Bennett Widyolar Performance of the Merced Demonstration

Bennett WidyolarSchool of Engineering,

University of California,

Merced, 5200 N. Lake Road,

Merced, CA 95340

e-mail: [email protected]

Roland WinstonSchools of Engineering and Natural Science,



Merced, CA 95340


Lun JiangSchool of Engineering,



Merced, CA 95340


Heather Poiry1

School of Engineering,



Merced, CA 95340


Performance of the MercedDemonstration XCPC Collectorand Double Effect ChillerA solar thermal cooling system using novel nontracking external compound parabolicconcentrators (XCPC) has been built at the University of California, Merced and oper-ated for two cooling seasons. Its performance in providing power for space cooling hasbeen analyzed. This solar cooling system is comprised of 53.3 m2 of XCPC trough collec-tors which are used to power a 23 kW double effect (LiBr) absorption chiller. This is thefirst system that combines both XCPC and absorption chilling technologies. Performanceof the system was measured in both sunny and cloudy conditions, with both clean anddirty collectors. It took, on average, about 2 h for the collector system to reach operatingtemperatures between 160 and 180 �C. When operated in this temperature range, theXCPC collector array collected solar energy with an average daily efficiency of 36.7%and reached instantaneous efficiencies up to 40%. The thermal coefficient of performance(COP) of the system (including thermal losses and COP of absorption chiller) averagedat 0.99 and the daily solar COP of the entire system averaged at 0.363. It was found thatthese collectors are well suited at providing thermal power to drive absorption coolingsystems and that both the coinciding of available thermal power with cooling demandand the simplicity of the XCPC collectors compared to other solar thermal collectorsmakes them a highly attractive candidate for cooling projects. Consequently, the XCPCtechnology is currently being commercialized in the U.S. and India. The XCPC’s numer-ous potential applications include solar heating, cooling, desalination, oil extraction,electricity generation, and food processing. [DOI: 10.1115/1.4027726]

1 Introduction

Current energy systems based on fossil fuels are largely responsi-ble for the present humanitarian, environmental, and economic cri-sis. World energy demand—and corresponding CO2 emissions—isexpected to rise with the rapid development of many countriesaround the world. The continued rise in both living and workingcomfort conditions and reduced prices for conventional (electri-cal) air conditioning units has led to a fast proliferation of thesesystems. In residential buildings, these are low efficiency systemsassociated with significant contributions to peak electricitydemand. At the same time there remains the looming question asto how much usable fossil fuel actually remains available. Thesetrends are the catalyst calling for new models in heat and powergeneration and encouraging strategies that include renewableenergies with higher levels of efficiency to reduce dependence onfossil fuels and the emissions they produce and to offset peakpower consumption. In this framework, solar and hybrid technolo-gies represent a potential solution in the near future.

Solar cooling has long been a desired goal in utilizing the solarresource for the benefit of people. The reasons are intuitive; theresource is well-matched to the load (high cooling demand daysare usually sunny days) and if deployed on a roof, solar radiationthat would otherwise heat the building is instead diverted to coolthe building. There is also an enormous potential to reduce energyconsumption and harmful emissions as buildings account forabout 41% of primary energy consumption in the United States(most of which is used for space heating and cooling, water heat-ing, refrigeration, and lighting [1]) and are associated with about40% of the United States’ carbon dioxide emissions [2]. Solarcooling also leads to a reduction of peak electricity demand,which is a benefit for the electricity network and could lead to

cost savings of the most expensive peak electricity. However, thetechnical barriers to implementation are well-known. Efficientcooling machines require relatively high temperatures which areoften beyond the range of flat plate solar collectors, while trackingcollectors are problematic for building applications.

During the 1970s and 1980s there was a significant amount ofwork done in the USA and Japan on the development of solarcooling components and systems and a number of demonstrationprojects proved technical feasibility. However, these systemsfailed to establish a significant global market for solar thermalcooling due to their high initial cost, lack of commercial hot waterdriven chillers, and a scarcity of demonstrations and impartialassessments by reputable institutions [3]. Recently, however, therehas been a renewed interest in the field of solar cooling with thedevelopment of high temperature solar receivers and the commer-cial availability of hot water driven chillers. Research and demon-stration projects have been carried out in many countries,particularly in the U.S., Germany, and Spain, and under the frame-work of the Solar Heating and Cooling Programme of the interna-tional Energy Agency. Yet despite the existence of long termdemonstration projects and even after 10 yr of numerous activitiesin the field of solar cooling, market penetration remains small [4].Currently there exists a good supply of robust, cost effective solarcollectors produced on an industrial level which operate with tem-peratures below 110 �C. However, solar collectors for higher tem-peratures above approximately 130 �C are still scarce as they are ayoung technology and not yet broadly applied and there exists apotential for important cost savings that may be achieved bydeveloping advanced materials and production technologies [4].Major progress in the last decade was made toward the develop-ment of small capacity thermally driven chillers down to 5 kWand there are several R&D projects focused on continuing to de-velop the technology of absorption chillers [5–7], adsorption chill-ers [8], and open cooling cycles using liquid desiccants [9,10].These developments have the potential to increase efficiencies,make such systems more compact, lower required operating tem-peratures, and lower system costs. Completely different solutions,

1Present address: E&J Gallo Winery, 600 Yosemite Blvd, Modesto CA, 95354.Contributed by the Solar Energy Division of ASME for publication in the JOURNAL

OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY

CONSERVATION. Manuscript received October 3, 2012; final manuscript receivedMay 13, 2014; published online June 3, 2014. Assoc. Editor: Werner Platzer.

Journal of Solar Energy Engineering NOVEMBER 2014, Vol. 136 / 041009-1Copyright VC 2014 by ASME

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such as new thermomechanical cycles, promise a significantincrease in efficiency and are under development but have not yetleft laboratory scale. Ultimately, continued development of bothcooling machines and high temperature solar collectors is neededto reach technical maturity and realize on potential cost savingsthat will make solar cooling systems economically competitive.

1.1 Technical Quantification. A basic parameter of anycooling system is the coefficient of performance (COP), which isdefined as the useful cold (Qcold) produced per unit driving heat(Qheat) and gives a measure of the system’s ability to producecooling from heat

COP ¼ Qcold

Qheat

(1)

There are several technologies that can produce cooling from adriving heat source (sorption chillers, desiccant systems, thermo-mechanical systems), but absorption chillers are the dominanttechnology for solar cooling primarily because the technology iswell developed and they are commercially available in a widerange of capacities [11]. Absorption chillers use a refrigerant-sorption chemical pair to transfer heat through a series of cham-bers of high and low pressure, the most common chemical pair forair conditioning applications being water–LiBr. The simplest ofthese machines are called single effect LiBr chillers and operatewith a COP of about 0.7 using a heat source between 80 and100 �C. These chillers are well matched for stationary solar collec-tors, such as flat plate collectors (FPCs) or evacuated tube (EVT)collectors, which can easily achieve this temperature range. Dou-ble effect Li–Br chillers require operating temperatures between140 and 160 �C but can produce nearly twice as much cooling(COP � 1.1–1.2) as their single effect counterparts for the sameenergy input. Thus a double effect system can produce the sameamount of cooling as a single effect system with half the collectorfield area if they can utilize the benefits of higher temperature heatthat can be provided by certain types of solar collectors. Tripleeffect chillers, which can potentially reach a COP of 1.7 with driv-ing temperatures above 160 �C, are an exciting possibility forfuture projects but are still under development [12].

The performance of a solar cooling system can be characterizedby three basic parameters: collector efficiency (gcol), thermal coef-ficient of performance (COPthermal), and the solar COP (COPsolar).Collector efficiency is the fraction of available solar power inci-dent on the collector (Qsolar) that is converted into usable heatpower (Qcol) by the collectors

gcol ¼Qcol

Qsolar

¼ _mCpDTcol

I�Acol

(2)

For high concentration tracking collectors it is customary to consideronly beam radiation (DNI) as the available solar energy incident onthe collector (I�) because this is the only radiation they can collect.For nonconcentrating collectors, or compound parabolic concentra-tors (CPCs) that collect most of the sky, total hemispherical radia-tion on the plane of the collector field is the relevant quantity.

We define the thermal COP of a solar cooling system to be theratio of cooling output per unit usable heat captured by the solarcollectors. This term includes the COP of the chiller as well asthermal losses through insulation, storage tanks, heat exchangers,and any other component between the collectors and the chiller

COPthermal ¼Qcold

Qcol

¼ _mCpDTchl

_mCpDTcol

(3)

The solar COP is the ratio of cooling output per available solarpower and characterizes the overall performance of the system,including all thermal losses throughout the system

COPsolar ¼Qcold

Qsolar

¼ _mCpDTchl

I�Acol

(4)

Parasitic losses from electrical pumping power and chiller powercan also be included to give a parasitic solar COP

COPsolar;parasitic ¼Qcold

Qsolar þWelectrical

(5)

When calculated for a single moment in time, these are consid-ered instantaneous quantities and can have significant variation overthe course of operation due to the high variability of solar radiationand the latency between components in a solar cooling system. Torectify this, an average or daily value is often presented.

Several solar assisted cooling projects have been demonstratedin the past 15 yr and they are presented here to gain a basic under-standing of the performance of solar cooling systems to date. AFPC system in Madrid, which drove a single effect water–LiBrabsorption chiller, achieved an overall solar COP of about 0.11[13]. An EVT collector system in Wales powered a small water–LiBr absorption chiller with an overall solar COP of about 0.4[14]. A combined FPC and EVT system powered a single effectwater–LiBr absorption chiller in New Mexico with an overall so-lar COP of about 0.25 [15]. Several high temperature solar coolingsystems powering double effect chillers have also been demon-strated. A linear Fresnel collector system in Seville drove a doubleeffect water–LiBr absorption chiller with a solar COP of approxi-mately 0.44 [16], and a parabolic trough system in Pennsylvaniadrove a double effect water–LiBr absorption chiller with a solarCOP of approximately 0.35 [17]. Also in 1998 a double effectwater–LiBr absorption chiller was run by a field of nontrackingintegrated compound parabolic concentrators in CA and achieveda solar COP of almost 0.5 [18]. Linear Fresnel and parabolictrough collector systems require single axis tracking mechanismsto concentrate by large factors and thus can achieve temperaturesmuch higher than those needed by a double effect absorptionchiller.

In 2009, researchers from the UC Solar group at University ofCalifornia, Merced built a solar cooling system to assess the feasi-bility of a new type of nontracking solar collector, the externalcompound parabolic concentrator (XCPC), in providing power fora double effect water–LiBr absorption chiller. This is the first sys-tem to combine the XCPC, which is a medium temperature (up to200 �C) nontracking collector, with a double effect chiller. Themain benefit of this collector lies in its ability to reach the operat-ing temperatures required by a double effect LiBr absorptionchiller without the need for tracking. The XCPC collectors andcooling system were designed, constructed, and tested, and theperformance results are presented in this paper.

2 System Description

This solar cooling system was designed, installed, and tested bythe UC solar group at the UC Merced castle research facility. Thesystem is powered by 160 XCPC type trough collectors (53.3 m2),installed facing true south at a 14 deg inclination to provide maxi-mum power during the Merced (37 deg N) summer (see Table 1).As a practical matter, a slight inclination assists in keeping the

Table 1 Collector array specifications

Aperture area 53.3 m2

Orientation North–SouthInclination angle 14 degConcentration 1.18Number of troughs 160Evacuated tube type U-tubeOperating temperature 160–180 �CThermal fluid Duratherm 600Thermal fluid flow rate 3.6 m3/h

041009-2 / Vol. 136, NOVEMBER 2014 Transactions of the ASME


collectors clean. They collect and convert solar energy by heatinga mineral oil (Duratherm 600) which is circulated by a pump.Heat is transferred to an aqueous solution containing 40% ethyl-ene glycol by a heat exchanger (90% efficiency at operating con-ditions) which is then circulated into a 23 kW LiBr–water doubleeffect absorption chiller (see Fig. 1 and Table 2). The intermediateethylene glycol loop was installed because the chiller manufac-turer would not guarantee their warranty if a paraffin-based fluid(mineral oil) was pumped through their chiller. However, theyhave since retracted their issue with paraffin-based substances andin the future an intermediate loop will not be necessary. Water iscooled by the chiller and pumped into a 500 gal (1.9 m3) storagetank. This water is circulated by an auxiliary pump from the tankinto an air handling unit that provides cooling for a small 720 ft2

(67 m2) trailer. A trim cooler was installed on the collector loop toreject heat from the collectors to the environment in case of emer-gencies or overheating.

2.1 The External Compound Parabolic Concentrator. Thecollector configuration is an evacuated tube receiver matched toan external nonimaging reflector, typically referred to as an XCPC(Fig. 2). Figure 3 is an image of the prototype. The XCPC pro-vides solar concentration without moving parts and can achieveoperating temperatures up to 200 �C. The design principle maxi-mizes the probability that radiation starting at the receiver wouldbe directed to a specific band in the sky we wish to accept. In ourcase (north–south orientation) this band is 120 deg in azimuth and180 deg in elevation. This corresponds to a nominal operationaltime period of 8 h per day with a concentration ratio of 1.18 [19].

Inside the evacuated tube receiver is a metallic absorber with aselective coating designed to absorb as much of the solar spectrumas possible while minimizing emission at operating temperatures.The space between the absorber and the glass shell is evacuatedso that convection and conduction are eliminated, and heat istransferred from the hot absorber to the external glass shell onlyby radiation. As a result, these collectors are efficient even at highoperating temperatures.

The XCPCs used in this solar cooling system were designed tooperate facing North–South and to accept light within a 60 deghalf angle. The efficiency of the XCPC over a range of acceptanceangles is presented in Fig. 5. This design was chosen so that theXCPCs would “see” the sun, which moves at approximately15 deg azimuthally every hour, for about 8 h. The selective coat-ing on the absorber had a solar weighted absorptance of 0.902 andan emittance that ranged from 0.05 to 0.14 at 25 and 200 �C,respectively. The troughs were produced locally from ABS plasticsheet and lined with ReflecTech, a silver film with spectral andhemispherical reflectance exceeding 94% [20]. A high optical effi-ciency (71.3%) coupled with the ease of nontracking make theXCPC collector an ideal collector for medium range temperatureapplications (up to 200 �C), the demonstration of which is themotivation for this project.

The operating performance of the XCPC was measured previ-ously at a separate testing loop at the UC Merced castle researchfacility. The measurement of the collector performance as shownin Fig. 4 critically depends on our use of the calorimetric tech-nique. This simultaneously calibrates both the flow rate and heatcapacity of the heat transfer fluid, both of which are prone to vari-ability as the heat transfer fluid (in our case Duratherm) degradesover time. The alternative is to frequently send a sample of theheat transfer fluid to a certified testing lab. However, since theheat transfer fluid properties change from day to day this is not apractical procedure. Flow meters similarly require frequent

Fig. 1 System diagram Fig. 2 Evacuated tube

Fig. 3 XCPC design rendering

Table 2 Heat exchanger specifications

Design duty 21.9 kWFluid 1 Duratherm 600Fluid 1 inlet/outlet temp 180/168 �CFluid 1 flow rate 3.6 m3/hFluid 2 Dowtherm 4000 (40% EG)Fluid 2 inlet/outlet temp 161/170 �CFluid 2 flow rate 2.23 m3/hEfficiency 0.9

Journal of Solar Energy Engineering NOVEMBER 2014, Vol. 136 / 041009-3


calibration, again, by a testing lab. For this reason, with the excep-tion of water collectors, we view testing of thermal collectors thatdo not use the calorimetric technique with skepticism. The effi-ciency in Fig. 4 is normalized to total solar radiation using a preci-sion spectral pyranometer (PSP) on the plane of the collectors. Ifnormalized by the direct solar radiation (DNI) as is customary forconcentrating collectors, the efficiencies are typically 25% higher.

Prior to the building of this solar cooling system, a stagnationtest was performed on a single XCPC collector that was drainedof fluid. Temperature inside the vacuum tube reached a maximumof 290 �C in a little over 1 h during a sunny day with an average of1030 W/m2 global radiation and approximately 20% diffuse frac-tion and an ambient temperature of 25 �C. Although no damage tothe tube was observed, the current XCPC collectors are coveredwhen not in use to prevent any possible damage from long termstagnation. At the writing of this paper, we also are in the processof installing a solar powered pump as a backup to prevent longterm stagnation.

2.2 The Absorption Chiller. The chiller used in this systemis a dual fired double effect water and Lithium Bromide (LiBr)absorption chiller rated with a COP of 1.1 for an output of 23 kW.It has a natural gas burner in its high temperature generator to pro-vide heat when solar energy is inadequate and a built in coolingtower to reject heat to the environment. It has a nominal powerconsumption of 1.8 kW which includes all internal components(pumps, controls, ventilation, cooling tower) and uses about0.06 m3 of water per hour for cooling.

The absorption chiller works by using thermal energy to poweran internal cooling cycle (see Table 3). The cycle starts when ther-mal energy vaporizes water from a dilute solution of water andlithium bromide at high temperatures and pressures in the genera-tor. This water is then condensed by rejecting its heat to a coolingwater loop in the condenser. The condensate water is pumped intoa low pressure chamber causing it to vaporize and absorb heatfrom a water loop and thus provides a cooling effect. This vapor-ized water is then recaptured into solution by a spray of

Fig. 4 XCPC collector efficiency

Fig. 5 XCPC angular acceptance



concentrated LiBr in the absorber. The now dilute solution ofwater and LiBr is then pumped back to the high temperature gen-erator and the cycle repeats (see Fig. 6).

2.3 Sensors and Accuracy. There were a total of 15 sensorsinstalled, including: two electromagnetic flow rate sensors (61% ofreading); one Coriolis Flow Meter (6 0.1%); 9 T-Type thermocou-ples (60.75% of range); 1 K-Type ambient temperature thermocou-ple (62.2 C); and one PSP (62.5%). The measurements collectedfrom these devices were used to characterize system performance.

3 Results

Performance of the cooling system was tested over 2 weeks inAug. 2012 and at various times in late summer and early fall of2011. These tests were performed in both sunny and cloudy condi-tions, and with clean and dirty collectors to gain an understandingof performance in these cases. Figures 7 and 8 depict a typicalday’s performance. Figure 7 shows inlet and outlet temperaturesfor the collectors and chiller (on the chilled water side) as well asthe ambient temperature.

Figure 8 shows the corresponding power provided by the sun,the power collected by the XCPCs, and the cooling power of thechiller. The calculated sun power is the product of total collectoraperture area and the global tilted solar irradiation (GTI), providedby a PSP sensor mounted at the same inclination as the collectors

on the collector frame (thus including the cosine effect). Oilpower is calculated as the product of the flow rate and temperaturedifference across the collectors. Water power is calculated as theproduct of the water flow rate and the temperature difference ofthe chilled water over the chiller. Glycol power typically fol-lowed, but was 1–2 kW lower than the water power output fromthe chiller. The glycol outlet temperature from the heat exchangerwas typically the same as the inlet temperature to the collectors.The time step for data collection was 10 s.

3.1 Performance Tests. During these tests, the collector andintermediate glycol loops were circulated for about 2 h until thetemperature was high enough to drive the double effect absorptionchiller. Then a bypass valve was switched so that the high temper-ature glycol passed through the chiller and provided thermalpower to drive the absorption cycle. No natural gas was used topower the chiller during these tests and as a result, there is a delaybetween when the collectors start losing temperature and when thechiller starts producing cooling. This is due to the time it takes forthe thermal fluid to warm up the high temperature generator insidethe chiller from ambient temperature to the required temperature.

As the system runs, the chiller cools water from a 500 gal stor-age tank. Water from this tank is circulated into an air handlingunit that absorbs heat from inside the trailer. This acts as the cool-ing load on the chiller but during our tests was not enough tomatch the cooling power provided by the chiller. As a result, tem-peratures in the cold water tank decreased until they reached theminimum outlet temperature of the chiller (6 �C). This had theeffect of decreasing the chiller’s COP later in the day because itbecomes increasingly more difficult for the chiller to cool waterthat is already cold [21]. Once the cold water inlet temperaturereaches the minimum limit of the chiller, the chiller cycles off fora short period of time to allow the chilled water to warm up. Thiscycling can be seen in Fig. 7. When the chiller cycles off, no ther-mal energy is removed from the high temperature fluids, causingtemperatures in these loops to increase. This became an issue inthe intermediate glycol loop where high enough temperaturessometimes caused the water in the loop to vaporize which createdenough pressure to stop the pump. The system was shut down

Fig. 6 Double effect absorption chiller schematic

Table 3 Broad micro chiller (BCTZH23) specifications

Cooling capacity 6.6 tons (23 kW)High temperature fluid Dowtherm 4000 (40% EG)HTF inlet 171 �CHTF outlet 161 �CHTF flow rate 2.23 m3/hChilled water inlet 13.9 �CChilled water outlet 7.2 �CChilled water flow rate 2.95 m3/hCOP 1.1Water consumption 0.06 m3/hPower (pumps, controls) 1.8 kW



when either the temperature in the intermediate glycol loopbecame too high or the solar input became too low. Then the hightemperature fluids were circulated through a trim cooler until thetemperatures dropped below 100 �C and the solar input droppedbelow 600 W/m2.

The expected solar COP of the system can be estimated accord-ing to Eq. (6) as the product of the efficiency of the collectors, ef-ficiency of heat transfer from the collectors to the chiller, and theCOP of the chiller

Estimated COPSolar ¼ gcol � gthermal � COPchiller (6)

Using a value of 45% for the collector efficiency based on operat-ing temperatures around 170 �C and an ambient temperature of30 �C and assuming a thermal efficiency of 0.9 (10% of heat cap-tured by collectors is lost on its way to the chiller), the estimatedsolar COP of the system is approximately 0.46. We can rewrite

the thermal COP in terms of its components. Thus, our estimatedthermal COP is approximately 0.99

COPthermal ¼ gthermal � COPchiller (7)

3.2 Typical Performance. Daily values for the collectorefficiency, thermal COP, and solar COP of the system werecalculated from the time the chiller began producing coolinguntil the system was shut down. For the typical performanceday (Figs. 7 and 8), this corresponded to the time periodfrom 11:45 a.m. to 4:20 p.m. Daily collector efficiency wasdetermined according to Eq. (2), by integrating the collectorpower and dividing by the integral of available solar powerover the operational time period. The daily thermal COP wascalculated according to Eq. (3) by integrating the coolingpower and dividing by the integral of collector power overthe operational time period. The total solar COP of the sys-tem was calculated according to Eq. (4), by integrating thecooling power and dividing by the integral of available solarpower over the operational time period.

When the collectors were operated between 150 and 180 �C,they reached instantaneous efficiencies between 34% and 40%.The instantaneous thermal COP and solar COP ranged from 0.769to 1.181 and 0.278 to 0.468, respectively. The maximum output ofthe absorption chiller was about 22 kW, although on average itproduced 15–20 kW of cooling (Table 4).

Values for collector efficiencies were somewhat lower thanexpected. This is likely caused by heat loss in the collectorarray between the temperature sensors used to measure theDTcol. Since this was an educational project, the collectorswere ground mounted and not optimally spaced, requiringmore pipe length in the collector array than anywhere else inthe system. A complex manifold configuration along the col-lector banks (supply and return piping runs, perpendicularvacuum tube connections, and flexible hoses every 10 tubesto accommodate thermal expansion) made it difficult to insu-late all surfaces effectively. These limitations are mitigated ina commercial system where piping runs are minimized andprofessionally insulated. The calculated thermal COP is ingood agreement with the expected thermal COP.

Fig. 7 Typical system performance (Aug. 22, 2012)

Fig. 8 Typical system performance (Aug. 22, 2012)



3.3 Cloudy Day Performance. Performance of the systemwas also measured on two cloudy days. Due to the optics of thedesign, the XCPC collector can collect a certain fraction of diffuselight (1/concentration). The collectors used in this system had aconcentration ratio of 1.18 and thus were able to collect 87% ofdiffuse light. This allows them to collect power on certain types ofcloudy days. Obviously no type of solar collector will work wellon an entirely overcast day but the XCPC, and other nontrackingcollectors such as flat plates, are still able to collect power. This isespecially true on days where clouds are interspersed with sun-shine because clouds act as giant mirrors, amplifying the amountof diffuse light just before crossing the sun (see Fig. 10), and a so-lar collector which accepts diffuse light can take advantage of thiseffect.

The system was run in the same manner on cloudy days as onclear days and no natural gas was used to power the chiller. Whileit is difficult to provide typical data for a cloudy day because ofthe wide variability of cloud cover with time of year and location,the data shown in Figs. 9 and 10 are promising because the collec-tors were still able to maintain temperatures above 140 �C and anappreciable amount cooling was still being produced. It is also im-portant to take note that on cloudy days there is usually less of aneed for space cooling and thus a decreased cooling output may insome cases be completely acceptable. Collector efficiency, ther-mal COP, and solar COP were calculated as before for an opera-tional time period between 11:40 a.m. and 4:30 p.m. and theresults are presented in Table 5. Instantaneous values are pre-sented, but should be regarded with an understanding that the highvariability of solar insolation and the latency of response in thetemperature sensors cause inaccurate calculations on an instanta-neous basis. They are included simply for completeness.

3.4 Collector Cleaning. The efficiency of the solar collectorsis directly affected by the amount of dust that has settled on thereflectors and vacuum tubes. Over time, the accumulation of dustparticles that absorb and scatter sunlight decreases the efficiencyof the collectors. The rate of accumulation depends on a variety ofenvironmental factors tied to the location of the array such aswind speed, vegetation, proximity to farmland, and climate. Thesolar array used in this cooling demonstration was built next to avacant plot of land and an orchard. Prior to taking data, the collectorshad not been cleaned for almost a year, the plot of land next to thecollectors was razed of vegetation, and the orchard was razed andreplanted with new trees. As a result, the collectors themselves wereextremely dirty and the data probably represents a worst case sce-nario. Testing was performed for 4 days with dirty collectors andthen for 4 days after cleaning the collectors. Prior to cleaning, thecollectors operated with an average daily efficiency (according toeach day’s specific operational time period) of 30% and after clean-ing operated with average daily efficiencies of 36.9%.

Figures 11 and 12 present data from a day when the collectorswere dirty, and Figs. 13 and 14 present data after they werecleaned. On August 15th (dirty collectors) the system was run for afull 8 h while on August 20th (clean collectors) the system was runfor about 61=2 h. The difference in run time was caused by our lowcooling load. When the collectors are dirty, they generate lesspower, maintain lower temperatures, and do not provide enoughcooling to overpower our cooling load. As a result we were able todemonstrate the full 8 h solar collection window (21=2 h warm-up,51=2 h run-time). However, once the collectors were cleaned, theywarmed up 30 min faster, maintained much higher temperatures,and quickly overpowered our cooling load causing a low chillerCOP and chiller cycling (Fig. 13). As mentioned before, this forced

Table 4 Results and definitions—Typical system performance

Range (instantaneous) Daily average

Collector efficiency Thermal power captured per available solar power 34.5–40.6% 36.7%Thermal COP Cooling power per captured collector power 0.769–1.181 0.990Solar COP Cooling power per available solar power 0.278–0.468 0.363

Fig. 9 System performance under conditions (Aug. 21, 2012)



Fig. 10 System performance under cloudy conditions (Aug. 21, 2012)

Table 5 Results and definitions—Cloudy day performance

Range (instantaneous) Daily average

Collector efficiency Thermal power captured per available solar power 19.6–64.1% 35.5%Thermal COP Cooling power per captured collector power 0.617–2.236 1.019Solar COP Cooling power per available solar power 0.204–0.886 0.362

Fig. 11 System performance with dirty collectors (Aug. 15, 2012)



us to shut down the system due to risk of vaporization in our inter-mediate water–glycol loop. Our inability to run for a full 8 h withclean collectors was simply due to our undersized cooling load andwould not be an issue in a properly sized system.

3.5 Warm-Up Time. The warm-up time of the system isdirectly related to the total mass of piping, valves, pumps, andfluid that need to be heated from ambient to operating tempera-tures. In 2011, the system included a large 50 gal (0.19 m3)

thermal storage tank with the purpose of evening out fluctuationsin solar radiation. This greatly increased the total mass of fluid inthe system and increased the system warm-up time by approxi-mately 1 h (see Figs. 15 and 16). Because the collectors have an8 h collection window, any increase in warm-up time cuts intopower production time and should be minimized. Therefore, it isimportant to design a system with a small heat capacity in thehigh temperature loops by minimizing pipe diameters, lengths,and fluid volume.

Fig. 12 System performance with dirty collectors (Aug. 15, 2012)

Fig. 13 System performance with clean collectors (Aug. 20, 2012)



During the 2011 tests, the chiller was powered by naturalgas during the system warm-up period and then switched tosolar power once operating temperatures were reached. As aresult, we were able to demonstrate the benefits of a hybridsolar powered cooling system by producing a seamless outputof cooling power from both natural gas and solar powerinputs. At the same time, the air handling unit on the chilledwater loop was not connected to the trailer, but was insteaddumping cooling power to the outdoors. This acted as amuch larger load on the chiller and prevented the chilled

water loop from reaching temperatures close to the minimumallowed by the chiller. As a result, we were able to maintaina much more stable cooling output and chiller COP.

3.6 Cooling Window. The XCPCs collect solar energy for8 h out of the day. With an average warm-up time of 2 h,this leaves 6 h of direct solar powered cooling. Depending onthe location this is approximately from 11 a.m. to 5 p.m.However, the typical cooling load lags behind the available

Fig. 14 System performance with clean collectors (Aug. 20, 2012)

Fig. 15 System performance with 50 gal storage tank and natural gas powered chiller(Sept. 23, 2011)



solar energy by a couple of hours and in some locations,space cooling may be needed before or after the direct solarpowered cooling window. Typical meteorological year (TMY)data was used to determine hourly cooling needs throughoutthe year for Merced, CA. A hybrid chiller can provide cool-ing from natural gas (Fig. 17) in the morning during thewarm-up time of the collectors (see Figs. 15 and 16) andcold storage can lengthen the cooling window into the lateafternoon and evening. The use of cold storage is recom-mended over hot storage due to a smaller temperature differ-ence with the ambient and therefore less heat loss (or gain).An oversized collector array can provide direct solar poweredcooling while also cooling a water storage tank. When thesystem no longer collects solar power, the cooled storagetank can then be circulated to provide additional cooling intothe evening. The storage tank and collector array can be sizedto provide the required duration of cooling.

3.7 Electrical Power Consumption and Parasitic Loss.Parasitic losses occur due to the fact that it takes power (pumps,chiller, fans) to produce cooling power in a solar cooling system.None of the electrical components in the system were monitoredfor electrical consumption, but a rough estimation can be calcu-lated based on the rated power consumption for the importantcomponents and their operating capacities (Table 6). The parasiticsolar COP is calculated according to Eq. (5). For the typical sys-tem performance day (Aug. 22, 2012), the chiller produced an av-erage of 16.25 kW cooling, corresponding to a cooling capacity of70% and an electrical consumption of 80% [21] of the ratedpower. The collector pump (1.5 hp) was oversized and at our flowrate and pressure operated at 63% capacity. The solar cooling sys-tem produced 76 kW h of cooling from 209 kW h of available so-lar energy over the operational time period. Total electricalconsumption was calculated accounting for a 2 h warm-up periodduring which the collector pump was running, and for a 5 h

Fig. 16 System performance with 50 gal storage tank and natural gas powered chiller(Sept. 23, 2011)

Fig. 17 Hourly cooling needs in Merced, CA based on TMY data



operational period of the chiller and collector pump, and totaled at12.17 kW h. None of the actual electrical usage was measured andthis is only provided as a rough estimate of electrical performanceof the system.

3.8 Economic Evaluation. A simple economic evaluation ofthe system was performed using commercially available materials(Table 7). Excluding the chiller, the most expensive componentswere the metal–glass evacuated tubes, the collector frame (thatholds the XCPCS in position), and manifolds (evacuated tube topiping connections).

For this demonstration project, the XCPC reflectors weremolded out of plastic and lined with a reflective film by hand.This is not an economic approach and would not be the method ofchoice for a commercially produced system. Reflector price wasestimated for a polished aluminum that is bent into the required

XCPC shape by a machine. The total cost per square meter of theXCPC collector system, including piping, insulation, pumps, andother miscellaneous equipment often referred to as the balance ofsystem, came to be about $270/m2. For an average insolation of1000 W/m2 and collector efficiency of approximately 37% (asachieved by this demonstration project), the cost of the collectorsand balance of system is about $0.72/W. Prices for double effectabsorption chillers range from $400/kW to $1000/kW dependingon the size. The target investment price, based on a large systemto make the absorption chiller price more economical, is approxi-mately $1.12/W.

4 Conclusion

In this paper, the design and performance of a solar cooling sys-tem using a new type of concentrator is presented and analyzed.The XCPC collectors, when operated in the temperature range of150–180 �C, achieved daily efficiencies of 36.7% with instantane-ous efficiencies up to 40%. The double effect absorption chillermaintained a COP between 0.97 and 1.14 and the thermal COP ofthe system averaged at 1.0. The solar COP of the entire systemwas 0.363 with very little variation. This is the first system of itskind to combine XCPC technology with double effect absorptionchilling technology. The purpose of this demonstration was toprove the XCPC’s competency at providing thermal power forspace cooling and in this sense it was a success. The XCPC (Fig.18) operates at efficiencies comparable to those reached by track-ing type solar collectors and the power production window coin-cides nicely with the desired window for space cooling. At thesame time, the XCPC is able to collect diffuse light, allowing forcooling during cloudy or hazy days, which is especially importantin regions with high diffuse percentage. The effects of dust settling onefficiency is briefly investigated and presented and a discussion onthe importance of designing a system with a low thermal mass todecrease warm-up times is presented. A discussion of parasitic lossesand a simple economic analysis are also presented for completion.Ultimately, while more research needs to be done in the areas of longterm and yearly performance, integration with building loads, and sys-tem level optimization, this project has successfully demonstrated theuse of the XCPC in providing thermal power for space cooling appli-cations. At this writing, the XCPC technology is currently being com-mercialized in the U.S. and India. The XCPC’s numerous potentialapplications include solar heating, cooling, desalination, oil extraction,electricity generation, and food processing.

Acknowledgment

The development of the XCPC collectors was supported in partby the California Energy Commission and this solar cooling pro-ject was supported in part by the California Community Founda-tion. Many UC Merced students and staff helped with this project.We particularly wish to acknowledge Kevin Balkoski, ChrisThomas, Tara Backman, Guenter Fischer, Bruce Johnston, andBrent Jerner.

Nomenclature

Acol ¼ solar collector inlet aperture area (m2)Cp ¼ specific heat capacity (J/kg K)I� ¼ solar irradiance (W/m2)_m ¼ mass flow rate (kg/s)

Q ¼ thermal power (W)

Fig. 18 XCPC array at UC Merced castle research facility

Table 6 Electrical consumption

Operational solar (kW h) Available solar energy during operational period 209.06Operational cooling (kW h) Cooling produced during operational period 75.98Total electrical (kW h) Total electrical consumption 12.17Solar COP (thermal) Solar COP considering only thermal terms 0.36Solar COP (parasitic) Solar COP including parasitic losses 0.34

Table 7 Price estimation

Cost per square meter ($/m2)

Reflector 18.27Frame 75.00Manifold 40.32Fluid 11.00Tubes 75.48Manifold insulation 5.68

Collector module 225.75

Pumps 4.16Pipe 4.58Pipe insulation 2.92Valves 2.67Controls 1.00Miscellaneous balance of system 2.00Engineering 14.26Installation 12.27

System total 269.61



T ¼ temperature ( �C)W ¼ work (W)

Greek Symbol

g ¼ efficiencySubscripts

chl ¼ chilled watercol ¼ collectors

Abbreviations

COP ¼ coefficient of performanceDNI ¼ direct normal irradianceEVT ¼ evacuated tube collectorFPC ¼ flat plate collectorGTI ¼ global tilted irradiancePSP ¼ precision spectral pyranometer

XCPC ¼ external compound parabolic concentrator

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Bennett Widyolar Performance of the Merced Demonstration