flat-plate and cpc floor heating - j sol en engr

Performance of Flat-Plate andCompound Parabolic ConcentratingSolar Collectors in UnderfloorHeating Systems

Sarvenaz Sobhansarbandi1

Mem. ASME

Department of Mechanical Engineering,

Eastern Mediterranean University,

Gazimagusa North Cyprus via Mersin 10, Turkey

e-mail: [email protected]

Ugur AtikolProfessor

Department of Mechanical Engineering,

Eastern Mediterranean University,

Gazimagusa North Cyprus via Mersin 10, Turkey

e-mail: [email protected]

There is a growing interest in using solar energy in underfloorheating systems. However, the large areas required for the instal-lation of solar thermal collectors array can be discouraging,especially in the apartment buildings where the apartments roofis a common area. The objective of this study is to investigate thepossibility of using compound parabolic concentrating (CPC)solar collectors instead of the commonly used flat-plate collectors(FPCs) in such systems. It is aimed to explore the feasibility ofarea reduction required by the collectors. Second, the temperatureprofiles of circulating water loop and the concrete slabs are soughtto be examined. The system consists of solar thermal collectors, astorage tank, and circulation of water to transport the heat to foursimilar floor slabs. The CPC collector outlet fluids temperaturecan reach a maximum of 95 C, compared to 70 C obtained fromthe FPCs. The results from the simulations show that a 2 m2 CPCcollector array can perform satisfactorily to match the job of an8 m2 FPC array, obtaining the same required circulating waterstemperature in the slabs. [DOI: 10.1115/1.4029229]

Keywords: solar energy, floor heating, TRNSYS, simulation

1 Introduction

Utilizing solar energy for underfloor heating of residentialhouses has been emerging as a viable solution in many countriesenjoying sufficient sunshine in winter. In Cyprus, which is wellknown as a sunny island located in Mediterranean Sea, twodifferent approaches are applied in the construction of these sys-tems. In the first practice, the solar energy is designed to be storeddirectly in the floor slabs of the house; whereas in the second, ahot water cylinder is used as the primary energy storage system. Itis observed that in many villas, where the storage tank is not used,the number of FPCs used for the practice would be 16 to 22 innumbers, in order to collect sufficient solar energy during the day,and transfer the energy into the floor slabs (storage media in thiscase). In the second practice, where typically a storage tank of1000 l is used as the storage media, the number of six FPCs wouldbe sufficient for providing the required heat. Figure 1 shows a typ-ical residential house in North Cyprus, where the first practice

was utilized. The 22 FPCs positioned toward south, occupying alarge area of the back yard.

Providing the heat through underfloor heating system has takena great interest in the recent years. Yeo et al. have investigated thechanges and recent energy saving potential of residential heatingin Korea. They showed that modern apartment buildings with hotwater radiant floor heating yield not only less heat loss due to thetighter envelope but also yield higher energy consumption due tothe use of energy more effectively [1]. Ghali investigated the eco-nomic feasibility of the underfloor heating system for the climaticcondition of Beirut. He developed two mathematical models plusthe economic feasibility of the underfloor heating system whenintegrated with solar energy [2].

A number of studies were performed regarding the applicationof solar energy in underfloor heating systems and its advantagesby experimental and simulation analysis. Badran and Hamdan dida comparative study for underfloor heating using solar collectorsor solar ponds. In their work, a theoretical and experimental studyis made for underfloor heating system using solar collectors. Also,a study for a similar system using solar ponds is made with thesame main conditions. The obtained results from their study showthat the solar collector systems are 7% more efficient than the so-lar pond systems [3]. Cuneyt et al. have worked on a floor heatingsystem using solar energy in Ankara, Turkey. The aim of theirstudy was to investigate a floor heating system for an office, byutilizing solar energy as the main source of energy. A thermalcomfort analysis is performed in their study using Fangersmethod. In their study, solar energy is collected by FPCs andstored in a storage tank. The results from their applied methodindicate that the thermal comfort can be provided with such aheating system [4].

Haddad simulated a model for a house equipped with a radiantfloor heating system connected to solar collectors used to evaluatethe potential of using solar energy for space heating in the North-ern Chicago climate. The solar fraction of the system is predictedwhen the supply temperature to the radiant loops is constant andwhen this temperature is changed according to outside tempera-ture reset control [5]. Athienitis and Chen investigated the effectof solar radiation on dynamic thermal performance of floorheating systems. They performed a numerical investigation oftransient heat transfer in floor heating systems using a three-dimensional explicit finite difference model by focusing on theinfluence of the cover layer and incident solar radiation on floortemperature distribution and on energy consumption [6]. Alkhalai-leh et al. present the development of modeling simulation andanalysis of a solar pond floor heating system. They stated that thesolar pond heating system could meet most of the winter season inJordan with solar fraction in the range of 80100% for at least twomonths of the season [7].

The aim of this study is to simulate a domestic underfloor heat-ing system and compare the performance of two types of solarthermal collectors in this system. One of the problems encoun-tered by the house owners is finding enough space for the

Fig. 1 A house using solar collectors for underfloor heating,Nicosia, North Cyprus

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

OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGYCONSERVATION. Manuscript received May 14, 2014; final manuscript received November17, 2014; published online December 23, 2014. Assoc. Editor: Werner Platzer.

Journal of Solar Energy Engineering JUNE 2015, Vol. 137 / 034501-1Copyright VC 2015 by ASME

Downloaded From: http://solarenergyengineering.asmedigitalcollection.asme.org/ on 12/23/2014 Terms of Use: http://asme.org/terms

placement of the collectors; therefore, it is highly desirable to usethe minimum possible space without losing performance. Thepresent work investigates and compares the use of FPC and CPCsolar collectors for a desired underfloor heating application inCyprus.

2 System Description

A schematic diagram of the system in the present study isshown in Fig. 2. The collector (A) receives the solar radiationavailable on a specific day. A stratified storage tank (B) has beenused that the thermal stratification improves the overall perform-ance of the system. In such solar thermal storage tanks, the coldfluid (water in this study) is withdrawn from the bottom, fluidsTemperature at the Bottom (TBottom), to be heated at the heatsource, i.e., solar collectors, and returns to the top of the tank at arelatively higher temperature [8].

An on/off differential controller (C) generates a control func-tion between the Outlet fluids Temperature of the Collector(TOColl) and TBottom. The temperature of the fluid entering thefloor slabs must be controlled not to be exceeded from a specificamount, considering the construction properties. In this regard,hot fluid flow withdrawing from TOP of the tank (TTOP) is mixedwith the return water with lower temperature at the Tee-Piece,reaching a moderate temperature before entering the floor heatingslabs (D). As the water passes through the slabs, the heat will betransferred to the room by convection, so the outlet water from theslabs will have lower temperature from that of the inlet. The slabsoutlet water is then directed to a three-way tempering valve that isused for moderating the temperature described in part (C) beforeentering the tank.

3 Modeling of the System

In this study, a simple indirect underfloor heating system usingsolar energy with weather data of Larnaca, Cyprus, is simulated.The TRaNsient SYStem (TRNSYS) software has been applied forsimulation section. The schematic of the systems configuration isshown in Fig. 3. As it can be seen in this figure, FPC and CPCsolar collectors can be interchanged in the systems arrangement,while the other components will be kept without any changes. Thecomponents have been chosen from the standard and thermalenergy systems specialists (TESS) component libraries fromTRNSYS software, where each component has its own propertiesand specific mathematical descriptions.

In system configuration, TMY2 is the component reading theweather data on hourly basis regularly and applies the solar radia-tion values to find the tilted surface radiation and angle of inci-dence for an arbitrary number of surfaces. Types 73 and 74 arethe components for thermal performance of a theoretical flat-plate

and CPC solar collectors, respectively, where the HottelWhilliermodel [9] is used for evaluating the thermal performance of bothcollectors. The specific properties of FPC and CPC solar collec-tors are adjusted in a way to be able to deliver comparableperformances (Table 1).

Type 2b is the differential controller component which iscontrolling the operation of the pump by monitoring TTOP andcontrolling the system performance by comparing the TOCollwith TBottom. Type 3d is a single speed pump which is eitheron or off based on the appropriate received signal from Type2b. Type 4c is the stratified storage tank (which consists of sixnodes with equal heights in this study), where the solar fluid isassigned to be stored in it during the night time. The tank is con-nected to the type 2b for specifying the Tl (lowest temperature)and Tin (monitoring temperature). Type 11h is the Tee-piece inwhich two inlet liquid streams are mixed into a single liquid outletstream. Type 11b is the tempering valve which has one inlet andtwo outlets that its performance depends on the outlet fluid tem-perature of the tank (TTOP). In a case that TTOP reaches a tempera-ture higher than the adjusted suitable temperature, the temperingvalve will transfer proper portion of the cold water to theTee-piece in order to moderate the outlet fluids temperature. Theoutlet fluid from the Tee-piece needs to be diverted to the floorheating slabs in each floor. Type 11f is the flow diverter, in whicha single inlet fluid is split into two outlet fluids according to a userspecified valve setting. On the other hand, the outlet fluids fromthe floor heating slabs need to be accumulated in order to return tothe cycle. By applying type 11d as the flow mixer, two inlet fluidsare mixed based on an internal control function, in order to main-tain the temperature at or below a user specified value. Type 653is the simple floor heating system that operates under the

Fig. 2 Schematic diagram of the system

Fig. 3 TRNSYS modeling scheme of the system using FPCs orCPC collectors

034501-2 / Vol. 137, JUNE 2015 Transactions of the ASME


assumption that the slabs can be treated as a single lump of iso-thermal mass. The energy transfer of the inlet fluid to the slabscan be modeled using a heat exchanger effectiveness approach[10]. The simple radiant slab parameters are shown in Table 2.

By assuming a lumped capacitance floor system, the energybalance of the slab is defined in the form dT/dt aT b, wherea and b terms are described as

a eCmin UAzone UAlossmslabCpslab

(1a)

where e is the effectiveness factor of the fluid/slab heat exchanger,which has the value between zero to one, Cmin is the minimum ofthe slab capacitance and the fluid, UAzone is the heat transfer coef-ficient between the slab and the zone, and UAloss is the overallthermal loss coefficient of the collector per unit area

b e CminTfluid;in

mslabCpslab UAtop

TtopmslabCpslab

UAbackTback

mslabCpslab(1b)

where UAtop is the heat transfer coefficient between slab and topzone and UAback is the heat transfer coefficient between the slaband the sink temperature.

Also, the energy transferred to the slab from the fluid stream iscalculated by

_Qin CpminTfluid;in Tslab (2)

where Cpmin is the minimum specific heat capacity of the slab andthe fluid.

And the energy that is transferred from the slab to the zone is

_Qzone UAtopTslab Ttop (3)

Consequently, the outlet fluids temperature of the slabs is calcu-lated by

Tfluid;out Tfluid;in _Qin

_mfluidCpfluid(4)

where Cpfluid is the specific heat of fluid passing through theslab [10].

4 Results and Discussion

The simulations have been accomplished on January as it is oneof the coldest months of the year. For better comparing the resultsof both selected collectors, Jan. 13 is chosen specifically. Thehourly ambient temperature (Ta) of the Larnaca airport, Cyprus(hypothetical location of the model system), and also total radia-tion on horizontal are examined. It is observed that at night time,

Ta can be as low as 34C, whereas during day time it is between

12 and 17 C, while the solar intensity at noon hours is consis-tently in the vicinity of 2100 kJ/h m2 (583 W/m2) for radiation onthe horizontal surface and 3700 kJ/h m2 (1027 W/m2) for radia-tion on tilted surface. The solar radiations on horizontal and ontilted surface can be seen in Fig. 4. The incidence angle for bothcollectors is 45 deg and also the assigned inlet water flow-rate is125 kg/h (0.035 kg/s).

The system is simulated with an 8 m2 array of FPC (commonrequired area of the collector for underfloor application) as thebase starting point, following with a trial and error method to findthe best optimized area of CPC collector. In order to be able tocompare the performance of the collectors, the simulation analysiswas applied under a controlled system configuration, where all thecomponents adjustments were keep fixed with the same opera-tional properties. The results suggest that a 2 m2 array of CPCcollector can match in the same final floor slabs temperature asthe FPC.

The obtained results from both solar collectors are shown inFigs. 5 and 6. The simulation shows that during the day time thesolar fluids temperature will increase by absorbing the solar radi-ation by the collectors. On the other hand, the water circulationwill be stopped during night time while the fluids temperaturewill decrease due to the thermal losses of the system. The advant-age of this method is that the outlet water from collectors is storedin storage tank during the day time and will provide the desiredheat which yields into the required slabs temperature in under-floor heating system.

The outlet fluids temperature of the FPC is between 25 and70 C, whereas for CPC collector this range is between 25 and95 C. As a result, total energy gained by the collectors is accord-ing to the inlet and outlet fluids temperature into the collector asfollows and is presented in Table 1:

_Qc _mcpTc;o Tc;i (5)

where _m is the solar fluid mass flow-rate (0.035 kg/s), cp is thespecific heat capacity of solar fluid (4.19 kJ/kg k, water specific

Table 1 Parameters of FPC and CPC collector

Parameters FPC CPC collector

Number is series 1 1Collector area (m2) 8 2Fluid specific heat (kJ/kg K) 4.19 4.19Intercept efficiency 0.8 Collector fin efficiency factor 0.7Absorbance of absorber plate 0.8Incidence angle 45 deg 45 degAxis orientation Transverse plane 90 deg

from the longitudinalInlet flow-rate (kg/s) 0.035 0.035Energy collected by solarcollector (kJ/s)

6.6 10.26

Table 2 Simple radiant slab system

Parameters Amount

Material ConcreteCapacitance of slab 7500 kJ/KSpecific heat of fluid 4.19 kJ/kg KSlab-to-ambient loss coefficient 500 kJ/h KSlab-to-zone heat transfer coefficient 2000 kJ/h K

Fig. 4 The hourly variation of total radiation on horizontal andtilted surface

Journal of Solar Energy Engineering JUNE 2015, Vol. 137 / 034501-3


heat capacity), and Tc,i and Tc,o are inlet and outlet fluids temper-ature into the collector.

The total energy absorbed by the CPC collector is much greaterthan that of the FPC, so concerning the limitation of the requiredspace for system installation; it is more preferable to apply a CPCcollector which yields to occupy less functional area while achiev-ing the desired output heat. As the differential controller has acrucial role in systems functionality of the first loop, a specificconfiguration has been made for its controlling operation. In thisregard, the TOColl is set as the upper input temperature (Th),TBottom as lower output temperature (Tl), and TOColl as themonitored temperature for the high limit cut out (Tin). The differ-ential controller is investigating the temperature differencebetween Th and Tl and sends the appropriate signal to the pumpaccording to the specified dead bands. There is a high limit cut outtemperature that can be defined as desired, which in this study isdefined as 100 C for TOColl. The irregular behavior in tempera-ture variation in Fig. 6 is basically due to the differential control-ler function which controls the performance of CPC solarcollector in order not to exceed the temperature limitation. Thedifferential controller configuration made it possible to preservethe energy by stopping the pump function accordingly.

The variations of inlet and outlet water flows temperature intothe slabs are shown in Fig. 7, where the inlet flows temperature isapproximately 45 C. The flow is providing the desired heat(around 24 C) through the slabs surface to the surrounding, sothe fluids inside of the slabs lose their heat and reach to a lowertemperature value. The outlet fluid from the slabs is then

transferred to the circulation loop using a flow mixer and theprocedure will repeat again.

5 Conclusion

The aim of this study was to simulate a solar underfloor heatingsystem by TRNSYS software in Cyprus. The hourly investigationsare performed for one specific day, Jan. 13, one of the coldestdays of winter. The main idea was to collect the sun radiation bymeans of solar collectors and transferring that to the demand sidewhich is floor-heating in this study.

Although FPCs are more common for domestic applications,they may have limitations in terms of spaces they occupy. Asthese collectors are not commonly used in high temperature appli-cations, so the collector arrays area should be optimized. Thecomparison between the collectors performances was appliedunder a controlled system configuration, which all the operationalproperties (including the components properties, weather data, so-lar radiation value, etc.) remain constant for simulation analysis.The simulation suggests that a 2 m2 CPC collector can performsatisfactorily to match the job of an 8 m2 FPC, which results in a24 C heat from floor slabs surface. It is observed that using CPCcollector can increase the thermal fluids temperature in a rangebetween 25 and 95 C, whereas for FPC this range is between 25and 70 C. As the required area for collectors installation is a crit-ical issue for each house owner, it can be concluded that usingCPC collectors by occupying less space compared to FPCs isbeneficial for the desired application.

Nomenclature

Cp specific heat capacity of solar fluid (kJ/kg K)Cpfluid the specific heat of fluid passing through

the slab (kJ/kg K)Cpmin the minimum specific heat of the slab and the fluid

(kJ/kg K)_m solar fluid mass flow-rate (kg/s)

Ta ambient temperature (C)Tback the temperature to which losses from the slab

occur (C)Tc,i temperatures of input collectors fluid (C)Tc,o temperatures of output collectors fluid (C)

Tfluid,in the temperature at which fluid enters the slab (C)Tfluid,out the temperature at which fluid exits the slab (C)

TSlab temperature of the slab (C)Ttop the temperature of the zone (C)

TBottom water temperature at the bottom of the tank (C)Th upper input temperature of differential controller (C)Tl lower input temperature of differential controller (C)

TOColl outlet temperature of the collector (C)UAback the heat transfer coefficient between the slab and the

sink temperature (kJ/h K)

Fig. 5 The hourly variation of TBottom, TOColl, TSlab, andTTOP-FPCJan. 13

Fig. 6 The hourly variation of TBottom, TOColl, TSlab, andTTOP-CPC collectorJan. 13

Fig. 7 The hourly variations of inlet and outlet water flowstemperature into the slabsJan. 13

034501-4 / Vol. 137, JUNE 2015 Transactions of the ASME


UAloss overall thermal loss coefficient of the collector perunit (kJ/h m2 K)

UAtop the heat transfer coefficient between slab and topzone (kJ/h K)

UAzone the heat transfer coefficient between the slab and thezone (kJ/h K)

e the effectiveness of the fluid/slab heat exchanger[0, 1]

References[1] Yeo, M.-S., Yang, I.-H., and Kim, K.-W., 2003, Historical Changes and

Recent Energy Saving Potential of Residential Heating in Korea, J. EnergyBuild., 35(7), pp. 715727.

[2] Ghali, K., 2007, Economic Viability of Underfloor Heating System: A CaseStudy in Beirut Climate, International Conference on Renewable Energies &Power Quality, Sevilla, Spain.

[3] Badran, A. A., and Hamdan, M. A., 2004, Comparative Study for Under-FloorHeating Using Solar Collectors or Solar Ponds, J. Appl. Energy, 77(1),pp. 107117.

[4] Kurtay, C., Atilgan, I., and Ataer, O. E., 2009, Performance of Solar EnergyDriven Floor Heating System, J. Therm. Sci. Technol., 29(1), pp. 3744.

[5] Haddad, K., 2011, Solar Energy Utilization of a Residential Radiant FloorHeating System, J. ASHRAE Trans., 117(1), p. 79.

[6] Athienitis, A. K., and Chen, Y., 2000, The Effect of Solar Radiation onDynamic Thermal Performance of Floor Heating Systems, J. Solar Energy,69(3), pp. 229237

[7] Alkhalaileh, M. T., Atieh, K. A., Nasser, N. G., and Jubran, B. A., 1999,Modeling and Simulation of Solar Pond Floor Heating System, RenewableEnergy, 18(1), pp. 114.

[8] Kusyi, O., and Dalibard, A., 2007, Different Methods to Model Thermal Strati-fication in Storage TanksExamples on Uses of the Methods, SolNET Ph.D.course, Technical University of Denmark, Denmark.

[9] Hottel, H., and Whillier, A., 1955, Evaluation of Flat-Plate Solar CollectorPerformance, Trans. Conf. Use of Solar Energy, Tuscon, AZ.

[10] TRNSYS Manual, 2005, Components Mathematical Reference.

Journal of Solar Energy Engineering JUNE 2015, Vol. 137 / 034501-5


http://dx.doi.org/10.1016/S0378-7788(02)00221-9http://dx.doi.org/10.1016/S0378-7788(02)00221-9http://dx.doi.org/10.1016/S0306-2619(03)00012-6http://dx.doi.org/10.1016/S0038-092X(00)00052-9http://dx.doi.org/10.1016/S0960-1481(98)00781-2http://dx.doi.org/10.1016/S0960-1481(98)00781-2

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flat-plate and cpc floor heating - j sol en engr

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