evacuated tube collector FPC flat plate collector GaAs gallium arsenide photovoltaics GAX generator absorber exchange G global irradiance (for non-concentrating collectors) or beam irradiance (for concentrating collectors) (J) HVAC heating, ventilation, air conditioning Kh atmosphere clearness index LiBr lithium bromide LiBr_DE double effect lithium bromide-water absorption chiller (water cooled) LiBr_SE single effect lithium bromide-water absorption chiller (water cooled) m parameter on the equation of PV modules efficiency NH3_Air air cooled water-ammonia absorption chiller NPW net present worth (V) OSE overall system efficiency p parameter on the equation of PV modules efficiency PDC parabolic dish collector PTC parabolic trough collector PV photovoltaics PV aSi amorphous silicon photovoltaics PV mSi mono-crystalline silicon photovoltaics

Solar thermal has been the most followed route for solar cooling in the last decades. With almost 2.4 GWth installed in 2012 (3.4 Mm2 solar collectors, slightly decreasing with respect to the previous year), the total installed capacity in Europe is (at the end of 2012) 28.3 GWth (40.5 Mm2) [44]. In Italy small size systems (less than 30 m2) are mainly spread for domestic hot water production in residential buildings. There are also various combi-systems (domestic hot water þ space heating) mainly located in the Northern part of the country. In the Southern part the use of combi-systems may be not convenient from both energy and economic point of view because of the low buildings thermal loads and the high solar radiation. This scenario may change when considering solar cooling. From 2005 incentives recognized in Italy to PV plants have given an impressive acceleration to the growth of PV market; consequently, Italy is now (data 2012) at the second place in the world for installed PV capacity. Data inherent cumulative installed power are: 3.5 GWp, 13 GWp and 16.3 GWp at the end of, respectively, 2010, 2011 and 2012 (about 17.5 GWp at the end of August 2013, whereas it was only 50 MWp at the end of 2006) (data from the Italian

PV pSi q r s SilGel T u VC_a VC_w

poly-crystalline silicon photovoltaics parameter on the equation of PV modules parameter on the equation of PV modules parameter on the equation of PV modules silica-gel adsorption chiller temperature ( C) parameter on the equation of PV modules air cooled vapor compression chiller water cooled vapor compression chiller

efficiency efficiency efficiency

efficiency

Greek symbols b tilt angle h efficiency Subscripts 0 refer to the zero-loss efficiency on the equation of thermal efficiency of solar thermal collector; reference state 1 refer to the first order coefficient on the equation of thermal efficiency of solar thermal collector (W m
2 K
1) 2 refer to the second order coefficient on the equation of thermal efficiency of solar thermal collector (W m
2 K
2) a air cell photovoltaic cell coll related to the solar thermal collectors cool cooling e electric H2O water in inlet inv inverter m mean out outlet peak peak conditions (cell temperature ¼ 25  C, air mass ¼ 1.5, specific radiation on tilted surface ¼ 1000 W m
2) PV photovoltaic modules system photovoltaic modules þ inverter th thermal wb wet bulb

Energy Services Manager, www.gse.it). Costs of the PV plants equipment (mainly modules and inverters) have considerably decreased in European market [45], probably just for the economy of scale allowed by so high a demand. For example, a polycrystalline PV modules plant in the 1-20 kWp peak power range could cost, in 2011, 4.5 V W
1 p (all inclusive) [46]; the forecast for 2013 is more than halved a cost (about 2 V W
1 p ) [45]. Besides this, PV modules nowadays have electrical efficiencies in the range of 13e17% for the “first generation” systems, based on singlecrystalline or multi-crystalline silicon cells. A small niche market is satisfied by the so called “second” and “third generation” systems, based respectively on the thin film and the advanced thin film technologies (CIS (copper indium diselenide photovoltaics), CIGS (copper indium gallium (di)selenidephotovoltaics), CIGSS (copper indium gallium sulphur (di)selenidephotovoltaics), CdTe (cadmium telluride photovoltaics), GaAs (gallium arsenide photovoltaics)). These are certainly possible applications in the near future. In this context the aim of this study is to compare the potential of these different technologies in delivering competitive sustainable solutions. The current commercial status of different solar

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3

Solar energy technologies

Solar electric PV Vapor Compression

Parabolic Dish Stirling Engines Vapor Compression

Solar thermal

Concentrating collectors

No-concentrating collectors

Parabolic Trough

Rankine Engines

Double Effect Absorption

Flat Plate

Single Effect Absorption

Adsorption

Evacuated Tube

Double Effect Absorption

Single Effect Absorption

Adsorption

Vapor Compression

Fig. 1. Alternative routes from solar energy into cooling effect.

cooling technologies has been evaluated by comparing the energetic performance (OSE (overall system efficiency), collectors efficiency, chillers EER (energy efficiency ratio), the economic specific investment cost and the economic profitability of the solutions (in terms of NPV (net present worth) and Discounted PayBack). The analysis has been performed by Trnsys dynamic simulation tool during a whole cooling season based on a typical office building coupled to a solar cooling/heating plant. In order to get free from the particular plant considered (type of utility, size of the solar section and of the cooling equipment) the results are reported in specific term per square meter of solar electric/thermal collector. The analyses have been extended at two different climates (Milan 45.5  N and Trapani 38  N, Italy). The choice of the resorts was done to be representative of the main Mediterranean climates according to Koppen classification (www.wikipedia.it): Milan for the Cfb climate (Cfb is a temperate and humid in all seasons climate with a hot summer e the hottest month has a mean temperature lower than 22  C) and Trapani for the Csa climate (temperate with dry summer climate with a hot summer, with the meaning just mentioned). The analyses have been carried out considering thermal collectors (PTC, ETC and FPC) coupled to single and double effect LiBr (lithium bromide) absorption chillers (water tower cooled), silicagel adsorption chiller (water tower cooled) and GAX (generator absorber exchange) ammonia-water chiller (air cooled) on one hand; mono-crystalline and amorphous PV modules coupled to

both water and air cooled vapor compression chiller have been considered on the other hand as well (Fig. 2). Only technologies actually available on the market have been considered. 2. Description of the models 2.1. Solar collectors and PV modules The solar collector is the device that converts solar radiation into thermal energy. Only liquid solar collectors will be dealt with here. In FPC (flat plate collectors) solar energy is usually absorbed by a channeled metallic plate. Thermal energy is produced at temperatures that can exceed 80e90  C, i.e. the thermal level requested by most sorption systems. For higher operative temperatures ETC (evacuated tube collectors) can be used where the convective losses are eliminated realizing vacuum between the plate (selective coated) and the glass. All these collectors are installed at a fixed tilt that optimizes the performance for a specified period: for a summer use a tilt equal to the latitude minus 10 can be considered as optimal. The thermal efficiency is defined as in Equation (1) [24,47] :


  * 2 * hth ¼ h0
a1 Tm
a2 Gb $ Tm

(1)

* is the reduced temperature defined as a function of the where Tm mean temperature of the water entering and leaving the collector

Fig. 2. Schematic of a thermal solar collector that drives a sorption chiller (a) and of a PV panel that drives a compression chiller (b).

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Tm, the air temperature Ta and the global solar radiation on the tilted surface Gb (Equation (2)): * Tm ¼

Tm
Ta Gb

(2)

h0, a1 and a2 are constants relative to the considered solar col* is zero when the average temperature lector. The co-ordinate Tm inside the collector is equal to the outside temperature. Then the losses are negligible and h0 (zero-loss collector efficiency) is the fraction of solar insolation transmitted through the shield and absorbed by the plate; the higher the values of the constants a1 and a2 the higher the collector thermal losses when the operative temperature is increasing. Even if tracking collectors are not so widespread, PTC (parabolic trough collectors) should be considered for operating temperatures as high as 160  C, suitable for driving double effect sorption chillers. In a parabolic concentrating collector a reflector focuses the direct solar radiation parallel to the collector axis onto the receiver placed on the focal line. The collector is equipped with an one-axis solar tracking system, usually with E-W tracking. The general equation describing a PTC behavior is quite similar to (1): the main difference is that Gb has to be considered as the direct solar radiation intensity since only a negligible fraction of diffuse radiation can be concentrated onto the focus [21]. In this case it has been considered that 75% is direct radiation. The three constants of the general Equation (1) differ greatly from one collector type to another, but even within the same typology of collector the differences can be significant. For the following evaluations reasonable average parameters are listed on Table 1 as representative of high performance products for the three considered solar collector typologies and refer to market available collectors. For PV collectors we have considered the semi-empirical efficiency formulation developed by Durish [48] because it is suitable for the two silicon PV technologies here considered (Equation (3)):  "  #    Gb Gb m Tcell AM AM u $ 1þr hPV ¼ p q þ þs þ AM0 AM0 Gb0 Gb0 Tcell;0

(3)

where the AM (air mass) and the cells temperature Tcell are calculated according to [49]; the parameters p, q, m, r, s, u and h have been determinated experimentally and are available in Ref. [49] as well, resulting in the electrical efficiencies reported in Table 1. As described in the same study, in a grid connected PV plant the type of the inverter must be correctly matched with the PV technology for the resort and tilt angle considered. We have determined the optimal sizing ratio, that is the ratio of the PV array peak power

(at STC (standard test conditions)) to the rated inverter power because both the extreme situations are negative: with low insolation, a PV array generates power at only a part of its rated capacity (peak) and the inverter thus operates under part load conditions with lower system efficiency; with high insolation or if the inverter's rated capacity is much lower than the PV peak power, PV efficiency is also affected adversely as the inverter would be operating at overload conditions, with the excess PV output lost (Fig. 3). In this study, a type 2 inverter (both standby and loaddependent losses are low) has been considered for all the climates [49]. Therefore the evaluations are based on the whole PV system efficiencies (PV modules þ inverter , hPV,system). In this analysis the not concentrating collectors are installed with a fixed tilt angle of 30 facing the South for all the resorts and the PTC is equipped with an one-axis E-W solar tracking system. The energy supplied hourly by the three collectors and PV has been thus evaluated considering also the IAM (incidence angle modifier), that is the reduction due to the variable incidence angle between the solar ray and the normal to the collector during the day with respect to normal incidence; the procedure is described in Ref. [24]. These are the theoretical basis on which the simulation model described in the next section has been developed. 2.2. The reference building/plant The case study concerns a typical office building, developed in three storeys (basement, ground floor and first floor, the last being identical) for a volume of 620 m3 and a total floor surface of 230 m2. The thermal transmittance of the external walls, roof and windows are, respectively, 0.72, 0.52 and 2.8 W m
2 K
1.The winter diurnal internal temperature set point is 20  C, whereas an attenuation down to 16  C is supposed when nobody is present during the night time. The summer temperature is set at 26  C, let rising up to 28  C following the schedule. No air relative humidity control has been provided. The heating and cooling loads have been calculated on the basis of Test Reference Years [50] with a time step of 1 h. In this study only cooling needs have been considered. For the cooling season (1st May e 30th Sept) internal heat gains have been considered, resulting in a nominal cooling load of 11.3 kW and 17.8 kW and cooling needs of 38 GJ y
1 and 60 GJ y
1 for Milan and Trapani respectively. Sanitary hot water demand is very small due to the building allocation and it is not further taken into account. The HVAC (heating, ventilation, air conditioning) system modeled in Trnsys is made of:

Table 1 Nominal efficiency parameters of the different solar collectors (thermal at EN 12975 conditions and PV at peak conditions). Collector type Flat plate collector (FPC) Evacuated tube collector (ETC) Parabolic trough collector (PTC) PV monocrystalline (PV mSi) PV amorphous (PV aSi)

h0

a1 (W m
2 K
1)

a2 (W m
2 K
2)

0.748

3.311

0.0087

0.718

0.974

0.005

0.6

0.36

0.0011

hPV, peak

Apeak (m2 kW
1 p )

15.4%

6.51

7.2%

13.83 Fig. 3. Efficiency curves of the three types of inverters (p is the ratio between inverter output power and rated output power) [49].

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 the cooling plant, consisting of the absorption or adsorption chiller (air or water cooling tower) plus an integration natural gas boiler in the case of thermal solar cooling or a vapor compression chiller (water or air cooled) in the case of electric solar cooling;  the solar field (thermal collectors or PV modules þ inverter);  the circuits (tubes, solar and chillers pumps, valves);  the cooling units (every floor is supplied by an independent circuit, the basement is supplied by fan-coil units only, the other two floors by both radiant floors and fan-coils). In the case of solar thermal, the solar field is connected to the cooling plant by a 1500 l sensible (water) tank, while the chiller is connected to the cooling units by a 1000 l sensible (water) tank (cold storage). 2.3. The scenarios Solar thermal can be interfaced with single effect LiBr absorption chillers (at a temperature around 90  C) or with double effect LiBr chillers (at a temperature of about 160  C) [24] [51]. In both cases a tower water cooling must be provided, as air cooling of absorber and condenser does not ensure an acceptable temperature for the absorption cycle (there is the risk of salt crystallization inside the cycle because of the high temperature of outside air in summer). A similar consideration is valid for adsorption chillers that can work at lower temperatures, say 70  C. At the temperature of 160  C GAX ammonia-water chillers can be operated even if air cooled. PV solar can resort to traditional vapor compression chillers. For a fair comparison tower water cooled chillers must be analyzed as well as air cooled.

Solar ratio ¼

Table 3 The different solar cooling solutions considered in this study. System

Description

FPC_SilGel FPC_LiBr_SE

Flat plate collectors coupled to silica-gel adsorption chiller Flat plate collectors coupled to single effect water-LiBr absorption chiller ETC_LiBr_SE Evacuated tube collectors coupled to single effect water-LiBr absorption chiller ETC_LiBr_DE Evacuated tube collectors coupled to double effect water-LiBr absorption chiller ETC_NH3_Air Evacuated tube collectors coupled to GAX ammonia-water absorption chiller PTC_LiBr_SE Parabolic trough collectors coupled to single effect water-LiBr absorption chiller PTC_LiBr_DE Parabolic trough collectors coupled to double effect water-LiBr absorption chiller PTC_NH3_Air Parabolic trough collectors coupled to GAX ammonia-water absorption chiller PV mSi_VC_w Mono-crystalline silicon PV modules coupled to water cooled vapor compression chiller PV mSi_VC_a Mono-crystalline silicon PV modules coupled to air cooled vapor compression chiller PV aSi_VC_w Amorphous silicon PV modules coupled to water cooled vapor compression chiller PV aSi_VC_a Amorphous silicon PV modules coupled to air cooled vapor compression chiller

The study regards the systems listed on Table 3. The design of the solar field (in terms of collectors area and solar circuit pump rated flow and power consumption) was done with the aim of reaching a solar ratio of 70% as a reasonable compromise between a significant supply to the building and a possible excess of solar heat during very sunny days. Solar ratio is defined as in Equation (4):

Solar_energy_to_thermal_chiller ðsolar thermal coolingÞ Cooling_needs

Chiller_electricity_from_PV ðsolar PV coolingÞ Solar ratio ¼ Chiller_electricity_total

Values of nominal Energy Efficiency Ratio (EERe for electric compression chillers, EERth for sorption chillers) and nominal capacity are reported on Table 2 (values are based on commercial equipment). The third column of the Table discriminates thermal equipment performance (subscript th) from electrical (e). Efficiencies of the chillers were supposed to vary with the outside air temperature and partial load on the base of producers' technical data.

5

(4)

Some solutions can be technically unfeasible (for example, adsorption chillers typically take no advantage by water hotter than 100  C, so it is useless to connect them to PTC or ETC); these solutions do not appear on Table 3 and in the next figures. Electricity demand of pumps and fans of the cooling tower was included in the evaluation. Finally, set points of the control logic have been suitably adjusted to be adapted to the characteristics of the equipment (chillers, collectors) for each case. 3. Energy, environmental and economic analysis: results and discussion

Table 2 Nominal efficiency index and capacity of the different chillers (at A35W7; W30W7). Chiller type

Adsorption H2Oesilica gel (SilGel) Absorption H2OeLiBr single effect (LiBr_SE) Absorption H2OeLiBr double effect (LiBr_DE) H2OeNH3 GAX (NH3_Air) Electric vapor compression (VC_w) Electric vapor compression (VC_a)

Rated EERth e EERe

Rated capacity (kW)

(Water cooled) (Water cooled)

0.45 0.70

15.0 17.6

(Water cooled)

1.10

23.0

(Air cooled) (Water cooled) (Air cooled)

0.60 3.33 2.75

17.7 17.0 16.5

3.1. Energy and environmental performance analyses All the simulations were worked out with a time step of 1 h; here the whole cooling season results are reported. A first significant comparison can be carried out evaluating the collectors/ modules thermal/electrical efficiency (Fig. 4). Concerning the thermal solar cooling, flat plate collectors coupled to “low temperature” single effect LiBr absorption chillers (FPC_LiBr_SE) perform as well as ETCs coupled to “high temperature” absorption chillers (LiBr_DE (double effect lithium bromide-water absorption chiller (water cooled)) and NH3_Air (air cooled water-ammonia

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Fig. 4. Seasonal thermal collectors efficiency (hth) and PV system (PV modules þ inverter) efficiency (hPV,system) for the different solar cooling solutions.

absorption chiller), thermal efficiency around 30% in Milan and over 38% in southern climate). FPCs coupled to adsorption chillers give a higher performance (thermal efficiency between 40% and 50%) because of the high h0 value of the collector and the lower operative temperature; by the way, they present a very poor performance when coupled to LiBr_DE and NH3_Air chillers (thermal efficiency close to zero), so they do not appear in Fig. 4 and following ones. Evacuated tube and PTC (parabolic trough collectors) are obviously more efficient when coupled to low temperature sorption chillers (LiBr single effect, seasonal efficiency around 55%). Concerning PV solar cooling, the crystalline modules allow a slightly higher efficiency in Milan with respect to Trapani because of the lower air temperature whereas the amorphous silicon modules instead perform substantially in the same way in the two resorts; in fact, for thin film module types the efficiency dependence on the temperature is much weaker [49]. The OSE (overall system efficiency) produces a better picture to compare the systems effectiveness (Fig. 5). It is the ratio between

the useful cooling effect and the incident solar radiation intensity integrated for an assigned time period. For thermal solar cooling the ratio can be correlated to the sorption chiller performance (characterized by the thermal energy efficiency ratio EERth) and to the collector efficiency hth as in Equation (5):

OSE ¼

Ecool Ecool Ecoll ¼ $ ¼ EERth $hth Gb Ecoll Gb

(5)

Similarly, for the PV solar cooling system (Equation (6)):

OSE ¼

EPV;system Ecool Ecool ¼ $ ¼ EERe $hPV;system Gb EPV;system Gb

(6)

For Milan the highest OSE is reached by LiBr double effect absorption chillers driven by parabolic through collectors (0.53) followed by water cooled chillers driven by mono-crystalline PV (0.51). According to the proposed evaluation the adsorption low

Fig. 5. Overall system efficiency of the different solar cooling solutions.

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7

Fig. 6. Monthly and seasonal mean clearness index (Kh) and dry-bulb air temperature (Ta,m) for the two climates.

temperature driven systems offer an OSE below the absorption systems, as the higher collector efficiency does not balance the lower EERth of this technology. While the best thermal driven systems OSE manage to be comparable to the PV systems with water cooling, the air cooled thermal systems are below the PV air cooled solutions; this is due to the combined effect of lower solar collectors efficiency (because of the high drive temperature of GAX ammonia-water chillers) and lower EERth of chillers. The OSE is around 0.30 for PTC_NH3_Air, even 0.20 for ETC_NH3_Air. The PV driven chillers, both air and water cooled, present an OSE strongly dependent by the PV technology. Southern resorts such as Trapani have a higher mean clearness index (or a higher direct solar irradiance) during summer (Fig. 6). The tracking systems efficiency is strongly dependent on the direct fraction of solar radiation. Nonetheless the performance difference between the two resorts is not so apparent on a seasonal basis.

Another way to assess the systems is to list the collecting surface area needed to obtain 1 MJ of cooling energy during one typical summer day. The days selected for the computations were the 17th of July for Milan and the 18th of August for Trapani because on these days the clearness index Kh is around 0.6 with a temperature slightly above the monthly average (Fig. 6). The comparison is depicted in Fig. 7: an estimate for the best systems for Milan is about 0.08 m2 MJ
1 per day both for electric system (mono-crystalline PV coupled to water cooled chiller) and thermal system (PTC_LiBr_DE). Water cooled PV driven chillers allow to use less area than air cooled (0.08e0.13 m2 MJ
1 per day vs 0.11e0.17 m2 MJ
1 per day respectively for mono-crystalline and amorphous silicon). Southern latitudes like Trapani allow a quite better performance for thermal solar cooling solutions, while for electric solar cooling improvements are limited: this is because the larger amount of electrical energy produced in Trapani thanks to

Fig. 7. Specific collecting surface area for the different solar cooling solutions for the typical summer day (average day of July for Milan and of August for Trapani).

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Fig. 8. Designing of the PV field in terms of peak power for the production of 1 MJ of cooling energy per day.

higher solar radiation does not balance the lower EERe caused by the higher air temperature. The results for PV in terms of peak power to produce 1 MJ of cooling energy per day are depicted in Fig. 8: the larger amount of solar radiation in southern resorts with respect to Milan allow the PV field to be smaller to face the same solar ratio. Obviously the final design of the PV field should take into account the other electrical needs of the building (illumination and other electrical users). The parasitic energy (consumed by pumps and fans) that must be supplemented to the solar thermal systems cannot be neglected in the energy balance: it accounts for about 10% in terms of primary energy in the present evaluation, with a slightly higher value for the double effect and for the ammonia-water chillers than for the other technologies. Finally, a comparison of the different solutions on the basis of CO2 emissions was done. In the cases of PV solar cooling, emissions were related to the electric energy purchased by the grid to produce the integration cooling energy (difference between the cooling

needs and the cooling energy produced by the solar plant) by an integration VC_a (air cooled vapor compression chiller) chiller; in the cases of thermal solar cooling, emissions were related to both the electric energy purchased by the grid to supply the pumps and fans and the natural gas purchased by the distributor to produce the integration cooling energy by the sorption chillers (fed by an integration boiler with 0.9 global mean seasonal efficiency). Emission factors here considered were 0.47 kg CO2 kWh
1 for the grid electricity and 2.22 kg CO2 Sm
3 for the natural gas combustion. Results are depicted in Fig. 9: environmental impact of PV solar cooling is much lower than thermal because of the presence of electric consumption of auxiliaries (pumps and fans) and the combustion of natural gas for integration for the latter. Note that in Trapani the CO2 emissions, that are quite representative of the norenewable primary energy consumption, are quite higher than in Milan: in southern resorts the higher solar radiation does not balance the increased cooling load of the building, so a higher integration cooling energy is required.

Fig. 9. CO2 emissions of the different solutions for the whole cooling season.

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M. Noro, R.M. Lazzarin / Energy xxx (2014) 1e12 Table 4 Average cost of solar thermal collectors and PV modules per m2 and cost of chiller technologies per kW. Collector cost

V m
2

Chiller cost

V kW
1

FPC ETC PTC PV mSi PV aSi

350 650 450 330 130

Adsorption H2O silica gel Absorption LiBr single effect Absorption LiBr double effect NH3 GAX VC_a

600 400 700 700 300

3.2. Economic performance analysis The economic analysis sums up the investment costs (for the solar collecting area and the cooling equipment) and the annual operative costs. The latter are:
cost of natural gas for the integration boiler for thermal solar cooling (0.9 V Nm
3);
cost of electricity for pumps (for solar thermal) and for vapor compression chillers (solar electric) supplied by the grid (0.18 V kWh
1); No gains are considered deriving from the electricity produced and not utilized in the cooling plant (almost negligible in all the cases analyzed). A reasonable estimate of collectors and PV panels specific cost [V m
2] and of chillers (absorption single and double effect, compression and adsorption) ones [V kW
1] is proposed on Table 4 with the nowadays average values [21,24]. For PV plants the cost per unit of area CPV,system expressed in V m
2 is calculated in Equation (7) (1000 is the global solar radiation in W m
2 at peak conditions):

CPV þ Cinv CPV;system ¼ hPV;peak $1000$ 1
Cgeneralþdesign

(7)

Following the indications by Ref. [45] concerning the values for 2013, the costs other than module CPV in VPV W
1 p þ inverter Cinv in Vinv W
1 p (that is costs for design, installation, frame of the plant, electrical system, Cgeneralþdesign) should weight for a 60% of the total cost (in Equation (7) it is expressed as percentage of

9

CPV,system). The investment cost for the inverter (in the range up to 10 kWp) is 0.2 V W
1 p and the ones for the PV modules are 0.66 and 0.52 V W
1 p respectively for mono-crystalline and amorphous silicon. The cost of other equipment (pumps, tubes, boiler, valves, etc.) has been considered to be the same for both solar thermal and solar PV so it is not included in the analysis: this favors the thermal system as its hydronic circuit is surely more complex. The interest rate and the period of the economic analysis are 3% and 20 years respectively. A first result is represented in Fig. 10 in terms of investment cost necessary for a daily cooling production of 1 MJ. The same typical summer days as previously described were considered. To better clarify how the computation is carried out, consider a single effect chiller driven at 90  C by an ETC. The specific cost of chiller capacity is evaluated in 400 V kW
1. To obtain 1 MJ during the typical summer day (17th July in Milan) 0.10 m2 of collectors are required. In fact the daily solar radiation on the collector is 26.7 MJ m
2, the daily efficiency of such collectors is 57.2% and the OSE is 0.36 (estimated EERth ¼ 0.64) so that the specific requested area is 0.10 m2 MJ
1 per day. By the previous calculation of the cooling needs, the investment cost expressed in V MJ
1 per day is therefore estimated in Equation (8):



328406 650 þ 400$ 31$10$3600

 $0:10 ¼ 69:9

V MJ=d

(8)

where: 650 V m
2 is the collectors specific cost; 400 V kW
1 is the chiller specific cost; 328,406 kJ m
2 is the specific cooling energy produced during July (31 days, 10 h per day) by the chiller; 0.10 m2 d MJ
1 is the collectors specific requested area. The cooling effect is supposed to be produced during 10 h of continuous operation of the chiller, so allowed by the storage: this hypothesis determines the chiller nominal capacity. The lowest investment cost is for water cooled PV aSi (amorphous silicon photovoltaics) driven chillers (25 V MJ
1 per day) followed closely by the other electric chillers (30e43 V MJ
1 per day) for Trapani. PTC driven double effect (53 V MJ
1 per day) and single effect absorption chillers (57 V MJ
1 per day) follow. Flat

Fig. 10. Specific investment costs to produce 1 MJ of cooling energy during the typical summer day.

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M. Noro, R.M. Lazzarin / Energy xxx (2014) 1e12

Table 5 Main results of the economic analysis. MI

PV mSi

VC_w VC_a PV aSi VC_w VC_a FPC SilGel LiBr_SE ETC LiBr_SE LiBr_DE NH3_Air PTC LiBr_SE LiBr_DE NH3_Air Traditional VC_a

TR

Investment cost (V)

Grid electricity cost (V/y)

6809 8069 5336 6198 20,894 18,220 18,025 20,907 31,927 13,094 11,794 16,796 3690

159 159 159 159 282 181 181 181 312 181 180 312 531

NG cost (V/y)

NPW cost (V)

DPB (y)

Investment cost (V)

Grid electricity cost (V/y)

642 412 412 196 338 412 195 337

2410 1151 3883 3021 e e e e e e e e

9.8 14.8 4.8 7.7 e e e e e e e e

9137 11,608 6811 8428 25,743 20,060 22,640 24,230 35,820 17,219 15,639 21,883 5340

257 257 257 257 482 291 290 296 493 290 295 493 857

plate and evacuated tube collectors are definitely more expensive. For resorts with a less clear atmosphere (lower fractions of direct solar radiation, Milan) the solar thermal options could significantly increase the investment cost (from 10% to 40%), whereas the PV electric options are 15% more expensive than in the clearer atmosphere of Trapani. Summarizing, the least expensive systems require an average investment cost of about 25e30 V MJ
1 cooling per day. The competition of the various technologies in terms of investment cost is less uncertain for air cooled chiller as the PV driven is evaluated at 30 V MJ
1 per day, whereas the best thermally driven costs 71 V (more than the double!). Furthermore, an economic analysis in terms of NPW (net present worth) and DPB (discounted payback period) of the different solutions was carried out with respect to a “reference solution” (air cooled electric vapor compression chiller). The main results of the analysis for a solar ratio of 70% are reported on Table 5. The solar thermal solutions are not in direct competition with the traditional solution: the lower grid electricity consumption for cooling production due to use of thermal solar cooling does not balance the expense for the parasitic electricity, the natural gas for the integration boiler and the higher investment cost due to the solar part

NG cost (V/y)

NPW (V)

DPB (y)

1097 662 661 321 534 660 320 534

5130 2659 7455 5838 e e e e e e e e

7.1 12.7 2.6 5.7 e e e e e e e e

of the plant. The PV solar cooling solutions seem instead to offer a possible economic advantage because of the fall of cost during the last years. The higher investment costs of PV solutions with respect to the traditional solution are balanced by the lower electricity bill. The Net Present Worth of electric solar cooling solutions are favorable with respect to the traditional solution; the discounted payback periods are all lower than the period of economic analysis above all for water cooled chillers (the longest period is 15 years arriving even at values as low as 2.6 years for amorphous silicon in Trapani). Payback periods are longer for air cooled chillers; all the same PV solar cooling is still advantageous. Anyway, it has to be pointed out that investment cost of the cooling tower is not considered in the analyses. Among thermal solar cooling, PTCs allow to get the best economic results when coupled to LiBr single and double effect absorption chillers.

4. Sensitivity analyses The previous analysis is based on cost assumptions taken from the nowadays market. However costs may be subject to large variations in few years as it was observed with PV systems.

Fig. 11. Investment cost (V per MJ cooling per day) for the different systems as a function of the % difference cost in solar collectors with respect to the costs of Table 4. Every system is coupled with the less expensive technology for that cost.

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M. Noro, R.M. Lazzarin / Energy xxx (2014) 1e12

Fig. 12. DPB as a function of the collectors cost for the PV technologies. The solar thermal cooling technologies are not reported because they have negative NPWs, so DPBs are greater than the reference period of the economic analysis (20 years).

Note that by chance the unitary cost (V kW
1) of the engines is very close to the unitary cost of the solar section (V m
2). Since three or more square meters are required to supplement the energy to drive 1 kW cooling capacity, the cost of the solar section is the prevalent investment voice (between 75% and 95% of the total). Then a sensitivity analysis of the specific investment cost (V MJ
1 per day) can be carried out regarding only the investment cost of collectors. Furthermore, economic results can vary as a function of the solar ratio: a sensitivity analysis of the payback period has been carried out as well. Finally, also interest rate for the economic analysis can vary during the years. The results are represented on Figs. 11e14. On the former the specific investment cost for the four considered collectors is given as a function of ±30% variation of the collector cost. Every system is reported for the least expensive result for the collector cost only. The diagram allows to compare the systems starting from a different cost assumption. PV driven solar cooling technologies are obviously less sensitive to variation of the collectors cost with respect to solar thermal ones. Even a cost reduction of 30% cannot produce a thermally driven system economically competitive with the actual and even 30% higher PV cost. Fig. 12 reports the DPB of the PV driven solutions with respect to the reference “traditional” solution varying the cost of the systems. The analysis is carried out for PV technologies only as solar thermal

Fig. 13. NPW and DPB as a function of the solar ratio for the PV technologies. The solar thermal cooling technologies are not reported because they have negative NPWs, so DPBs are greater than the reference period of the economic analysis (20 years).

11

Fig. 14. NPW and DPB as a function of the interest rate variation with respect to the nominal value (3%) for the PV technologies. The solar thermal cooling technologies are not reported because they have negative NPWs, so DPBs are greater than the reference period of the economic analysis (20 years).

ones present negative NPWs, so that DPBs are longer than the reference period of the analysis (20 years). A further comparison is proposed in Fig. 13: both the NPWs and the DPB periods are represented as a function of solar ratio. This sensitivity analysis states that NPWs are essentially indifferent to solar ratio (except for mSi in Milan), and sensitivity is higher in Cfb climate; DPBs are instead rapidly increasing with solar ratio particularly for PV aSi systems, and sensitivity is higher in Csa climate. Finally, Fig. 14 reports both the NPWs and the DPBs varying the interest rate with respect the nominal value (3%). In this case DPBs are essentially indifferent to interest rate variation above all for amorphous silicon modules (the less expensive); NPWs are instead rapidly decreasing with interest rate variation particularly for southern climates. 5. Conclusions The analysis here reported is based on a typical office building and conducted by hourly time step transient simulations; two great families of solar cooling technologies have been considered: solar thermal and PV driven. On a whole cooling season the solar collector efficiency and the overall system efficiency were evaluated. While the first rewards the conversion of solar energy into useful thermal/electrical energy (so the evacuated tube and concentrating thermal collectors result to be more competitive), the second index rewards the ability of the whole plant to convert solar energy into cooling effect: in this case the PV solutions using silicon cells are generally better than thermal ones in Milan climate. Only the parabolic trough collectors allow a comparable performance when coupled to double effect LiBr absorption chillers. The comparison of the specific area required to produce 1 MJ cooling per day has demonstrated the most favorable values for the VC_w (water cooled vapor compression chillers) and the PTC driven LiBr absorption chillers (0.07e0.08 m2 MJ
1 d for Milan). Very poor results are permitted by the FPCs (0.16e0.19 m2 MJ
1 d for Milan for LiBr single effect and adsorption chillers respectively) due to the low thermal efficiency. In southern climates solutions driven by flat plate and evacuated tube collectors present the best improvement (decrease) of the specific required area. This is due to the appreciable increase of thermal efficiency of the collectors that compensates the slightly decrease of the chillers efficiency (both effects are due to the higher air temperature in Trapani with respect to Milan).

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M. Noro, R.M. Lazzarin / Energy xxx (2014) 1e12

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