journal paper on natural converction

Lin ChenDepartment of Energy and

Resources Engineering,

College of Engineering,

Peking University,

Beijing 100871, China

Xin-Rong Zhang1Department of Energy and

Resources Engineering,

College of Engineering,

Peking University,

Beijing 100871, China;

Beijing Key Laboratory for Solid Waste

Utilization and Management,

Peking University,

Beijing 100871, China

e-mail: [email protected]

Experiments on NaturalConvective Solar ThermalAchieved by SupercriticalCO2/dimethyl ether Mixture FluidThe current study proposed an experimental investigation into the basic characteristicsof solar thermal conversion using supercritical CO2dimethyl ether (DME) natural con-vection. The main goals are to reduce the operation pressure while maintaining relativehigh solar thermal conversion efficiency. Experimental systems were established andtested in Shaoxing area (around N 30.0 deg, E 120.6 deg) of Zhejiang Province, China.Due to the preferable properties of supercritical fluids, very high Reynolds number natu-ral convective flow can be achieved. Typical summer day results are presented and ana-lyzed into detail in this paper. It is found that the introduction of DME has successfullyreduced the operation pressure and the increase in DME fraction leads to further reduc-tion. Different from pure supercritical CO2 systems, the collector pressure follows thetrend of solar radiation with its peak value at noon, instead of continuously increasingmode. The mass flow rate and temperature are typically more stable and also more sensi-tive than pure supercritical CO2 tests due to the moderation of supercritical fluid proper-ties when DME is introduced. At the same time, the averaged collector efficiency is lessaffected by the DME mass addition. It is also found that there possibly exist some optimalof DME mass fraction when both the system suitability and stable natural circulation canbe achieved. [DOI: 10.1115/1.4026920]

Keywords: solar thermal energy, supercritical fluid, natural convection, experiment, car-bon dioxide, DME

1 Introduction

Solar thermal conversion has become one of the most promis-ing choices for convenient utilization of solar energy. As oneexhaustless kind of renewable energy on earth, more and moreattention has been attracted into the various methods/systems ofsolar thermal utilization, like its concentration, conversion,upgrade/storage, and transportation, etc. Those solar thermalsystems are generally categorized into low-temperature systemswithout sunlight concentration, medium-temperature, and high-temperature systems. Low-temperature solar thermal systems ownthe potential to supply general households and commercial unitswith simple heating and/or cooling capacities. Among those, solarwater heater is one of the most wanted and promising kinds inmarket [17]. It is reported that both in developed and developingcountries, solar water heating technology can be one economicalchoice for its simplicity as a renewable source and has gained avast market around the world [13].

In recent years, a lot of studies have been carried out in thesolar thermal field and the main focus laid on the absorberplate design, selective coatings, thermal insulation, tilt angle ofcollector, working fluids, specific geometric design, etc. [711].Status reviews [1,4,5] and detailed studies [1217] on the devel-opment in solar water heating technologies have also come out inrecent years. The majority of them focused on both solar thermalconversion cycles and the economics of them. In addition, specialattention of solar thermal conversion systems is paid to solar

water heaters of water-in-glass evacuated tube type [12,14,16,17],where relative higher collecting efficiency are generally foundwith proper coating strategies and evacuated tube design/manufacturing.

In the above mentioned reviews, circulation mode and workingfluid are both very important factors for flow friction factors andheat exchanger efficiency in the conversion system. Pumping isneeded in water based solar thermal conversion because naturalconvection of water fluid is relatively weak with low circulationrate and conversion efficiency. In recent years, as one substituteworking fluid, supercritical carbon dioxide has been proposed inenergy conversion (including solar thermal) systems [68,1822].As one old working fluid, CO2 is nontoxic and with negligibleozone layer depletion (ODP) and global warming potential, it hasalso been used in air conditioning and marine refrigeration sinceearly 20th century but later replaced by chloro-fluoro-carbon(CFC) or hydro-chloro-fluoro-carbon (HCFC). In recent years,CO2 is put forward again due to the severe environment impacts/problems of CFC/HCFCs. CO2 fluid has its advantages as a natu-ral working fluid: when it goes near the critical point(TC 304.13 K and PC 7.38 MPa), the thermal-physical proper-ties experience dramatic changes even the temperature/pressurechange only a little, especially for density and specific heat, whichcan be seen in Fig. 1 (data obtained from NIST Standard Refer-ence Database-REFPROP, Version 8.0.). Large density changescan enhance the natural circulation potential and the high specificheat changes can introduce high heat capacity in the evacuatedtube collector flow [2023]. Such properties are rarely seen in tra-ditional or natural working fluids and can help avoid the problemsrelated as well as improvement of system efficiency [510]. Fur-thermore, for supercritical CO2, its critical temperature is 31.1

C;

1Corresponding author.Contributed by the Solar Energy Division of ASME for publication in the

JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received October 15, 2012; finalmanuscript received January 17, 2014; published online March 6, 2014. Editor:Gilles Flamant.

Journal of Solar Energy Engineering AUGUST 2014, Vol. 136 / 031011-1CopyrightVC 2014 by ASME

Downloaded From: http://solarenergyengineering.asmedigitalcollection.asme.org/ on 04/29/2014 Terms of Use: http://asme.org/terms

and that is low enough to be easily reached in the low-temperaturesolar thermal conversion system.

In previous studies, natural convective kind of solar collectorsystems and related energy conversion systems [68,18,19,23]which utilized supercritical CO2 fluid have been established andtested under some representative conditions. Such kind of naturalcirculation based solar collector is less studied and/or comparedwith forced convection kind [1,5]. The basic design of solar waterheater using supercritical fluid comprised a supercritical fluid loopand a water loop, which are coupled with each other by a heatexchanger. In the closed loop with supercritical fluid, the naturalconvective flow develops as a result of heating by solar radiationand the cooling by the heat exchanging with water side. Accordingto the first series tests in Yokohama region in Japan and newlydeveloped systems in Zhejiang Province (around 30.0 N, 120.6 E)of China [8,18,19], supercritical systems are different from watersystems: supercritical fluid state was achieved at the collector outletwith high Reynolds flow (Re Number around 1900) [8]; later the ba-sic system characteristics were experimentally investigated [18,19]and found supercritical system pressure is nonstable during opera-tion, its flow rate is less sensitive to solar radiation changes and bet-ter system performance is found in spring and winter whilegenerally high solar thermal conversion efficiency (averaged above60%) than water system was found across four seasons.

However, one major concern of the current kind supercritical solarthermal conversion is the high operation pressure, which may bringabout extra cost of manufacturing and maintenance. In the current ex-perimental study, DME is chosen to premix with supercritical CO2with proper volume fraction, attempting to reduce/adjust the operationpressure and test the system performance of such kind of mixtureworking fluids. Indeed, DME is also one substitute working fluid ofCFCs/HCFCs with its preferable compression/condensation proper-ties. DME fluid is also of less corrosive and has good miscibility withother polar/nonpolar fluid (like CO2) but it is very flammable and ex-plosive. In recent years, some groups have tried to mix DME withCO2 as working fluid to reduce the high supercritical CO2 operationpressure and in turn the CO2 fluid help reduce the flammability ofDME [24]. For example, Koyama et al. [25] experimentally studiedthe behaviors of CO2DME (with 10% DME) mixture in heat pump

systems, where high standard coefficient of performance (COP) wasmaintained and considerably lower discharge pressure was achieved.Later, Onaka et al. [26] reported that besides operation pressurereduction, CO2DME mixture based heat pump COP increases withDME mass fraction (in the range of 0%10% in their analysis).Indeed, the thermal-physical properties of refrigerant can be differentwhen DME is introduced. The above studies referred to the increasein thermal conductivity and viscosity of CO2DME mixture as shownin Fig. 1 (data obtained from NIST reference package of fluid thermo-dynamic and transport properties REFPROP version 8.0; in Fig. 1pure CO2 and pure DME fluid properties are also plotted for compari-son), which may play an important role in respective systems whencompared with pure CO2 or pure DME single fluid system.

The present study is one continuation of our previous experi-mental studies of transcritical experimental study on CO2 solarwater heater [18,19] and preliminary natural experimental proto-type solar water heater [8,18,19]. In the present study, DME isalso proposed to be mixed into CO2 fluid within the explosivefraction limit thus to take advantages of both fluids. Respectiveexperimental systems are designed to investigate the systembehaviors when CO2DME mixture fluid is used in solar thermalconversion. The following parts will introduce the basic systemdesign and system operation process; then the basic results on thepressure reduction effect of DME introduction; after that the sys-tem features/parameters are compared with previous pure CO2based tests and the seasonal efficiency, feasibility study as well asfuture development are also discussed into detail in this paper.

2 Experimental Set-Up and System Operation

2.1 Basic Solar Thermal Conversion System Design. Inprevious theoretical and experimental tests, several experimentalset-ups of the solar to thermal collection are established and oper-ated [8,18,19]. Though the first stage experiments on supercriticalfluid based natural convective solar thermal were not optimalones, good seasonal performance and application feasibility havebeen identified. Due to the potential of high Reynolds natural con-vective flow of supercritical fluids, related systems have shown

Fig. 1 Variation of thermophysical properties of the CO2DME mixture refrigerant with dif-ferent DME mass fractions at 8.0 MPa. (a) Density; (b) specific heat; (c) thermal conductivity;and (d)viscosity.

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advantages over the traditional ones. The present study is focusedon the exploration of system behaviors when DME is mixed withsupercritical CO2 as working fluid, trying to reduce the operationpressure and change the unstable pressure behaviors of puresupercritical CO2 system [8,18,19]. Following the previous trialsof mixture fluid in supercritical states operated in related energyconversion systems [24,26], the current system is designed forusing supercritical CO2DME mixture fluid natural convection. Aschematic diagram of the experimental set-up is shown in Fig. 2.Similar with previous supercritical systems, the new model hasno pumping devices and is mainly consisted of three parts: thesupercritical CO2DME mixture based solar collector circulationsystem and the water heating and control/data collecting system.The supercritical CO2 natural collector side has two parts: (1)evacuated tube solar collector panel; (2) heat exchanger for super-critical CO2DME and water loop. The built experimental controlsystem comprised three flow valves (13) and measurement anddata acquisition system. Other control parts include pressure con-trol valves mounted between the heat exchanger outlet and evac-uated tube collector inlet, and in inlet and outlet of water loop,valves are also mounted specifically and connected to the hotwater tank.

The design of collector panel is one critical part in solar thermalconversion systems. In the collector panel, solar energy is trans-ferred to heat contained in supercritical CO2DME mixture andthe mixture fluid itself should be heated into supercritical state inorder to sustain high natural circulation flow in the loop. To sup-port this working fluid characteristics and the demand of highenergy conversion efficiency, the collector panel is carefullydesigned and tested. In the experimental set-up, all-glass evac-uated solar collector with a U-tube heat removal system is used.The collector consists of a glass envelope over a glass tube coatedwith a selective solar absorber coating. On the vacuum side of theinner glass tube, selective coating is achieved with a high solar ab-sorbance of 0.918 and a low emissivity of 0.189. The transparenceof the glass envelope is 0.92. The current design of the collectorcan sustain a maximum operating pressure of 15.0 MPa, which issufficiently safe for the current supercritical CO2DME mixtureoperation range. In the present set-up, evacuated solar collectorof 1.50 m2 (gross area) is used and it is manufactured basicallybased on the previous ones using pure supercritical CO2 naturalconvection [23,27,28]. The every U-shaped heat removal fluidtube utilized is 3.5 m long and 0.006 m internal diameter. Otherdetails of the experimental system can be found in previous solarthermal conversion systems with supercritical CO2 fluid [8,18,19].Related system design and tests experiences in supercritical CO2fluid systems [18,19,2932] should be referred as they may

provide both experimental and theoretical information for the con-venience of novel system development and manufacturing.

The heat exchanger between supercritical CO2DME side andwater side is one traditional concentric tube counterflow kind withsupercritical CO2DME flows in the inner pipe circulated bywater counterflow in outer pipe. The inner pipe is made of stain-less steel with internal diameter of 6.0 mm and wall thickness of1.0 mm. The internal diameter of outer pipe is 22.0 mm with wallthickness of 1.0 mm. Total heat exchange area here is estimated tobe 1.0 m2 [19]. This structure is maintained from previous studiedwith good performance [8,18,19]. After the heat exchanger, thehot water flows into a hot water tank, a valve is set at the tank tocontrol the water flow as shown in Fig. 2.

2.2 System Operation and Data Collecting. Previous stud-ies showed that supercritical loop flow is suspect to oscillations orunstable operation [18,19,2729]. To avoid this, the current loopsystem is specially designed and tested according to our previousnumerical and experimental studies [18,19,2729,33,34]. Besides,for positive controlling of system, several pressure valves andflow valves are mounted in the flow loop, as shown in Fig. 2. Inthe supercritical fluid circulation loop, valve 1 is operated byhand. Valve 1 can be used to adjust flow rate of the CO2DMEfluid and in the current study as the basic response of CO2DMEmixture fluid to solar radiations is focused, valve is only used tocontrol the maximum flow rate (but indeed it can be used to con-trol the flow rate of CO2DME side and thus as one method tocontrol the system [611]). As seen in Fig. 2, a hand-operationvalve 2 is also installed in the water loop, which is self-adjustedaccording to heat exchanger outlet water temperature. In the waterloop, hot water from the outlet of the heat exchanger flows into ahot water tank (made of stainless steel and its volume is 80 l). Onehand-operated valve 3 is also set at the outlet of the hot watertank. A mass flow meter was used to measure the mass flow ofCO2DME mixture, which has a maximum operating pressure of15.0 MPa. The flow meter was installed in the downstream of theheat exchanger in the supercritical fluid loop, as shown in Fig. 2.It provides a measurement range from 0.02 kg/min to 5.0 kg/minwith an accuracy of 60.1%.

In data collecting system, two T-type thermocouples and twopressure transmitters are, respectively, mounted at the inlet andoutlet of the solar collector to measure CO2DME temperaturesand pressures, with an accuracy of 60.1 C for temperature meas-urements and 60.2% for pressure measurements. Two platinumresistor temperature sensors are also mounted to measure the inletand outlet water temperatures of the heat exchanger. Its accuracycan be expressed as 60.15 0.0002 t C. Thermal insulation coat-ing is applied on all the CO2DME and water loops to reduce heatlosses from the loops. A meteorological measurement system wasalso installed, which mainly comprised sun radiation sensor, ane-mometer, and air temperature gauge, and so on. In the presentstudy, measured solar radiation data are presented and discussedtogether with the system parameters. This pyranometer is basedon a thermopile sensor and typically the pyranometer outputsignal does not exceed 20 mV, and sensitivity is around10lV/(W/m2), thus to provide an accuracy of 60.3% for solarradiation measurement. Also, here the measured solar radiationvalue is the global horizontal solar irradiation value, and forexperiments with collector panel inclination angles, the radiationvalue is adapted to the global radiation value for respectiveinclination angles, which process is used and calibrated as generalbasis for comparison and discussion in this study [8,18,19]. Inaddition, a data acquisition system is used in the experimentalset-up, which can be used to obtain real-time data measurement,acquisition, processing, and share.

In the experiment test, the solar collector is first adjusted towardthe direction of the Sun. Then premixed CO2DME mixture isinjected into the circulation loop. After that cool water flow iscirculated into the heat exchanger, the experiment system is then

Fig. 2 Schematic diagram of the supercritical CO2DMEnatural convection based solar water heater

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started. It should be noted that in the current design, no pumpingdevices are used and only natural circulation mode is used. Duringthe experiment, the two major sources of data uncertainties are (1)temperature, pressure, and flow rate measurement accuracy; and(2) errors resulting from data logging and reading by the com-puter. The error of data logging and reading are analyzed and theirvalues are listed as follows: 6 0.1 C for CO2DME temperature;60.2% for CO2DME pressure; 60.2% for CO2DME mass flowrate; 60.2 C for water temperature; 60.2% for water flow rate;60.2% for solar radiation. Based on the above accuracies, accura-cies of all the parameters defined in this paper are calculated to beless than 62.0% using the following equation:

x x21 x22 x2n

n

r

Xni1

x2i

n

vuuut(1)

where x is the average of the parameter x; xi is accuracy of everyparameter involved in the calculation equation of x; and n is thenumber of the parameters in the calculation equation of x. Otherdetails and processing methods are similar with previous studieswith pure supercritical CO2 tests [8,18,19].

3 Results and Discussion

The solar collector is set declined, with an angle of 45 deg withrespect to the horizontal level in the present study, which indicatesthe U-tubes in the collector has an angle of 45 deg from the hori-zontal level. Also, the measured solar radiation data are alsoadapted to the global radiation value for 45 deg inclination anglecollector panel in the data collecting and analysis in this study[8,18,19]. During the experiments, DME mass fraction is set at10% and 20% without reaching the explosive limit of DME fluid[25,26]. The current experimental tests will also be comparedwith pure supercritical CO2 tests (namely, DME mass fraction is0%) in the same system. The CO2DME charge amount is 3.70 kgand 3.77 kg for tests with 10% DME and 20% DME in theexperiments, respectively. The following parameters are mainlymeasured: the solar radiation, the CO2DME fluid temperatures atthe inlet and outlet of the heat exchanger, the CO2DME fluidpressures at the inlet and outlet of the solar collector, and the massflow rate of CO2DME mixture. Based on those measured quanti-ties, the following parameters are defined to further discuss thecollecting efficiency, which evaluates the solar thermal energyconversion performance

C Tf TaI

(2)

QC _mCO2DMEhCO hCI (3)

gC QCIA

(4)

In the above equations, C is a parameter that describes the ambi-ent and solar radiation conditions; QC is the heat collected;_mCO2DME is the mass flow rate of CO2/DME mixture flow; hCI

and hCO are the inlet and outlet enthalpy values of the fluid,respectively; gC is the defined collector efficiency; I is the solarradiation; and A is the effective collecting area. It also should benoted here that the current study is the first trial to lower down theoperation pressure of supercritical CO2/DME solar thermal con-version systems. More detailed theoretical analysis and relatedmodelizations can be found in pure supercritical CO2 studies[68,18,19]. For the current development, we only focus on theeffect of DME addition on the basic solar conversion efficiencyand operation pressure.

In this paper, CO2DME enthalpy values at the different moni-toring points were calculated based the measured temperature andpressure values using a program package for thermophysical

properties of fluids: NIST Reference of Fluid Thermodynamic andTransport Properties (REFPROP 8.0). The experimental set-upwas tested during the summer season of 2010 in Shaoxingi area(around N 30.0 deg, E 120.6 deg), Zhejiang Province of China. Asthe focus of current experimental tests is the effect of CO2DMEmixture on system pressure behavior and solar thermal conversionfeasibility, optimized structure and operation are not claimed. InSecs. 3.13.3, we present new behaviors through analysis of rep-resentative operation curves as well as gross parameters, wherevery different system behaviors from pure fluid cases are found insupercritical CO2DME based solar thermal conversion.

3.1 Transient Characteristics of Solar ThermalConversion System

3.1.1 Solar Radiation and Pressure Behaviors With 10%DME Tests. The operation pressure in previous supercritical solarcollector showed very high value and unstable behaviors. As thefirst target of the present experiments with CO2DME mixture,the pressure behaviors along with different solar radiation condi-tions are analyzed. The collector panel inlet and outlet pressuresare plotted against test time in Fig. 3. Also, plotted is the respec-tive variation of solar radiation during the tests. In Fig. 3, two typ-ical tests during the summer season of 2010 are shown,respectively, for July 8, 2010 (Fig. 3(a)) and Aug. 9, 2010(Fig. 3(b)). For comparison and convenience of feasibility discus-sion of current system, the test date of Fig. 3(a) was sunny and thedate of Fig. 3(b) was cloudy. The total amount of CO2DME mix-ture injected into the system was 3.70 kg, which contained 10%mass fraction of DME for the above tests. The averaged ambienttemperatures were around 27.5 C for July 8, 2010 and 25.4 C forAug. 9, 2010, which were, respectively, monitored during eachexperiment days.

It is seen in Fig. 3 that the general trend of pressure variationwith time is quite different between the two tests. For the sunnyday of Fig. 3(a), the solar radiation is around 600 W/m2, while in

Fig. 3 Variations of the measured solar radiation and fluidpressures with test time. The mass fraction of DME is 10%. (a)July 8, 2010 and (b) Aug. 9, 2010.



the cloudy day of Fig. 3(b), it is only around 400 W/m2. Both ofthe above radiation conditions are typical for the weather in themoderate-to-high latitude regions. The most interesting informa-tion from Fig. 3 is that in Figs. 3(a) and 3(b), the pressures seemto follow the general trend of solar radiation. Such pressurebehaviors are very different from pure supercritical CO2 basedsolar collectors studied, where the supercritical CO2 pressure allshow monotone increasing with test time, regardless of solar radi-ation [8,18,19]. The present CO2DME system shows possiblecorrelation between solar radiation and operation pressure. Asshown in Fig. 3(a), the test on July 8, 2010 was measured from11:00 to 17:00 and after the initial state of heat accumulation from11:00 to 12:00, the collector pressure began to increase along withthat of solar radiation. In previous theoretical and experimentalstudies, such heat accumulation period was also identified and thesudden increase in system parameters is typical for supercriticalfluids [8,18,19,2729]. In the afternoon, the solar radiation beganto decrease gradually from 13:30 to 17:00, the collector pressuresalso showed gradual decrease. As discussed above, comparingwith the pressure monotone increase mode for pure supercriticalCO2 based system [8,18,19], the current supercritical CO2DMEmixture fluid brings about pressure dependence and thus it mayhelp reduce the overall system operation pressure. Indeed, thecontinuous increasing mode in pure CO2 system may not be safeafter a long days operation as in the late afternoon, the operationpressure is at the peak values, while for current CO2DME tests,the peak is found at noon.

Another feature for the current system is that the delay of pres-sure changes according to solar radiation. As shown in Figs. 3(a)and 3(b), the variations of pressure curves are later than typicalvariations of solar radiation. For example, in Fig. 3(a), when thesolar radiation becomes very low around 15:30, the smooth pres-sure drops are seen after 16:00, and when the solar radiation isback at regular value around 16:00, the pressure increase happensaround 16:30. Similar pressure variation trend can be also seen inFig. 3(b). From Figs. 3(a) and 3(b), it is also seen that the opera-tion pressure for supercritical CO2DME mixture can be main-tained at a certain level for a relative long operation time, like inFig. 3(a), the pressure is around 9.2 MPa and in Fig. 3(b), it isaround 10.0 MPa. It is unusual for solar conversion system tosustain relative low operation pressure under relative high solarradiations as shown in Fig. 3, which indicates the advantageousof supercritical CO2DME mixture in the natural circulationloops. Also, compared with pure CO2 tests [8,18,19], it has beensuggested that this phenomenon may contribute to a stable main-tenance of real solar thermal system.

As the pressure effect of supercritical CO2DME mixture is thecore topic in the present study, it can be also interesting to com-pare the collector inlet and outlet pressure changes shown inFig. 3 with previous pure CO2 studies. From Fig. 3, the collectorinlet and outlet CO2DME mixture pressure differences are verysmall (indeed less than 0.1% of the operation pressure), while thepure CO2 tests show relative larger differences (around 1% of theoperation pressure). The above result indicates that CO2DMEmixture has smaller pressure loss when flowing through the col-lector panel than pure CO2 tests. This characteristic is partly dueto the relative higher thermal conductivity and moderate densityvariation curves as shown in Fig. 1, which may enhance the heattransportation from collector panel to the CO2DME mixture andreduce the thermal expansion and self-acceleration of refrigerant.Therefore, the friction losses across the collector panel can bereduced while the mixture viscosity is still increased comparewith that of pure CO2 fluid as shown in Fig. 1(d).

3.1.2 Supercritical Mixture Mass Flow Characteristics With10% DME Tests. As discussed in Sec. 3.1.1, the typical changesin CO2DME mixture properties compared with pure CO2 can bevery important for the current solar thermal system behaviors. Inparticular, the mass flow rate, the solar thermal system, can induceis another important target besides operation pressure control. For

convenience of comparison, the supercritical CO2DME mixturemass flow rate and the variations of solar radiation are plottedagainst test time in Fig. 4. The data in Figs. 4(a) and 4(b) aremonitored during the same date and same test with those shown inFigs. 3(a) (typical sunny summer day) and 3(b) (typical cloudysummer day), respectively.

In Fig. 4, the supercritical CO2DME mixture mass flow rate isrelatively higher than previous water based system [1,5,8] and itshows different variation trends from that of pressure. Instead ofcochange mode with solar radiation found for pressure curves inFig. 3, the mass flow rates of CO2DME mixture exhibit generalstable trend. Besides the initial heat accumulation periods andovershoots in the beginning that have also been identified in previ-ous studies [8,18,19], the CO2DME mixture flow rates tend toshow much smaller fluctuations after some level of flow rate isreached, as shown in Fig. 4. In previous pure supercritical CO2tests, the flow rate changes are summarized to have several changemodes according to the amplitude of radiation fluctuations[8,18,19]: if the radiation fluctuation is slow and smooth, the flowrate follows with slow and smooth curve; if the radiation fluctua-tion is quick and small, the flow rate is less affected; if the radia-tion fluctuation is quick and lasting, the flow rate follows a similartrend but with less fluctuation. In the current tests with CO2DMEmixture, the condition is found to be different. As shown inFig. 4(a), from 12:00 to 13:00, the mass flow rate can be main-tained around 10.0 kg/h and from 13:30 to 16:00, it is around12.5 kg/h, which is different from pure supercritical CO2 tests,where the flow rate is not so well sustained [8,18,19]. Also, therelatively high flow rate in this study is maintained when com-pared with previous systems [8,18,19]. And it is due to the strongnatural convection of supercritical CO2DME mixture fluid thathigh circulation rate and compact systems can also be achievedwhen flowing across collector panels, thus to win its potentialadvantages over traditional collectors [23].

Fig. 4 Variations of the measured solar radiation and massflow rate with the test time. The mass fraction of DME is 10%.(a) July 8, 2010 and (b) Aug. 9, 2010.



Similar behaviors can also be seen in the cloudy day curves asshown in Fig. 4(b). In Fig. 4(b), though the solar radiation fluctu-ates a lot in the afternoon, the flow rate is less affected and main-tained around 7.0 kg/h10.0 kg/h and changed gradually.However, when the very small fluctuations of CO2DME massflow rates are examined, it can be found that the mass flowchanges are very sensitive to the fluctuations of solar radiation,which is not found for pure CO2 tests [8,18,19]. This new trendfor supercritical CO2DME mixture can be explained as thatthe effect of increased heat transportation by increase in heat con-ductivity when DME is introduced. Therefore, the temperaturegradient in the heat transfer boundary layers is reduced and moresmooth properties curves can be established across the bounda-ries to reduce the fluctuations from unstable heat transfer. Theabove effect of properties can be of critical importance for super-critical natural circulation systems, which is also identifiedand explained in previous studies [8,18,19,2729]. For the currentCO2DME mixture, the viscosity changes across the low Reyn-olds boundary and the relative high Reynolds main flow can affectthe stability of viscous boundary contact layer and nonequilibriumhot/cool pockets are formed due to unstable boundary heat trans-fer. In addition, the general increase in CO2DME mixture viscos-ity also contribute to the stability of collector fluid flow while theflow rates still left sensitive to ambient radiation changes.

3.1.3 Mixture Fluid Temperature Behaviors and CollectorSystem Efficiency in 10% DME Tests. Collector fluid temperatureincrease is critical for the efficiency of solar thermal conversion.It has been found typically in previous pure CO2 natural convec-tive collector that much higher collector outlet temperature(around 90.0 C) than that of water based system [1,5]. The collec-tor inlet and outlet temperature variations of CO2DME mixturerefrigerant are plotted in Fig. 5, where the data are monitored dur-ing the same date and tests as shown in Figs. 3 and 4. In Fig. 5,the mass flow rates are also shown against with test time.

In Figs. 5(a) and 5(b), the collector inlet CO2DME mixturetemperatures are around 25.0 C to 30.0 C for each and both ofthem experience moderate increase during the operation. Compar-ing with the inlet refrigerant temperature, the outlet temperaturedetermines how much heat can be recovered in through the solarpanel and also the temperature of hot water supply. In the currentsupercritical CO2DME mixture based system, the collector outlettemperature reached around 70.0 C for the sunny day data shownin Fig. 5(a) and the peak outlet CO2DME mixture temperaturecan reach 80.0 C as shown in Fig. 5(b). Those high collector tem-peratures indicate the good performance of the current mixturerefrigerant heat conversion, which is also comparative with thecollector outlet temperature of pure CO2 based tests [8,18,19]. Inaddition, the outlet temperature curves of CO2DME mixture fol-low the trend of mass flow rate, which is also different from pureCO2 system. In solar thermal conversion systems with pure super-critical CO2 tests, the refrigerant flow rate is close related to thatof refrigerant temperature and not sensitive to the fluctuations ofsolar radiation [8,18,19]. As discussed in the Sec. 3.1.2, theincrease in heat transfer by CO2DME mixture would make themain flow temperature more sensitive to that of solar energyreaching the collector panel, thus the current collector temperatureis much more sensitive to the variations of solar radiations com-pared with pure CO2 tests. And due to the same reason, thecochange trend of collector temperature shows less delay com-pared with the trend of pressure as shown in Fig. 3.

The collector efficiencies of the current CO2DME system areplotted in Fig. 6, the dates and test conditions are the same withFigs. 35. The efficiency points are calculated as defined inEqs. (3) and (4) and plotted against parameter C, which is definedin Eq. (2) as a comprehensive parameter of operation condition.The main influencing factors to this solar thermal conversion sys-tem are solar radiation, ambient temperature, CO2 temperature,

Fig. 5 Variations of the measured mass flow rate and fluidtemperatures with the test time. The mass fraction of DME is10%. (a) July 8, 2010 and (b) Aug. 9, 2010.

Fig. 6 Variations of the collector efficiency with the compre-hensive coefficient. The mass fraction of DME is 10%. (a) July 8,2010 and (b) Aug. 9, 2010.



CO2 pressure, and CO2 flow rate. Parameter C is usually used toconsider those influencing factors, which include the solar radia-tion, CO2 temperature in the collector, and ambient temperature.For water based solar conversion systems, the collector efficiencygenerally decreases with the increase in C parameter, whichmeans that higher fluid to ambient temperature difference andlower level radiation conditions usually lower down the systembehavior, according to the previous discussions and definition ofC parameter [8,18,19]. The feasibility of solar thermal conversionsystem is also dependent on the trend of efficientC parameter,by which we can judge around what level of ambient temperatureregions and solar radiation levels is suitable for one kind of solarcollector. For water based systems, the trend discussed aboveconcludes that it is more suitable for summer season and low-to-moderate latitude regions and possibly not performs well forwinter season or mid-to-high latitude regions.

It can be seen from Fig. 6 that the general collector efficiencyfalls within 0%100%, while some points go near (or above)100% due to the transient measurement of collector inlet/outlettemperature and the delay of fluid status across the collector inletand outlet. Nevertheless, the defined collector efficiency can berepresentative for judging the basic performance of natural con-vective solar thermal conversion and has also been used in previ-ous studies [8,18,19]. In Fig. 6, the general trend for both sunnyday and cloudy day shows an increase in collector efficiency withC parameter, which means that higher efficiency can be found forhigher collector and ambient temperature difference and forlower radiation level according to the definition of parameter C inEq. (2). Such kind of evolution trend is fundamentally differentfrom water based solar thermal conversion system, where the col-lector efficiency decreases with the increase in C parameter.Accordingly, it can be concluded that the current supercriticalCO2DME mixture based natural convective solar thermal systemcan still achieve high performance in winter season and in mid-to-high latitude regions, where the conditions are generally consid-ered not suitable for water based systems as discussed formerly.In addition, the flow rate and heat recovery efficiency are alsohigher than that of water based collectors as discussed in previousstudies [8,18,19] and also the current system as shown in Fig. 6.

It should also be noted that for the sunny day data in Fig. 6(a),the points are more concentrated around 0.020.04 of C valuethan those of cloudy data in Fig. 6(b). That can be ascribed to theunstable solar radiation data during the cloudy day, thus the tran-sient efficiency points are distributed in a wide range of C valuesand even very low efficiency are seen. However, the averaged col-lector efficiency of the current CO2DME based system is calcu-lated to be 50.1% for the sunny day shown in Fig. 6(a) and 48.2%for the cloudy day shown in Fig. 6(b). Such collector efficiency isstill comparative with pure supercritical CO2 system and muchhigher than water based system [1,58,18,19], indicating that thecurrent kind of natural convective solar thermal conversion can beone potential choice in the future development.

3.2 Transient Characteristics of System With 20% DMEMass Fraction. In order to test the feasibility of the current super-critical CO2DME solar system, the DME mass fraction isincreased to 20% in the second series of experiments. In this sec-tion, the basic system parameter performances are shown for 20%DME test and compared with 10% DME tests and 0% DME (puresupercritical CO2) tests. For convenience of analysis, two typicaldays of Sept. 8, 2010 (sunny day with some cloud in the after-noon) and Sept. 12, 2010 (sunny day) with 20% DME additiondata are chosen and compared. The averaged ambient temperaturefor the test day of Sept. 8, 2010 is around 26.8 C to 31.2 C, whilefor Sept. 12, 2010 it is around 16.8 C to 20.8 C. The total injec-tion for premixed CO2DME mixture working fluid is around3.77 kg for the current solar thermal system. The tests are stillconducted in Shaoxing area (around N 30.0 deg, E 120.6 deg),Zhejiang Province of China, the same spot with 10% DME tests.

3.2.1 Pressure and Mass Flow Comparisons for 10% and20% DME Tests. The solar radiation and pressure variation curvesfor the above two days are shown in Figs. 7(a) and 7(b), respec-tively. In Fig. 7(a), the date is sunny with some cloud in the after-noon, so large fluctuations in solar radiation are seen and lowlevel solar radiation around 200 W/m2 are seen in the afternoon.In Fig. 7(b), the date is sunny and more stable pressure variationcurves are seen. The averaged solar radiations for Fig. 7 arearound 350 W/m2 and 550 W/m2 for Sept. 8, 2010 and Sept. 12,2010, respectively. Similar pressure behaviors of pressure varia-tion can be seen from Fig. 7 for 20% DME tests. The pressure var-iation trend in the current tests shows smooth curves and changesalong with that of solar radiation, which is still different from thatof pure supercritical CO2 tests. Also, the CO2DME mixture pres-sure behavior is different from 10% tests when the solar radiationlevel is low. In Fig. 7(a), when solar radiation drops in the after-noon, the collector pressure also drops below 7.50 MPa and thefluid turns to be pressurized liquid state. However, the currentpressure cochange trend with solar radiation becomes less sensi-tive when compared with that of 10% DME tests, as shown inFig. 7. For example, in the afternoon of Sept. 12, 2010 (Fig. 7(b)),the pressure still maintained around 9.0 MPa, while the solar radi-ation experienced gradual decrease during which time. Also, itcan be seen from Fig. 7 that the collector inlet and outlet pressuredifferences are still very small and even smaller than that of 10%DME tests discussed formerly in this paper.

The variations of CO2DME mass flow rates during 20% DMEtests are plotted in Fig. 8. Seeing from the two typical daysbehaviors in Fig. 8, very different mass rates can be found for thecloudy day and the sunny day: for the cloudy day, relative highflow rate is induced, while for the sunny day, the flow cannot bestably maintained. Compared with the tests with 10% DME shownin Fig. 4, the current 20% DME tests become more dependent onsolar radiation. For example, around 400 W/m2 solar radiationscan sustain 8.0 kg/h10.0 kg/s mass flow for 10% DME test as

Fig. 7 Variations of the measured solar radiation and fluidpressures with the test time. The mass fraction of DME is 20%.(a) Sept. 8, 2010 and (b) Sept. 12, 2010.



shown in Fig. 4(b), while higher solar radiation cannot sustain sta-ble mass flow for 20% DME test as shown in Fig. 8(b). The resultsindicate that the 20% DME tests need stable and high solar radia-tion to maintain the supercritical CO2DME mass flow, which isnew for CO2DME based natural circulation system.

Indeed, the ambient temperature and initial CO2DME mixturetemperature for the above two days are quite different. The moni-tored data in Fig. 9 show that for the cloudy day, the initialCO2DME mixture temperature is around 28.0

C (Fig. 9(a)),while the sunny day of Sept. 12, 2010, the initial CO2DME tem-perature is only around 17.0 C (Fig. 9(b)), which means that themixture fluid of the later test is more dense and with higher vis-cosity as shown in Fig. 1 (and that effect is greater than the effectof thermal conductivity when DME mass fraction is increasedfrom 10% to 20%). This initial operation temperature inducedinertia phenomena which demand higher solar radiation for stableoperation when DME mass fraction is increased should be veryimportant to the design of such kind solar thermal conversion sys-tem. Further discussion/summary on this can be seen in the laterpart of this paper.

3.2.2 Collector Efficiency Comparisons for 10% and 20%DME Tests. The collector efficiency variations against C parame-ter can be seen in Fig. 10 for the two typical tests with 20% DMEmass fraction. It is found that the averaged transient collectorefficiencies of the cloudy day of Sept. 8, 2010 and sunny day ofSept. 12, 2010 are around 70.0% and 40.0%, respectively. How-ever, the general trend of the efficiency dependence is differentfor the current tests with 20% DME (Fig. 10) with that of 10%DME tests (Fig. 6): the collector efficiency does not increasemonotonically with the increase in C parameter and higher Cvalue does not mean to have higher efficiency as shown inFig. 10. Even the CO2DME mixture natural convective flowcannot be sustained under this condition as shown in Fig. 8(b).The above results show the fact that 20% DME mass fraction testsmay not increase the system performance of the current solarthermal conversion system and higher fraction DME into the

Fig. 8 Variations of the measured solar radiation and massflow rate with the test time. The mass fraction of DME is 20%.(a) Sept. 8, 2010 and (b) Sept. 12, 2010.

Fig. 9 Variations of the measured mass flow rate and fluid tem-peratures with the test time. The mass fraction of DME is 20%.(a) Sept. 8, 2010 and (b) Sept. 12, 2010.

Fig. 10 Variations of the collector efficiency with the compre-hensive coefficient. The mass fraction of DME is 20%. (a) Sept.8, 2010 and (b) Sept. 12, 2010.



supercritical mixture may not be the optimal choice for cold andhigh latitude regions.

3.3 Feasibility Study on System ComprehensiveParameters

3.3.1 Pressure Effect of CO2DME Mixture Flow in theSystem. Based on the above discussion on pressure behaviors ofsupercritical CO2DME mixture, it is found that the pressuredependency can be very different from previous pure CO2 system.It is seen from Figs. 3 and 7 that the pressure changes to similartrend with solar radiation instead of monotonically increasingmode for pure CO2 system [8,18,19]. Due to the cochange modeand the pressure drop effect in the afternoon, the overall operationpressure is successfully reduced in CO2DME mixture based solarcollector tests, assuring the mixture fluid as one possible choice inthe future development.

The system pressures during summer season tests in 2010 areplotted against DME mass fraction in Fig. 11. In Fig. 11, the aver-aged operation pressure during the season tests is also added nearthe operation pressure points for each DME mass fraction. It canbe seen from Fig. 11 that the average operation pressure decreaseswith the increase in DME mass fraction: the averaged operationpressures for pure CO2, 10% DME, and 20% DME tests are9.47 MPa, 8.95 MPa, and 8.75 MPa, respectively. Therefore, themethod of CO2DME mixture natural convection solar collectorcan be one choice to reduce the operation pressure of supercriticalsystem, as also suggested in related studies [2426]. However,DME mass fraction higher than 20% to 30% can be difficult forsupercritical solar thermal system due to the explosive limit ofDME fluid.

In addition, the introduction of DME into the supercritical CO2system also brings about new evolution trend of mass flow rate inthe collector, which becomes more sensitive but still stable withvariation of solar radiation due to the changes of thermal-physicalproperties like thermal conductivity, viscosity, etc. Similar toprevious studies, the difficulties of high precision parameter cap-turing and near-critical region analysis still need further develop-ments [68,18,19]. Also, though the pressure reductions havebeen achieved, the absolute operation pressure (higher than8.0 MPa) is still relatively high. Future designs of similar systemsshould pay attention to those characteristics different from pureCO2 based systems.

3.3.2 Collector Efficiency Variations With Different DMEMass Fraction. Besides the operation pressure reduction, systemefficiency is also generally maintained at relatively high levelwhen DME is introduced, as discussed in Secs. 3.1 and 3.2. It isessential that the current solar thermal conversion just by naturalconvective flow can have efficiencies generally above 40%, even20% DME is mixed in the system. The collector efficiency varia-tions with comprehensive parameter C for different DME mass

fractions are plotted in Fig. 12. It can be seen from Fig. 12 thatwith the increase in DME mass fraction, the distribution of collec-tor efficiency is less expanded. Also for pure CO2 cases and 10%DME cases, the efficiency generally increase with C parameter,while 20% DME cases do not show similar trend. It is also foundin the previous discussion that though the pressure behavior ispreferable and the temperature sensitivity and stability of flow arealso good, higher DME percentage may make it difficult to main-tain stable flow as shown in Fig. 9(b). Table 1 presents theseasonal tests of 2010 summer for average system efficiency ofdifferent DME mass fraction. From Table 1, the average efficien-cies are around 50% and they seem less affected by the DMEmass fraction, which indicates the effect of DME fraction on thecurrent solar conversion tests is limited during the summer/autumn season.

3.3.3 Discussion on Feasibility and Optimization ofSupercritical Fluid Based Solar Thermal. In the current tests withCO2DME solar collector, the pressure is successfully reducedand the system efficiency not clearly affected by this DME addi-tion. Basically, relatively high efficiency is maintained in the cur-rent seasonal tests (with the outlet water temperature even higherthan 80 C while traditional water systems can hardly achievehigher than 60 C). For other aspects and further comparisons ofstructures, efficiencies of the current supercritical kinds and thewater based ones, further references can be useful [68,18,19]. Itis found feasible for the utilization of DME into the supercriticalCO2 system with more tests done on the various effects of it.However, the increase in DME mass fraction changes the depend-ency correlations of system parameters, due to the moderation ofsupercritical fluid properties when DME is introduced. Further-more, it should also be paid attention that system feasibility forcold/high latitude area of pure CO2 and low fraction DME systemcan be different from higher mass fraction DME systems. Forexample, though the collector outlet temperature can be still highas shown in Fig. 9(b), the very small mass flow rate makes the so-lar thermal conversion system difficult for real operation when theambient/initial temperature is very low, which is different frompure CO2 tests [8,18,19]. That is to say, on the one hand, theincrease in DME mass fraction reduces the operation pressure ofsupercritical solar thermal system; on the other hand, the systemefficiency is also different when DME fraction is increased.

Fig. 11 Variations of the time-averaged pressures with themass fraction of DME

Fig. 12 Variations of the measured collector efficiency withthe comprehensive coefficient under three different massfractions of DME, 0%, 10%, and 20%. Every point shown in thisfigure represents a time-averaged value during one day.

Table 1 Collector efficiency under different DME fractions

Mass fraction of DME 0 10% 20%Collector efficiency 51.7% 49.9% 50.8%



Also, system failures and low flow rates can be seen (as shownin Figs. 8 and 9) and the collector efficiency trend (which islargely different from water based collector) probably reverse andbecome not preferable for cold weather and mid-to-high latituderegions as discussed for pure supercritical CO2 system [8,18,19].The efficiency and C parameter change trend also indicate that thesystem with high DME percentage is not suitable when the ambi-ent/initial temperatures are relatively low, which is one typicalsignal of a transition behavior of collector efficiency with theincrease in DME fraction. According to the above discussion, itcan be concluded that there possibly exist some optimal point ofDME mass fraction, which considers both the suitability incold and mid-to-high latitude regions and the stable circulation/operation. Still, further experiments and developments on the col-lector system are needed and optimal system design and improvedperformance are to be tested in the future.

4 Concluding Remarks

In this paper, an experimental work is introduced and con-ducted to study the basic characteristics of solar thermal conver-sion made by supercritical CO2DME mixture natural circulation.Based on the measured and arranged data in the transient and sea-sonal patterns, the following concluding remarks are made:

(1) The introduction of DME successfully reduced the opera-tion pressure and it is found that higher DME mass fractionleads to lower average operation pressure; different frompure supercritical CO2 systems, the mixture fluid pressurefollows the fluctuations of solar radiation with the peakvalue at noon, which contributes to the reduction of overalloperation pressure;

(2) Collector efficiency is found to be around 50% in the cur-rent tests, which is higher than that of water based system;also contrary to water based systems, collector efficiencyincreases with comprehensive C parameter, indicating thatbetter performance can be found in winter season and mid-to-high latitude regions, where the conditions are generallyconsidered not suitable for water based systems;

(3) Supercritical CO2DME mixture natural convective massflow rate and temperature is generally more stable but moresensitive than pure supercritical CO2 tests, which is due tothe moderation of supercritical fluid properties when DMEis introduced. In detail, the overall effect of moderate den-sity and increase in thermal conductivity and viscosity con-tribute to the stable mass flow;

(4) The increase in DME fraction should also be paid attentionas the averaged collector efficiency is less affected by theDME addition during the experimental results. Judgingfrom the tests, there possibly exist some optimal of DMEmass fraction, which compromises both the system suitabil-ity in cold and mid-to-high latitude regions and the stablecirculation/operation.

Acknowledgment

The support from the National Science Foundation of China(51276001) and Common Development Fund of Beijing aregratefully acknowledged.

Nomenclature

A projected area of the collector (m2)C comprehensive coefficient (K/W)h enthalpy (J/kg)

hCI enthalpy values of the CO2DME fluid at the inletof the solar collector (J/kg)

hCO enthalpy values of the CO2DME fluid at the outletof the solar collector (J/kg)

I solar radiation (W/m2)

mCO2DME mass flow rates of CO2DME fluid (kg/s)QC collected heat quantity by the solar collector (W)Ta ambient temperature (K)Tf fluid average temperature in the solar collector,

which is calculated based on the inlet fluidtemperature and outlet fluid temperature (K)

xi accuracy of every parameter (%)gC solar collector efficiency (%)

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