A study of the economic perspectives of solar cooling schemes




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A STUDY OF THE ECONOMIC PERSPECTIVES OF SOLAR COOLING SCHEMES


  1. Syed1 BEng(Hons), MBA, Graduate Member

G. G. Maidment1 BEng(Hons), PhD, CEng, MIMechE, AMInstR, FRSA

R. M. Tozer1,2 MSc, PhD, CEng, MASHRAE, MInstR, Member

J. F. Missenden1 BSc, PhD, CEng, FRSA, Member


1South Bank University, School of Engineering, 103 Borough Road, SE1 0AA, London,

United Kingdom.

Tel: +44 (0) 207 8157634, Fax: +44 (0) 207 8157699, Email: syedaa@sbu.ac.uk

2Waterman Gore – Mechanical and Electrical Consulting Engineers, Versailles Court, 3 Paris Garden, SE1 8ND, London, United Kingdom.


ABSTRACT

This paper provides an economic evaluation and comparison of ejector, absorption and vapour compression systems driven either partly or fully by solar energy. Life cycle costing has been used to assess the relative economic ranks of eight solar cooling schemes. It has been shown that the capital cost of solar collectors is the dominant capital cost item in the total inventory of solar cooling plant, which has a strong influence on the system life cycle costs. Lower collector costs are therefore critical in establishing economically viable solar cooling systems. Primary energy analysis has been carried out and the running costs of chillers were determined for the maximum range of thermal and electric solar fractions and a number of annual equivalent full load hours of operation.


The results indicate that two low temperature flat-plate collector assisted single-effect absorption chillers compete favorably with a PV assisted centrifugal mechanical compression chiller across the maximum range of thermal and electrical solar fractions. Low temperature options for solar cooling have been shown to be more economical than their high temperature counterparts. At current collector prices, solar cooling systems are still not cost effective compared with conventional centrifugal cooling systems, however, it is shown that at a collector cost of £57/m2 for thermal energy and £1.8/Wp for electrical energy, single-effect solar absorption and PV-centrifugal compression could become cost effective within an annual EFLH of 5840 hours.


As such, the paper presents technical guidance on the procurement and operation of solar cooling plant. The novel energy and cost calculation methodology developed here can be applied globally to a wide range of solar collectors, chillers, heat rejection and ancillary subsystems.


KEYWORDS

Absorption, coefficient of performance, ejector, equivalent full load hours, life cycle cost, photovoltaics, plant on hours, primary energy, solar energy, solar fraction, thermal collectors, vapour compression


NOTATION

C Costs £/kWh

Cp Specific heat capacity kJ/kg-K

COP Coefficient of Performance dimensionless

CPC Compound Parabolic Concentrator -

crf Capital recovery factor -

cwtd Cooling water temperature difference oC

DE Double Effect

E Energy Cost £/year

EE Electrical Energy Cost £

EEN Electric Energy kWh

EFLH Equivalent Full Load Hours hours

ETC Evacuated Tube Collector -

FPC Flat Plate Collector -

HT High Temperature (120oC / 110oC)

Annual interest rate %

I Irradiation W/m2

LCC Specific life cycle costs £/kW Capacity-Year

LT Low Temperature (85oC / 80oC)

mass flow rate l/s

M Maintenance cost £

MT Medium Temperature (85oC / 80oC)

 Efficiency dimensionless

n Amortisation period Years

PER Primary Energy Ratio dimensionless

POH Plant On Hours hours

PV Photovoltaic

pd Pressure drop kPa

Q Heat kW

R Running costs £

SCOP Solar Coefficient of Performance dimensionless

SE Single Effect

SOLFthePercentage thermal solar fraction %

SOLFw Percentage electrical solar fraction %

T or t Temperature K or C

W Work kW

Q Heat kW

z Plant Cost £


Suffixes

a Ambient or absorber

abs Absorption

appr Approach

c Collector or condenser

chw Chilled water

comp Compressor

cw Cooling water

e evaporator

g Glogal irradiation at normal incidence angle

g Generator

gb Gas burner

hrr Heat rejection rate

l Leaving

m average fluid

ng Grid natural gas

nm Motor efficiency

np Pump efficiency

p Power output or pump

ps Power station

vc Vapour compression

wg Grid power

pv Photovoltaic


1.0 INTRODUCTION

Solar air-conditioning with LiBr/H2O absorption chillers has been widely demonstrated in a variety of buildings (Machielsen et al. 1999; Duff et al. 1999, Hammad and Zurigat, 1998). Laboratory scale solar ejector cooling systems have also been demonstrated for air-conditioning (Wolpert and Riffat, 2002, Mostofizadeh, Ch. and Bohne, D., 2001) and simulated for refrigeration (Huang, et al., 2001; Sun, 1997) applications. However, the use of PV to power vapour compression chillers is rarely found. This is primarily due to the high cost of PV modules and their low conversion efficiencies (presently 15-16%) [Green, 2001]. This has led contemporary researchers to investigate the designs and economics of solar-thermally driven absorption cooling systems (Löf and Tybout, 1974; Warren and Wahlig, 1985).


However, continually reducing PV prices (presently £2.50/Wp to £5.30/Wp) [NCPV, 2001], have recently created an opportunity to utilise PV-compression cooling systems. To this end, solar-electric refrigeration systems have been developed for residential and light commercial air-conditioning applications which use either PV power in stand alone applications (Jourde, 1999) or to supplement grid electrical power, since autonomous PV operation is not yet considered to be economical (Wright, 2001).


It is widely accepted that neither PV-compression nor solar-absorption is presently cost competitive with conventional air-conditioning plant driven with grid electricity, since the cost of solar energy collection/ conversion is the dominant capital cost item in the total solar system costs (Critoph, 1999; Kellow and Jarrar, 1991, Yellott, 1981). This effectiveness gap is presently 17.6% at the end of a full year of operation at 100% thermal or electrical solar fraction on life cycle cost basis compared to conventional centrifugal compressors driven by grid-electricity. Therefore, the additional investment in solar energy is not recovered annually within a reasonable economic life span of solar cooling systems. However, with continually rising fossil fuel prices and decreasing solar collector prices (as a result of government incentives and manufacturing economies to scale), the cost difference between natural gas and PV in the EU Member Countries is expected to become negligible by the year 2016 on reaching a threshold value of approximately £0.0267 per kWh (Awerbuch, 2002). However the present economic status of solar-absorption appears to favour a reduction in peak electrical demand for cooling to implement demand side management strategies (Kulkarni, 1994) from which the utilities could benefit as well as end-users. Therefore, the objective of this essentially sifting study is to identify the preferred solar cooling systems as those having a particular mix of solar and conventional equipment that minimise costs to the customer and provide the utilities some leeway to manage their loads by helping to reduce their generating capacity (Purcell, 1979), which are the two main advantages of adopting solar absorption cooling technologies.


Whilst efforts are underway to improve the efficiencies of solar collectors (Brunold et al, 2002; Winston, et al. 1998; Collares Pereira, 1995) and PV and develop economic absorption chillers for solar operation (Fineblum, 2001; Gee et al. 2001; Sumathy and Li, 2000; Lamp et al. 1998), further theoretical investigations are required to assist chiller and collector manufacturers to establish a place in the niche market of solar cooling. Therefore, a holistic and concise approach to determining the relative economic ranks of available solar cooling options has been developed and consists of primary energy analysis and life cycle costing (LCC).


The primary energy calculations for solar-assisted absorption cycles consider the efficiencies of the power station and gas burner depending on the thermal solar fractions given by Ziegler (1999) and Filipe Mendes et al. (1998). They also consider the electricity demand of PV-assisted vapour compression chillers depending on the percentage electrical solar fraction given by Syed et al. (2002). Whereas, LCC (Leboeuf, 1980; Perino, 1979) is an established technique, which extends the cost evaluations to include, the fossil fuel energy costs, capital costs and maintenance costs over the useful life of solar-assisted absorption and PV-assisted compression chillers.


Based on the above techniques, this paper presents a novel methodology for energy and cost comparison of solar air-conditioning systems. Tozer (1995) first presented the given technique, which enabled the calculation of the life cycle cost of cooling systems driven by a CHP plant. So far, there have been no publications, which have adopted Tozer’s techniques in the analysis of solar cooling systems. This paper makes use of this technique and therefore novelly applies the same to solar cooling systems. It also presents a new parameter of electrical solar fraction to assess its effect on the energy consumption and life cycle cost of PV-vapour compression cycles whereas previously, the thermal solar fraction was used only with solar absorption. The methodology is applied to solar-thermally driven LiBr/H2O single- and double-effect absorption and steam ejector cooling cycles and mechanically driven screw and centrifugal vapour compression cycles.

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