yba

40, P

Helical screw expanderParabolic trough solar collectorSilica gelwater adsorption chiller

hywitandemp

mal energy and power for remote off-grid regions. A case study was carried out to evaluate an annual

and 70.4%, respectively. Also found is that the studied system can have a higher solar energy conversion

d thewer geecade

tems have been introduced for many applications with provenenvironmental and economical benets. The most popular one isthe hybrid PV/T system which has twofold purposerstly, to coolthe PV module and thus improve its electrical performance andsecondly to collect the thermal energy which would have other-wise been lost as heat to the environment [36]. As reported byTripanagnostopoulos et al. [3], the electrical efciency of the basic

power is used to drive a mechanical chiller, this system also canbe regarded as a solar hybrid power and cooling system.

Not only hybrid solar heating and power system but also hybridsolar cooling and power system is faced with a major problem thatonly producing heating or cooling is not very consistent with theorder of nature. So in several months, the recovery energy willnot be used efciently. Nowmany hybrid solar heating and coolinghas been used in green buildings to make the utilization solar col-lectors more sufcient in a whole year [14,15]. Thus is can be seenthat a tri-generation system including cooling, heating and power

* Corresponding author. Tel.: +86 21 34204358; fax: +86 21 34206814.

Applied Energy 86 (2009) 13951404

Contents lists availab

lseE-mail address: yjdai@sjtu.edu.cn (Y.J. Dai).of renewable, clean and abundant energy, and can meet the de-mands of both thermal and electricity energy. In the process, solarenergy can be transformed to electric power directly or indirectly,through the use of some sort of heat engine [1,2]. But the usuallyexisting commercial solar thermal plant capacity is more than500 kW and far away from residential districts. The large scale unitlimits its application in lack-electricity remote districts where elec-tricity consumption is small (less than 50 kW). Also, it prevents theefcient utilization of the waste heat.

Over several decades, multi-production hybrid solar energy sys-

output of 1.5kW and approximately 12 kW h of heat energy willbe supplied daily from this system. This suggested an overall con-version efciency of 15%. On the other hand, Goswami [10] pro-posed a novel ammoniawater binary mixture thermodynamiccycle capable of producing both power and refrigeration in 1998.Further experimental and theoretical studies [11,12] showed thatthe exergy efciency of this kind of system was more than 50%.Gordon and Ng [13] proposed a thermodynamic cascade unit thattakes maximal advantage of high temperature input heat producedby a solar ber-optical mini-dish system. Though the generated1. Introduction

With the onset of energy crisis anmental protection, solar thermal poand more attention during the past d0306-2619/$ - see front matter 2008 Elsevier Ltd. Adoi:10.1016/j.apenergy.2008.11.020efciency than the conventional solar thermal power generation system alone. The energy efciency canbe increased to 58.0% from 10.2%, and the exergy efciency can be increased to 15.2% from 12.5%. More-over, the economical analysis in terms of cost and payback period (PP) has been carried out. The studyreveals that the proposed system the PP of the proposed system is about 18 years under present energyprice conditions. The sensitivity analysis shows that if the interest rate decreases to 3% or energy priceincrease by 50%, PP will be less than 10 years.

2008 Elsevier Ltd. All rights reserved.

pressure from environ-neration attracts mores because it is one kind

polycrystalline silicon (pc-Si) PV/T model was found to be 3.2%higher than that of the simple pc-Si PV module. Besides photovol-taic CHP system, Riffat et al. [79] developed a novel hybrid heatpipe solar collector CHP system to provide electricity and heatingfor buildings. The experimental results showed that an electricityKeywords:energy and exergy efciency of the system under the climate of northwestern region of China. It is foundthat both the main energy and exergy loss take place at the parabolic trough collector, amount to 36.2%Energy and exergy analyses on a novel hand power generation system for remote

H. Zhai, Y.J. Dai *, J.Y. Wu, R.Z. WangInstitute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 2002

a r t i c l e i n f o

Article history:Received 18 March 2008Received in revised form 9 November 2008Accepted 12 November 2008Available online 31 December 2008

a b s t r a c t

In this study, a small scalebolic trough solar collectorchiller, etc., was proposedgeneration cycle at lower t

Applied

journal homepage: www.ell rights reserved.rid solar heating, coolingreas

R China

brid solar heating, chilling and power generation system, including para-h cavity receiver, a helical screw expander and silica gelwater adsorptionextensively investigated. The system has the merits of effecting the powererature level with solar energy more efciently and can provide both ther-

le at ScienceDirect

Energy

vier .com/locate /apenergy

ergyis a more rational utilization mode. But there is very a little of re-search on hybrid solar cooling, heating and power system thoughcombined cooling, heating and power (CCHP) technology has beendeveloped for more than 100 years. Mittelman et al. [16] has pro-posed combined heating and power approach that employs CPV/Ttechnology and the thermal energy is wide enough to satised the

Nomenclature

A collector area (m2)_EX exergy rate (W)C cost (RMB)COP coefcient of performanceCRF capital recovery factorcp,w specic heat of water (J/kg K)f dilution factorh enthalpy (J/kg)I solar radiation (W/m2)_m ow rate (kg/s) fuel natural gasi interest rateLCC life cycle cost (RMB)n life cycle period (year)PP payback period (year)PWF present worth factor_Q energy rate (W)s entropy (J/kg K)T temperature (OC)UA unacost (RMB)W power (W)x quality

Greek letterg efciency

1396 H. Zhai et al. / Applied Enrequirements of cooling and heating because the IIIV PV cellsoperated well under 240 C.

In many rural locations of western China, grid-connected elec-tricity is unavailable because the harsh topography (mostly desertand mountain) and low population density result in high cost. Notsurprisingly, therefore, there is little prospect of improvement inthe standard of living of villagers from their present low level with-out the support of electricity. Fortunately the solar energy resourcehas been found to be very rich, which is in the range of 54008400 MJ/m2 every year. So solar energy can effectively resolvesthe no-electried problem in these remote areas, meanwhile, sup-ply cooling and heating for peasant household. Moreover, this kindof system is also suitable for other electricity user such as sentry,communication base station or hotel in nature reserve.

For supplying the cooling, heating and electric demands forthese remote areas, this paper presents a small hybrid solar cool-ing, heating and power generation system of a 10s of kW by usinghelical screw expander and silica gel adsorption chiller. The powergeneration technology in our proposed system is by using helicalscrew expander which was invented in USA and used in some geo-thermal power plants since 1970s years [1720]. In China, the50 kW to 1.5 MW products of such machine are available in mar-ket. Compared with the steam turbine, it has three advantages:no limit of working uid (no need for dry saturated steam), lowrequirement of uid quality and steady internal efciency. But ithas a shortcoming of higher outlet temperature and pressure,which results in serious outlet thermal loss and lower thermal ef-ciency. However, if this kind of engine is used in a cogenerationsystem and the extra high temperature latent thermal energy ofexhaust steam can be recovered, the energy utilization efciencywill be improved largely and the advantage of helical screw expan-der in medium temperature solar thermal power generation appli-cation will emerge.

The purpose of this paper is to investigate the performance ofthis system and explore its feasibility for using in western regionsof China. This system uses two kinds of energy (primary solar en-ergy and assisted natural gas) and produces three kinds of energy

Subscripta ambientB condensing boilerb beam solar radiationC parabolic solar collectorcool coolingDispose abandoned equipmentE electrical energye energyex exergyfuel natural gasG natural gasHE1 heat exchanger oneHE2 heat exchanger twoheat heatinghotwater hot waterIC initial capitalOM operation and managementPG power generatorS solarst steamSE helical screw expander engineVC vapor compressor chiller

86 (2009) 13951404productions (cooling, heating and power). So the exergy analysiswhich is based on the second law which allows not only a quanti-tative but also a qualitative consideration in an energy conversionprocess is introduced. In the following sections, energy and exergyanalysis based mainly on the rst law and second of thermody-namics along with a case study of a solar hybrid cooling, heatingand power system has been presented. On the other hand, anexhaustive cost analysis is processed to discuss the feasibility ofthis system in practice.

2. System description and materials

The studied solar heating, cooling and power generation systemis shown in Fig. 1. It consists of three main subsystems, namely,steam generation subsystem, power generation subsystem andwaste heat utilization subsystem.

The rst subsystem includes a series of parabolic trough solarcollectors and an assisted gas-red condensing boiler. Here, theparabolic trough solar collector with cavity absorber is adoptedconsidering its cheap cost, reliability and good performance [21].Our previous results shown in Ref. [22] indicated that the triangleshape cavity absorber which shown in Fig. 2 had the best opticalperformance and can reduce the convective heat loss effectively.If the beam solar radiation is large enough to drive the whole sys-tem, no assisted gas is needed. Conversely, a gas-red condensingboiler begins working as an assisted unit to ensure the steady en-ergy input to the power generation subsystem.

The second subsystem is the power generation unit including ahelical screw expander engine and a generator. With the parabolictrough collector to collect and supply the heat energy, dry satu-rated, superheated, even wet-saturated steam can be employed

ing,

H. Zhai et al. / Applied Energy 86 (2009) 13951404 1397as the working uid in the power cycle. The high-pressure workingmedium pushes against the vanes of a screw-type expander, con-verting some of its heat energy into mechanical power, turningthe shaft of a generator and producing a net power output.

The third subsystem mainly consists of two heat exchangers, ahot water tank, eight 10 kW silica gelwater adsorption chillers (insummer) and radiation heating system (in winter). The relativelyhigh temperature exhaust steam from the helical screw expanderis separated in steam separator, and dry steam ows into the rst

Fig. 1. Sketch diagram of the solar heatheat exchanger. Considering that the waste heat at the tempera-ture of 7090 C abounds in the exhaust steam of helical screw ex-pander, an adsorption cooling system designed and built by SJTU[23,24] is chosen to offer cooling in summer. The photograph of acommercial 10 kW adsorption chiller is shown in Fig. 3. In this chil-ler, the environment friendly silica gel/water is used as the work-ing pair. The working principle is that the silica gel adsorbswater vapor, so the water continuously evaporates to producecooling, and the heat produced due to the adsorption of silica gel

Fig. 2. Photograph of triais removed by cooling water. When the adsorption nishes, the sil-ica gel is heated by hot water and desorbs water vapor to the con-denser. The two adsorption/desorption chambers of each chillerwork alternatively in order to produce the cooling effect continu-ously. In winter, the rst heat exchanger produces heat for radia-tion heating system. If the uid leaving the rst heat exchangeris still in two-phase state, the steam enters the second heat ex-changer to release the heat and produce hot water for domesticuse (see Fig. 4).

cooling and power generation system.Some technical parameters used in the analysis are given in Ta-ble 1. The unique merits of the proposed solar cooling, heating andpower generation system are further summarized below.

Since the helical screw expander can use low temperature wet-saturated steam, the requirements on the tracking accuracy, theabsorber and temperature level of the trough solar collector arelow, this provides a chance of using low cost parabolic troughsolar collector with cavity absorber.

ngle cavity absorber.

ergy1398 H. Zhai et al. / Applied En Solar energy can be more efciently utilized by recovering thelatent energy in the exhaust steam to supply cooling andheating.

Use of the novel silica gelwater adsorption chiller can drivecooling cycle with hot water at temperature level 6090 C.

It is a good stand alone energy system, and is feasible for remoteand off-grid areas where solar energy resource is abundant.

3. Analysis

A simple hybrid solar energy system has been schematicallyshown in Fig. 1. Some typical state points in the following illustra-tion are also shown. The Ts diagram of the arrangement for thecase that the reheat pressure is greater than that of the regenera-tive one is schematically shown in Fig. 2. Saturated water fromthe feed water tank is heated from state 6 to state 1 in solar collec-tor and gas boiler and then is supplied to the helical screw expan-der engine. In the helical screw expander, wet steam does work byexpansion and expands to state 2. The separated dry steam is sent

Fig. 3. Photograph of silica gelwater adsorption chiller.

Fig. 4. Thermodynamic cycle in Ts diagram.to the rst heat exchanger and is condensed to point 4. After heatexchanging, the latent energy of dry steam is used to produce hotwater which can drive silica gelwater adsorption chiller in sum-mer or supply heating in winter. Subsequently, the wet steamows into the second heat exchanger and produce domestic hotwater. The nally condensed water at state 5 ows into feed watertank and is sent to solar collector by a pump. In order to performthe energy and exergy analyses, following assumptions areinvolved:

The expansion process in the helical screw expander is nonisen-tropic and the loss in the heat engine is considered. The relativeinternal efciency and generator efciency are assumed to beconstant. Neglect the heat loss of the heat exchangers and the effective-ness of two heat exchangers is constant and has the same value.

It is assumed that the cooling capacity and efciency of adsorp-tion chiller are just affected by the temperature of driving hotwater. The effects of environment temperature on adsorptionchiller are completely neglected.

Because the pump work in this system is very small, it isneglected and the states of point 3, 5 and 6 are the same.

Neglect the pipeline heat loss in the system.

3.1. Energy analysis

Based on the rst law of thermodynamics, the energy analysis isprocessed in this section.

(1) solar collector and gas assisted subsystem

Incident beam solar radiation on parabolic trough solar collec-tor is

_QS AIb 1The useful solar energy transferred to uid media is

_QC AIbgC 2where the thermal efciency of parabolic trough solar collectorwith cavity receiver is

gC 0:70 0:41T1 T6=2 Ta

Ib3

If the solar energy is not enough to drive total system, neededassisted natural gas is

_QG maxh1 h6 _mst AIbgC=gB; 0 4

(2) Power generation subsystem

The mechanical energy received from steam of the screw ex-pander is

_QSE _mstgSEh1 h021 x2s h002x2s 5The power output (neglecting the pump work) from by the

screw expander is

_WE gPG _Q SE 6

(3) Latent heat recovery

86 (2009) 13951404In summer, the latent heat of the exhaust steam from the heatengine can be recovered to drive the aforementioned adsorptionchiller. The performance of silica gel adsorption chiller with different

If the heat is rich enough, the extra energy will be condensed in

Table 1Technical parameters of the facility and reference energy parameters.

Parameters N

Atmosphere parametersAverage temperature in summer/winter TBeam solar radiation Ib

Designed hybrid solar systemParabolic trough solar collector area AElectrical of helical screw expander engine WInternal efciency of helical screw expander engine gsteam mass ow _mHeating capacity of assistant boiler QEfciency of assistant boiler gInlet/outlet steam temperature of helical screw expander TEfciency of electrical generator gRefrigerating capacity of eight 10 kW adsorption chiller QChilled inlet/outlet water temperature of adsorption chiller TInlet/outlet water temperature of oor heating TDomestic hot water temperature T

Conventional solar power generation systemInlet/outlet steam temperature of turbine T

g

H. Zhai et al. / Applied Energydriving temperature is tested under xed conditions. The cycletime includes the 900s for heating/cooling, 120s for mass recovery,20s for heat recovery and 40s for valves switching. The coolingtemperature and the chilled water inlet temperature are 30.7 Cand 20.6 C, respectively. Based on the test results shown inFig. 5, the cooling capacity and the COP of the adsorption chillerare relevant to the hot water temperature, and can be curve ttedas

_Q cool 5:45358 0:17373T7 7

COP 0:21821 0:00251T7 8The energy balance in the heat exchanger which involves the

exhaust steam in the condensation side and the hot water suppliedto the adsorption chiller in the other side can be listed below

_Q cool=COP T7 T8 _mcoolcp;w 9

T7 T8 gHET2 T8 10

Efciency of Rankine heat engineAfter the steam temperature T2 and the ow rate of hot water in thechiller side _mcool are known, the other four unknown variables T7, T8,Qcool and COP can be solved from Eq. (7)(10).

Fig. 5. Refrigeration power and COP vs. hot water temperatures.the second heat exchanger and can be used to heat water fordomestic use

_Qhotwater gHET4 T10 _mhotwatercp;w 12Thus, the total energy efciency of the studied system is

ge _WE _Qcool_QS _QG

; summer_WE _Qheat _Qhotwater

_QS _QG; winter

8