Socio-economic performance of a novel solar photovoltaic/loop-heat-pipe heat pump water heating system in three different climatic regions

Abstract

h i g h l i g h t s We predict the system's socio-economic performance in three climatic regions. The system achieved the highest energy efficiency in Hong Kong area. The system had the best economic revenue in London area. The system obtained the most environmental benefits in Shanghai area. The system's suitability is dependent on the priority order of the three factors. a b s t r a c t This paper aimed to study the socio-economic performance of a novel solar photovoltaic/loop-heat-pipe (PV/LHP) heat pump water heating system for application in three different climatic regions, namely, cold area represented by London, warm area represented by Shanghai, and hot (subtropical) area represented by Hong Kong. This study involved prediction of the annual fossil-fuel energy saving, investment return period and carbon emission reduction of the new system against the traditional gas-fired and electrical boilers based water heating systems. An established dynamic model developed by the authors was uti-lised to predict the system's energy performance throughout a year in the three climatic regions. A life-cycle analytical model was further developed to analyse the economic and environmental benefits of the new system relative to the traditional systems. Analyses of the modelling results drew out several conclusive remarks: (1) the system could achieve the highest energy efficiency when operating at the hot (subtropical) climatic region (represented by Hong Kong), enabling the heat output of as high as 922 kW h/m 2 yr and water temperature of above 45 °C, while the grid power input is only 59 kW h/ m 2 yr; (2) the system is worth for investment when operating at the high energy charging tariff area (rep-resented by London), with the cost payback periods of 8 and 5 years relative to the traditional gas-fired and electrical boilers based systems, respectively; (3) the system could obtain the most promising envi-ronmental benefits when operating in Shanghai where the energy quality (embodied carbon volume of per kW h energy) is relatively poor, enabling reduction in life-cycle carbon emissions of around 4.08 tons/m 2 and 17.87 tons/m 2 respectively, relative to the gas-fired and electrical boilers. Answer to such a question on which area is most suitable for the system application is highly dependent upon the priority order among the three dominating factors: (1) energy efficiency, (2) economic revenue, and (3) environmental benefit, which may vary with the users, local concerns and policy influence, etc. The research results will be able to assist in decision making in implementation of the new PV/thermal technology and analyses of the associated economic and environmental benefits, thus contributing to realisation of the regional and global targets on fossil fuel energy saving and environmental sustainability.

Full text

Socio-economic performance of a novel solar
photovoltaic/loop-heat-pipe heat pump water
heating system in three different climatic regions
Xingxing Zhang
a,
⇑
, Jingchun Shen
a
, Peng Xu
a,b
, Xudong Zhao
a,
⇑
, Ying Xu
c
a
School of Engineering, University of Hull, HU6 7RX, UK
b
Beijing University of Civil Engineering and Architecture, Beijing 100044, China
c
Shanghai Pacific Energy Centre, Shanghai 200001, China
highlights

We predict the system’s socio-economic performance in three climatic regions.

The system achieved the highest energy efficiency in Hong Kong area.

The system had the best economic revenue in London area.

The system obtained the most environmental benefits in Shanghai area.

The system’s suitability is dependent on the priority order of the three factors.
article info
Article history:
Received 19 April 2014
Received in revised form 10 July 2014
Accepted 18 August 2014
Keywords:
PV
Loop heat pipe
Simulation
Energy performance
Economic
Environment
abstract
This paper aimed to study the socio-economic performance of a novel solar photovoltaic/loop-heat-pipe
(PV/LHP) heat pump water heating system for application in three different climatic regions, namely, cold
area represented by London, warm area represented by Shanghai, and hot (subtropical) area represented
by Hong Kong. This study involved prediction of the annual fossil-fuel energy saving, investment return
period and carbon emission reduction of the new system against the traditional gas-fired and electrical
boilers based water heating systems. An established dynamic model developed by the authors was uti-
lised to predict the system’s energy performance throughout a year in the three climatic regions. A
life-cycle analytical model was further developed to analyse the economic and environmental benefits
of the new system relative to the traditional systems. Analyses of the modelling results drew out several
conclusive remarks: (1) the system could achieve the highest energy efficiency when operating at the hot
(subtropical) climatic region (represented by Hong Kong), enabling the heat output of as high as
922 kW h/m
2
yr and water temperature of above 45 °C, while the grid power input is only 59 kW h/
m
2
yr; (2) the system is worth for investment when operating at the high energy charging tariff area (rep-
resented by London), with the cost payback periods of 8 and 5 years relative to the traditional gas-fired
and electrical boilers based systems, respectively; (3) the system could obtain the most promising envi-
ronmental benefits when operating in Shanghai where the energy quality (embodied carbon volume of
per kW h energy) is relatively poor, enabling reduction in life-cycle carbon emissions of around
4.08 tons/m
2
and 17.87 tons/m
2
respectively, relative to the gas-fired and electrical boilers. Answer to
such a question on which area is most suitable for the system application is highly dependent upon
the priority order among the three dominating factors: (1) energy efficiency, (2) economic revenue,
and (3) environmental benefit, which may vary with the users, local concerns and policy influence, etc.
The research results will be able to assist in decision making in implementation of the new PV/thermal
technology and analyses of the associated economic and environmental benefits, thus contributing to
realisation of the regional and global targets on fossil fuel energy saving and environmental
sustainability.
Ó2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.apenergy.2014.08.074
0306-2619/Ó2014 Elsevier Ltd. All rights reserved.
⇑
Corresponding authors. Tel.: +44 (0)1482 466684; fax: +44 (0)1482 466664.
E-mail addresses: Xingxing.zhang@hull.ac.uk (X. Zhang), Xudong.zhao@hull.ac.uk (X. Zhao).
Applied Energy 135 (2014) 20–34
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy

1. Introduction
In contemporary energy sector, solar photovoltaic (PV) and
solar thermal are the fundamental pillars to assist in transition
from the traditional fossil fuel energy structure to a renewable
energy system. Recent governmental schemes addressed that by
2050, the solar PV will generate nearly 11% of global electricity
[1] while the solar thermal will deliver about 50% of the low and
medium temperature heat in the EU [2]. At present, the technical
drawback of the traditional PV systems lies in the relatively lower
electrical efficiency, which is in the range 10–20% [3]. Furthermore,
PVs’ electrical efficiency varies in an inversely linear trend with the
PV cells’ surface temperature, leading to around 0.5% efficiency
declining per degree rise in the cells’ temperature [4]. The PV/ther-
mal (PV/T) technology was therefore developed to control the tem-
perature of the PV cells and make advanced utilisation of the heat
trapped within the PVs simultaneously.
Technologies for this purpose have been developed substan-
tially but meanwhile exhibited by some inherent problems [5].
The most common way to cool the PV cells/modules is the one uti-
lising the naturally/mechanically ventilated air [6–10]. This
method has poor heat removal effectiveness due to the low ther-
mal mass of the air. Alternatively, PV modules could be cooled by
using the loop circulated water that runs through the backside
coils of the PVs [11–15]. This approach has also very limited
improvement in efficiency owing to a higher temperature rise of
the water within the loop and potential piping freezing occurring
in cold climatic regions. Another PV cooling approach was to place
the direct expansion evaporation coils beneath the PV module that
allow a refrigerant to pass through [16–20]. As a result, the PV cells
could be cooled to a very lower temperature, leading to a greater
increase in the PVs’ electrical efficiency. This approach, regarded
as the significant step-forward in the PVs cooling technology,
was also identified with several practical challenges: e.g., multiple
welding joints making a complex manufacturing process, high cost
by using the copper coils, and uneven refrigerant distribution
across the multiple coils in a large area [5]. In recent years, heat
pipes were applied to cool the PV cells/modules by extracting the
heat trapped within the PVs and delivering it to the passing fluid
employing the self-driven evaporation & condensation cycling of
the heat pipe working fluid [21,22]. However, traditional design
relating to this concept is to use multiple heat pipes as the thermal
absorbers, which have the relatively complex structure and higher
cost. Furthermore, the fluid passing across the heat-pipe condenser
is usually water, which has the gradually growing temperature
along the flow path thus leading to reduced heat transfer rate
between the heat pipe working fluid and passing water. It is there-
fore essential to find a solution to enhance the overall heat-transfer
efficiency of the PV/T system and meanwhile, to simplify its struc-
ture thus enabling reduced cost and broader service applications.
To remove the above addressed technical barriers remaining
with the existing heat pipe based PV/T technology, loop heat pipes
(LHP) have been introduced into the PV/T systems. A LHP is a spe-
cial type of heat pipe that combines the principles of thermal con-
ductivity and phase transition. It has a large heat transport
capacity enabling heat to be transferred along a relatively long dis-
tance by circulating the working fluid in a closed loop. The LHP has
also some particular features that makes it appropriate to cooling
of the PV cells/modules, e.g., effective heat transfer using the least
pipings, use of the anti-freezing medium within the loop fluid, her-
metically sealed loops and homogeneous capillary force [23].It
was understood that the LHPs have been widely used in thermal
controls of satellites, spacecrafts, electronics, and LEDs [23,24].
However, use of LHPs in solar collecting systems was not often
reported, with a couple of cases concerning application of the grav-
itational LHP [23]. It has also been noted that the gravitational
LHPs have the ‘dry-out’ problem caused by limited capillary effect
of the wicks within the heat pipe evaporation section, resulting in
significantly reduced heat transfer capacity.
To tackle the above addressed ‘dry-out’ problem, a novel LHP
structure comprising the top-positioned vapour–liquid separator
was developed by the authors and this new type of LHP solar ther-
mal systems have been studied using both simulation and experi-
mental methods, resulting in the dedicated parametrical
characterisation of the specific LHP and associated thermal and
power systems [25–29]. On basis of the authors’ previous research
achievement on this topic, the social economic issues of the system
for use in three different climatic regions, namely, cold area – rep-
resented by London (0.1°W, 51.3°N), warm area – represented by
Shanghai (121.8°E, 31.2°N), and hot area (subtropical) – repre-
sented by Hong Kong (114.2°E, 22.2°N), were studied in the paper.
This study involved prediction of the potential fossil fuel energy
Nomenclature
cspecific heat capacity (J/kg K)
Ccost (€)
CS cost saving (€)
CR carbon reduction (kg)
eroot mean square deviation
ffactor
Mmass (kg)
PP pay-back period (year)
Qenergy rate (W)
rcorrelation coefficient
ttime
Ttemperature (K)
xwidth parameter of fin sheet
X
e
experimental results
X
s
simulation results
Greek
g
energy efficiency
Subscript
au auxiliary energy
c,PV/LHP capital of PV/LHP system
eelectricity
el-CO
2
CO
2
emission of electric heater
gas-CO
2
CO
2
emission of gas boiler
LHP loop heat pipe
m,PV/LHP maintenance of PV/LHP system
m,wh maintenance of water heating system
RE renewable incentives
th thermal
ooverall
o,PV/LHP operation of PV/LHP system
o,wh operation of water heating system
wwater
wh,CO
2
CO
2
emission of water heating system
w,load water heating load
X. Zhang et al. / Applied Energy 135 (2014) 20–34 21

saving, return time on investment and carbon emission reduction
of the new system relative to the traditional gas-fired and electrical
boilers based water heating systems. The research results will be
able to assist in decision making in implementation of the pro-
posed PV/T technology and analyses of the associated economic
and environmental benefits, thus contributing to realisation of
regional and global targets on fossil-fuel energy saving and envi-
ronmental sustainability.
2. System description
Fig. 1 illustrates the target PV/LHP heat pump water heating
system, which comprises a PV/LHP module, the vapour/liquid
transportation lines, a flat-plate heat exchanger (LHP condenser
and heat pump evaporator), an electric compressor, a coil con-
denser immersed into a water tank, an expansion valve and an
electric control & storage unit (controller, inverter and battery).
The PV/LHP module is the key component of the system that con-
sists of a single tempered glazing, a PV lamination layer, the LHP
evaporator, a fin sheet and a polystyrene board for thermal insula-
tion. These layers are positioned into an aluminium-alloy casing
using the clamping and argon-arc welding methods. The LHP evap-
orator plays an essential role in transferring the heat from the PV
layers to the flat-plate heat exchanger. It is placed into an alumin-
ium
X
-shape fin sheet and then attached to the rear surface of PV
layer using thermal-conductive silicon sealants. Detailed descrip-
tions about the LHP evaporator structure, the system working prin-
ciple and the system design parameters have been fully addressed
in previous work of the authors [27,28].
Through dedicated computer numerical simulation and experi-
mental testing, the operational performance of such a novel LHP
and associated thermal and power systems has been characterised
[25–29], giving the following conclusions respectively: (1) heat
transport capacity of the new LHP was around 1.08 W/cm
2
(900 W in total), nearly 86% higher than that for common heat
pipes operated in gravitational field [30]; (2) solar thermal effi-
ciency of the new LHP based solar thermal facade system was
about 48.8%, over 18% higher than that for the conventional solar
thermal system [31]; (3) solar thermal and electrical efficiencies
of the new LHP based PV/T system were 39.25% and 9.13% respec-
tively, resulting in 15.02% exergetic efficiency, nearly 24% higher
than that for the common PV/T systems [32] and (4) Coefficient
of performance (COP) of the new LHP based solar heat pump sys-
tem was 5.51, about 1.5 times of conventional solar heat pump sys-
tems [33]. Performance characterisation results of the new LHP and
associated thermal and power systems are summarised in Fig. 2.
Such PV/LHP heat pump system may have several distinct char-
acteristics: (1) configuration of the PV/LHP module is simplified
with only one LHP, leading to a corresponding low cost; (2) the
LHP can passively transport heat for a long distance that removes
the need for a circulation pump; (3) the heat pump controls the
PV cells in a relatively low-temperature operation mode; (4)
the generated PV electricity can offset part/full power load for driv-
ing the compressor, thus creating a low/zero-carbon water heating
operation; and (5) this system can be either installed on a building
by mounting the PV/LHP module onto the facade and connecting PV
electricity to the national grid or installed as an independent heat
and power cogeneration unit to meet the energy load.
Fig. 1. Schematic of the solar PV/LHP heat pump water heating system.
22 X. Zhang et al. / Applied Energy 135 (2014) 20–34

3. Social economic performance analyses – simulation model
development
3.1. Energy-performance prediction model
The dynamic modelling had the aim of evaluating the perfor-
mance of the integrated PV/LHP heat pump system in real climatic
operational conditions, which are simultaneously affected by sev-
eral critical factors, i.e., solar radiation, air temperature, air velocity
and operating time. This modelling enabled (a) a prediction of sys-
tem performance in real climatic operational conditions; (b) a fore-
cast of seasonal system performance; (c) a recommendation for an
appropriate climate region suitable for the operation of such a PV/
LHP heat pump system; and (d) analyse the system’s economic and
environmental benefits.
For the PV/LHP heat pump water heating system, the transient
operational model involved six energy balance equations: (1) a
heat balance equation for the glazing cover; (2) a heat balance
equation for the PV layer; (3) an one-dimensional unsteady-state
heat conductance of the fin sheet; (4) a heat balance equation for
the LHP operation; (5) heat balance equations for the heat pump
evaporator and (6) the water tank [34–36]. The full mathematical
descriptions of the energy-balance equations were presented in
authors’ previous work [28]. To resolve the equation system using
a numerical method, the differential equations can be rewritten in
the formats of the MATLAB’s ‘‘ode15s’’ and ‘‘pdepe’’ solvers using an
implicit backward difference formula and the finite element meth-
ods (FEM) and [37]. The MATLAB ‘‘pdepe’’ solver is normally
applied for the initial-boundary value problems to solve the para-
bolic and elliptic partial differential equation (PDE) systems in one
space variable ‘‘x’’ and time ‘‘t’’. This solver converts the PDE into
ordinary differential equation (ODE) using a second-order accurate
spatial discretization based on a set of nodes, which is recognised
as a classical FEM method. The time integration is completed with
the ‘‘ode15s’’ solver, which is a variable order solver to resolve the
algebraic equations according to the backward differentiation for-
mulas. Such numerical method is based upon the built-in subpro-
grams in the MATLAB software, which consumed much less
computing time than the difference method in previous work [28].
3.2. Operational cost saving and payback-time prediction model
Prior to evaluating the socio-economic benefits, the annual hot
water demand, Q
w,load
, should be calculated as the baseline of
energy requirement for the comparison between such system
and traditional water heaters. The initial water temperature in
the 35-L water tank was assumed to be the same as the ground
water temperature. The eventual hot water temperature criteria
are all considered at 45 °C. It can be predicted that more or less
the energy deficiency of such PV/LHP heat pump system may
occur, which could be matched through an auxiliary gas boiler or
electric heater as the backup.
Q
w;load
¼M
w
c
w
D
T
w
ð1Þ
The cost payback period for operating such a prototype system
to replace conventional water heaters can be estimated by [38]
PP
PV=LHP
¼CapitalCost Incentives
AnnualðOperational &maintenanceÞCostSaving
¼C
c;PV=LHP
C
RE
ðC
o;wh
þC
m;wh
ÞðC
o;PV=LHP
þC
m;PV=LHP
Þð2Þ
Fig. 2. Performance characterisation results of the new LHP and associated thermal and power systems.
X. Zhang et al. / Applied Energy 135 (2014) 20–34 23

To install a new solar water heating system, it might be possible to
receive grants through the government’s renewable policy, such as
the Renewable Heat Incentive (RHI) scheme in London, which is
intended to encourage the uptake of renewable heating technolo-
gies within households, communities and businesses through the
provision of financial incentives. The financial support available
for installing such a solar thermal system are ‘‘€0.24/kW h yr heat
(7 years)’’ for London and ‘‘reduction in 13% of capital cost’’ for Shang-
hai while there is no renewable tariffs found in Hong Kong right
now [39,40]. The final cost of the installation of such a solar proto-
type system equals the value achieved by subtracting the local
incentive amounts for renewable projects from the capital cost.
The maintenance cost of a solar heating system is normally esti-
mated at 2% of the initial system cost [38] due to its low mainte-
nance requirement. An electric water heater is considered free in
terms of its maintenance during its life cycle [41].
As a solar photovoltaic system is usually considered to have a
life span of 25 years [42], the life-cycle net cost saving, CS
PV/LHP
,
of this solar system in energy bills can be determined by
CS
PV=LHP
¼ðLifetime paybacktimeÞ
AnnualðOperational &maintenanceÞCostSaving
¼ð25 PP
PV=LHP
Þ½ðC
o;wh
þC
m;wh
ÞðC
o;PV=LHP
þC
m;PV=LHP
Þ ð3Þ
3.3. Environmental benefit prediction model – life cycle carbon
emission
Environmental benefits can be simply estimated by using the
annual CO
2
emission factor when operating this PV/LHP heat pump
system (including the required auxiliary energy and the net system
electricity consumption) to replace a conventional water heater
[38].
CR
PV=LHP
¼f
wh;CO
2
Q
w;load
f
gas-CO
2
Q
au;PV=LHP
f
el-CO
2
Q
au;PV=LHP
ð4Þ
The gas-to-CO
2
emission factor is estimated to be the same at
0.26 kg CO
2
/kW h heat for the three regions, as gas is directly
burned for heat generation [43]. While the electricity-to-CO
2
emis-
sion factor of should be different for the three regions due to the dif-
ferent efficiencies of the national power plants, which are 0.545,
0.997 and 0.840 kg CO
2
/kW h heat respectively in London, Shanghai
and Hong Kong [44–46].
3.4. Module and system performance evaluation
In this paper, performance of the PV/LHP module was examined
by both the energetic and exergetic efficiencies while the perfor-
mance of the whole system was assessed by the advanced thermal
performance coefficients (COP
PV/T
). All the mathematical descrip-
tions of these evaluation parameters could be found in authors’
previous work [28].
3.5. Simulation model operation
The corresponding initial and boundary conditions, i.e., solar
radiation, air temperature, wind speed and water temperature,
were extracted from the weather database of Energy-Plus software,
which are respectively the ‘037760_IWEC’ for London, the
‘583670_IWEC’ for Shanghai, and the ‘450070_CityUHK’ for Hong
Kong [47]. The monthly diurnal averages for solar radiation and
air temperature for these regions are respectively shown in Figs. 3
and 4.Table 1 gives the monthly average weighted wind speed for
the three climatic regions. It is seen that Hong Kong has a medium
level of solar radiation, is hot in summer and warm in winter;
Shanghai also has a medium level of solar radiation but is hot in
summer and cold in winter; while London has a low level of solar
radiation, is warm in summer and cold in winter. During the sim-
ulation, it was assumed that the system starts operation from
08:00 and ends its operation at 16:00 for a single day. The heat
pump was considered to operate at 0 °C/55 °C in winter, 10 °C/
55 °C in summer, and 5 °C/55 °C in spring and autumn. The PV/
LHP panel installation angle was set to the same level as the local
altitude in the three selected regions. The initial temperature of the
water stored in the tank was considered to be the ground water
temperature at a height of 0.5 m below ground level, as in Table 2.
An initial temperature distribution (T= 0) from the module
cover to the water in the tank is desired before starting the itera-
tion. The time step size,
D
t, and the space step size,
D
x, were,
respectively, given at 1 min and 22 mm because a smaller step
would consume much more time to calculate and would not affect
the results very much, while a larger time step would lead to an
unstable calculation process [31]. As the temperature profile of
the fin sheet is assumed to be symmetrical at two sides of the
LHP evaporator in the centre, there are then only considered to
be 11 elements from the left-hand edge to the fin centre along
the fin width. To overcome the potential energy deficiency of such
PV/LHP heat pump system, an auxiliary gas boiler or electric heater
was applied as the system backup. The socio-economic figures,
such as capital cost, renewable tariffs, system life span and gas/
electricity-to-CO
2
emission factors, were also input into the pro-
gram for simulation. The algorithm is presented in a flow diagram
in Fig. 5, which is also interpreted as follows:
(1) Assign the design, operating parameters and socio-economic
figures into program code.
(2) Input the external boundary conditions from the weather
data file.
(3) Assume the initial parameters’ values: temperature distribu-
tion and mass flow rate.
(4) Set up the time step size,
D
T, and space step size,
D
x, and
start the calculation.
(5) Carrying out heat analysis on each module/system compo-
nent and rewriting them in the required format of the
‘‘ode15s’’ solver.
(6) Rewrite the Eq. (3) in the required format of the ‘‘pdepe’’
solver.
(7) Input the boundary condition of the fin sheet into the PDE
and analyse the transient heat conductance on the fin sheet.
(8) Ensure the PDE results’ accuracy achieve the criteria of 10
3
.
(9) Calculate the hot water load by Eq. (1) based on the initial
weather data.
(10) Estimate the annual energy saving and required auxiliary
energy by Eq. (2) based on the operational results.
(11) Identify the life-cycle cost and carbon savings by Eqs. (3) and
(4).
(12) Carry out the energetic, exergetic and system performance
calculation.
(13) Complete the operation of the program until time end and
export the results.
(14) Program stops.
4. Validation of the simulation model
4.1. Validation by using the published data
The simulation model was initially validated for its suitability
and accuracy by comparing the modelling results with the pub-
lished experimental data of a PV/solar-assisted heat pump/heat
pipe (PV–SAHP/HP) system developed by Fu et al. [47]. They
attached groups of PV cells to a flat-plate heat-pipe thermal
24 X. Zhang et al. / Applied Energy 135 (2014) 20–34

collector and a direct-expansion evaporator in a heat pump for hot
water generation. There were three operating modes during their
testing, and the heat pipe operating mode with the greatest simi-
larity to the proposed PV/LHP system was selected to validate
the simulation model. The system performance indicators were
defined congruously according to the mathematical descriptions
in this section. The correlation coefficient (r) and the root mean
square percentage deviation (e) defined in below equations were
Fig. 3. Monthly averages for solar radiation in three climate regions.
Fig. 4. Monthly averages for air temperature in three climate regions.
Table 1
Monthly averages for wind speed in three climate regions (m/s).
Location January February March April May June July August September October November December
London 3.4 2.7 4.6 4.2 2.5 3.2 2.6 3.0 3.1 2.4 2.3 3.2
Shanghai 3.5 3.5 3.5 3.5 3.9 3.7 3.3 3.4 3.6 3.7 3.9 3.8
Hong Kong 3.1 3.7 3.4 3.5 2.8 3.5 3.3 3.3 4.2 3.5 3.3 3.1
Table 2
Monthly averages for ground water temperature in three climate regions (°C).
Location January February March April May June July August September October November December
London 4.2 5.3 7.5 9.6 13.6 15.7 16.2 15.2 12.7 9.7 6.7 4.7
Shanghai 5.5 7.5 11.4 15.2 22.3 26.0 27.0 25.1 20.8 15.3 10.0 6.4
Hong Kong 17.3 18.4 20.5 22.5 26.3 28.3 28.8 27.8 25.5 22.6 19.7 17.8
X. Zhang et al. / Applied Energy 135 (2014) 20–34 25

applied to analyse the difference between the theoretical and the
published experimental results.
r¼n
R
X
e
X
s
ð
R
X
e
Þð
R
X
s
Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
n
R
X
2
e
ð
R
X
e
Þ
2
q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
n
R
X
2
s
ð
R
X
s
Þ
2
qð5Þ
e¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P½100 ðX
e
X
s
Þ=X
e

2
n
sð6Þ
where nis the number of experiments implemented; and X
e
and X
s
represent the experimental and simulation results, respectively.
Fig. 5. Flow chart for the modelling set-up.
26 X. Zhang et al. / Applied Energy 135 (2014) 20–34

Fig. 6 presents the transient simulation results of the module
outputs against operating time by inputting the design, operation
and weather conditions of the referenced PV–SAHP/HP experimen-
tal rig [48]. The original experimental data in Fig. 6 was derived
from the reference [48], which was further compared with the sim-
ulation results in this paper. The correlation coefficients (r) and the
root mean square percentage deviation (e) for the modelling and
testing of the module electrical/thermal outputs were 0.9938/
0.9973 and 4.95%/12.87%, respectively. The reason for errors may
exist in the utilisation of simplified assumptions/empirical formu-
las, and an inaccurate estimation of the heat loss coefficient due to
a lack of wind data. However, the accuracy achieved by this
dynamic simulation model was acceptable from the engineering
point of view, and could therefore be applied to evaluate the sys-
tem performance in the real climates and for recommending
appropriate regions for operation.
4.2. Validation by using the experimental results
In addition, the accuracy of this simulation model was also ver-
ified using the dedicated experiments outlined in authors’ previous
work [28]. Through a parallel comparison between the modelling
and real-time test results, the established simulation model was
validated with a reasonable accuracy of mean error less than 9%.
Owing to the good level of agreement achieved, this simulation
model is appropriate for predicting the annual operational perfor-
mance of the PV/LHP heat pump system and recommending
regions for operation.
5. Results and discussion
5.1. Energy-performance prediction results
In order to establish which climate best suits the system, this
section investigates the annual operational performances of the
prototype system in three climate regions: London, Shanghai and
Hong Kong. Fig. 7 presents the monthly average temperatures of
the PV layer in the three regions. It was observed that the PV tem-
perature had a similar trend of variation to the solar radiation and
air temperature, which achieved a maximum figure in summer and
a minimum figure in winter. In London, the monthly PV tempera-
ture was in the range from 7.57 °C (in February) to 35.76 °C (in
July). In Shanghai, the monthly PV temperature was in the range
from 15.93 °C (in January) and 47.12 °C (in August). In Hong Kong,
the PV temperature was in the range from 25.23 °C (in February) to
43.41 °C (in July). The PV temperature was the lowest in London
and the highest in Hong Kong throughout the year. The annual
average PV temperatures in London, Shanghai and Hong Kong were
18.97 °C, 31.45 °C and 34.66 °C, respectively.
The monthly electrical efficiencies (
g
e
) of the PV/LHP module
varied inversely to its temperature, as shown in Fig. 8a, which were
in the range from 8.42% to 9.35% in London, 8.02% to 9.08% in
Fig. 6. Comparison of the simulation results with the published testing data.
Fig. 7. Temperatures of the PV layer in three climate regions.
X. Zhang et al. / Applied Energy 135 (2014) 20–34 27

Shanghai, and 8.16% to 8.77% in Hong Kong. The annual mean elec-
trical efficiency was highest at 8.94% in London, while it was rela-
tively lower in Shanghai (8.57%) and Hong Kong (8.42%). However,
the module’s thermal efficiency (
g
th
) varied in the opposite manner
to its electrical efficiency, as displayed in Fig. 8b. The monthly ther-
mal efficiency of the PV/LHP module was in the range from 14.13%
to 34.63% in London, 18.06% to 61.41% in Shanghai, and 43.13% to
59.86% in Hong Kong. Operation of the prototype system in Hong
Kong was found to be the most stable, with the highest annual
average thermal efficiency of the module at about 51.65% owing
to the warmest air temperatures existing in this area. In Shanghai
and London, the annual average thermal efficiencies of the module
were much lower than in Hong Kong at around 38.99% and 26.99%,
respectively. After adding the electrical and thermal efficiencies
together, the overall energetic efficiencies (
g
o
) of the module in
Fig. 8c varied as a similar way to the thermal efficiency, which
had annual average values of 35.93%, 47.57% and 60.06% in London,
Shanghai and Hong Kong, respectively.
Fig. 9 illustrates the overall energy output of the module. The
quantity of the energy output was found primarily to depend on
Fig. 8. (a) Electrical energetic efficiencies, (b) thermal energetic efficiency and (c) overall energetic efficiency of the PV/LHP module in three climate regions.
28 X. Zhang et al. / Applied Energy 135 (2014) 20–34

Fig. 9. Overall energy output of the PV/LHP module in three climate regions.
Fig. 10. Condensation capacity of the heat pump in three climate regions.
Fig. 11. Overall assessment of the prototype system in three climate regions.
X. Zhang et al. / Applied Energy 135 (2014) 20–34 29

the amount of available solar radiation and the surrounding air
temperature in the three regions. The module electricity genera-
tion in London varied from 1.42 kW h/m
2
to 10.70 kW h/m
2
, with
an average value of 5.43 kW h/m
2
. The monthly electrical output
range in Shanghai was from 8.48 kW h/m
2
to 13.15 kW h/m
2
, with
an average performance of 10.76 kW h/m
2
. In Hong Kong, the sys-
tem generated electricity in the range of 7.49 kW h/m
2
to
12.56 kW h/m
2
, with an average value of 10.06 kW h/m
2
. The ther-
mal output ranges of the module were 4.07–43.99 kW h/m
2
,
19.51–89.77 kW h/m
2
and 41.04–79.50 kW h/m
2
, respectively in
London, Shanghai and Hong Kong, and their corresponding
monthly average values were 16.44 kW h/m
2
, 51.40 kW h/m
2
and
61.82 kW h/m
2
. According to these figures, Hong Kong was found
to have the highest energy output for the module, including an
almost equivalent electricity output to Shanghai and the highest
monthly average heat generation for the three regions.
By adding together the heat pump electrical consumption and
the heat output of the module (heat source), the monthly condensa-
tion heat outputs (heat sink) of the integrated PV/LHP heat pump
system are presented in Fig. 10, giving a figure in the range
from 5.37 kW h/m
2
to 54.96 kW h/m
2
in London, 25.75–
112.15 kW h/m
2
in Shanghai, and 52.54–102.28 kW h/m
2
in Hong
Kong, respectively, and the corresponding monthly average figures
are 20.89 kW h/m
2
in London, 65.59 kW h/m
2
in Shanghai, and
79.33 kW h/m
2
in Hong Kong.
An overall assessment of the PV/LHP module and the associated
heat pump system is presented in Fig. 11. From the module point of
view, the exergetic efficiency was relatively stable for all three
regions. Owing to the highest thermal output, the annual average
exergetic efficiency of the PV/LHP module was highest in Hong
Kong at 13.08%, followed by 12.35% in Shanghai, and 11.85% in
London. From the integrated system point of view, the annual aver-
age COP
PV/T
value was highest at 9.44 in London because of the
largest ratio of electricity to heat output, while relatively lower
COP
PV/T
values were found in Shanghai (8.35) and Hong Kong
(6.97). It needs to be addressed that as the evaporation and con-
densation temperatures of the heat pump operation were set up
at the same level during the simulation in all three regions, the sea-
sonal system COP
th
value will be the same as that of Shanghai at
4.57, 5.10 and 5.75, respectively, in the winter, transit, and summer
seasons.
Table 3 presents the monthly eventual water temperature after
a single day’s operation for the three climatic regions. The area
shaded grey in the table indicates the months that the temperature
of the tank water could not be heated above the hot water temper-
ature criterion of 45 °C by the prototype system alone. This system
could provide hot water service for nearly seven months per year
in Shanghai. However, there was only one month in London (July)
in which this system could achieve the water temperature crite-
rion. In Hong Kong, this system could reach the required water
temperature throughout whole year.
Table 4 gives the total annual operational output of the proto-
type system in the three areas. Hong Kong was found to have the
highest thermal energy output from the module at 741.85 kW h/
m
2
yr, which was then upgraded to 921.70 kW h/m
2
yr by input-
ting electricity at 179.85 kW h/m
2
yr into the heat pump compres-
sor. After subtracting the solar electricity generation of nearly
120.74 kW h/m
2
yr, this system would require additional grid elec-
tricity of 59.11 kW h/m
2
yr to meet the heat pump consumption in
Hong Kong, and would not need any other auxiliary heater in the
system to achieve the hot water demand. Owing to the highest
solar radiation level and conspicuous seasonal air temperature in
Shanghai, the prototype module could generate the most electric-
ity at 129.14 kW h/m
2
yr but have a lower heat output than the
operation in Hong Kong at 616.78 kW h/m
2
yr. And in Shanghai,
additional grid electricity of 16.15 kW h/m
2
yr will be required to
meet the heat pump consumption, further delivering the conden-
sation heat at 762.07 kW h/m
2
yr. In London, this module produced
the lowest energy quantity for both electricity (65.11 kW h/m
2
yr)
and heat (197.23 kW h/m
2
yr), due to the lowest level of solar radi-
ation and the coldest air temperature for the three regions.
Although this prototype system produced a net amount of electric-
ity (19.64 kW h/m
2
yr) in London, it would consume much more
energy from the back-up heater to meet the hot water demand.
The output of condensation heat was also the least at only
242.70 kW h/m
2
yr in London. The simulation results offer the
interpretation that this prototype system would perform best in
a subtropical climate, such as the Hong Kong area.
5.2. Bill saving and return time on investment prediction
5.2.1. Capital cost
5.2.1.1. Capital cost of the prototype PV/LHP heat pump system. The
capital cost of the prototype PV/LHP heat pump system was esti-
mated by adding together the individual prices of all the system
components and taking into account appropriate commercial prof-
its. Table 5 provides a list of cost breakdowns and indicates that the
initial cost of such a system is €512.69. Furthermore, the cost
details of the different system components are presented in
Fig. 12. The PV/LHP module was the most expensive of the system
components, accounting for nearly 44% of the total system cost.
It needs to be addressed that this system was a second proto-
type whose capital cost was much less than that of the previous
one [29], mainly owing to the fast reduction of PV price recently.
5.2.1.2. Renewable Earning (RE). The financial support available for
installing such a solar thermal system in the three regions is given
in Table 6 using figures extracted from Tables 4 and 5. The final
cost of the installation of such a solar prototype system equals
the value achieved by subtracting the local incentive amounts for
renewable projects from the capital cost.
Table 3
Monthly final tank water temperature in three climate regions.
Table 4
Total annual output of the prototype system in three climate regions.
Location London Shanghai Hong
Kong
Solar radiation (kW h/m
2
yr) 737.35 1515.78 1439.26
Solar electricity generation (kW h/m
2
yr) 65.11 129.14 120.74
Solar heat output of module (kW h/m
2
yr) 197.23 616.78 741.85
Heat pump work consumption
(kW h/m
2
yr)
45.47 145.29 179.85
Heat pump condensation heat
(kW h/m
2
yr)
242.70 762.07 921.70
System net electricity output
(kW h/m
2
yr)
19.64 16.15 59.11
30 X. Zhang et al. / Applied Energy 135 (2014) 20–34

5.2.2. Annual operational cost and saving
In the potential case of the unsatisfactory operation of the pro-
totype system in certain terrible weather conditions, an auxiliary
heater could be switched on to heat up the water until its temper-
ature achieves the expected 45 °C. A gas boiler and electric heater
are, respectively, considered as the auxiliary heater of the proto-
type system when comparing it with a conventional gas boiler
(efficiency of 80%) and a typical electric heater (efficiency of 90%)
[43]. The annual operating cost of these systems can be estimated
as shown in Table 7 and Fig. 13. It needs to be noted that the total
heat (required only to be 45 °C here) produced by the prototype
system was estimated to be less than the amount of heat pump
condensation heat in Table 4, because the water was heated up
to more than 45 °C in a number of circumstances. The combination
of the prototype system with an auxiliary gas boiler was the most
economical and, when replacing conventional gas/electric water
heaters, such a combined operation could save nearly €13.00/
73.44, €11.16/32.61, and €71.46/49.25 per year in the regions of
London, Shanghai and Hong Kong, resulting in an annual cost sav-
ing ratio of 34.43%/74.79%, 72.03%/88.27%, and 92.95%/90.08%,
respectively.
5.2.3. Annual maintenance cost
The maintenance cost of a solar heating system is normally esti-
mated at 2% of the initial system cost due to its low maintenance
requirement [38]. The maintenance cost of a gas boiler was esti-
mated at €31.25/yr, €10/yr and €12.50/yr, respectively, in the
regions of London, Shanghai and Hong Kong [48,49]. An electric
water heater was considered free in terms of its maintenance dur-
ing its life cycle [41].
5.2.4. Cost payback period and life-cycle net cost saving
Table 8 gives the estimated payback period and life-cycle net
cost saving of such a prototype PV/LHP heat pump system when
supported by a conventional gas boiler. When replacing a typical
gas heater, this system has the shortest cost payback period of
up to 7 years and the highest life-cycle net cost saving of nearly
€2174 per m
2
in Hong Kong. In London, the cost payback period
will be around 8 years with a life-cycle net cost saving of about
€985 per m
2
after considering the renewable award of installing
a new solar thermal system. About 41 years (more than its life
span) would be required in Shanghai to reclaim the initial invest-
ment due to the current lowest gas charging tariff, which indicates
that it would be uneconomical to replace a gas water heater with
the proposed PV/LHP heat pump system in this area at the
moment. When replacing a conventional electric water heater,
the system’s payback periods were estimated at nearly 5, 20 and
14 years, respectively, in London, Shanghai and Hong Kong owing
to the different electricity charging tariffs and governmental sup-
port policies. The net cost saving would be around €2151, €184
and €756 per m
2
accordingly throughout the system life span.
Table 5
Capital cost breakdown of the prototype PV/LHP heat pump system.
No. Component Quantity/size Unit price (€) Cost (€)
PV/LHP module
1 PV layer 1 [89 Wp] 106.25 106.25
2 Loop heat pipe 3.2 [m] 20.00 20.00
3 Treated Al-alloy sheet 1 [piece] 18.75 18.75
4 Tempered glazing 1 [piece] 7.50 7.50
5 Aluminium frame 1 [piece] 8.75 8.75
6 Aluminium fin sheet 1 [piece] 13.75 13.75
Other components
7 Flat-plate heat exchanger 1 [1 kW] 18.75 18.75
8 Heat pump compressor 1 [1 HP] 125.00 125.00
9 Expansion valve 1 [piece] 6.25 6.25
10 Refrigerant 1 [300 g] 7.50 7.50
11 Water tank 1 [35 L] 6.88 6.88
12 Solar controller 1 [12V10 A] 10.00 10.00
13 Electric wire 1 [coil] 5.00 5.00
14 Battery 1[12V100 AH] 18.75 18.75
Other accessories
15 Thermal insulation 4 [piece] 7.50 7.50
16 Module bracket 1 [piece] 10.00 10.00
17 Silicon seal 2 [bottle] 3.75 3.75
Subtotal
Total system fabrication cost (€) 394.38
Additional profit (30% of total fabrication cost) 118.31
Capital cost (€) 512.69
Fig. 12. Cost breakdown of the PV/LHP heat pump prototype system.
Table 6
Renewables tariffs for the installation of a solar thermal system.
Location London Shanghai Hong
Kong
Tariff €0.24/kW h yr
heat [39]
Reduction in 13% of
capital cost [40]
Not found
Years 7 1 N/A
Total earning (€) 249.54 66.25 N/A
X. Zhang et al. / Applied Energy 135 (2014) 20–34 31

The analytical results in Table 8 demonstrate that it would be
most cost-effective for such a PV/LHP heat pump system with a
backup gas boiler to replace a gas water heater in Hong Kong
and an electric water heater in London. It would be very uneco-
nomical for the PV/LHP heat pump system to replace either type
of conventional water heater in the Shanghai area.
5.3. Environmental benefit prediction – life cycle carbon emission
reduction
The gas-to-CO
2
emission factor was estimated to be the same
for the three regions, as gas is directly burned for heat generation.
The CO
2
emission factor of electricity was considered different for
Table 7
Annual operating costs of different water heating systems.
Location London Shanghai Hong Kong
Energy price and demand
Gas price (€/kW h) [50–52] 0.06 0.03 0.19
Electricity price (€/kW h) [53–55] 0.18 0.08 0.15
Feed-in tariff (€/kW h) [39,40] 0.19 0.13 0.18
Total heat demand for hot water (kW h/yr) 519.91 431.13 328.04
PV/LHP heat pump system operational performance
Heat produced from system (kW h/yr) 148.02 317.58 328.04
Energy required from auxiliary heater (kW h/yr) 371.89 113.55 0.00
Total electricity output from module (kW h/yr) 39.85 79.03 73.89
Electricity consumed by heat pump (kW h/yr) 27.84 82.30 110.04
Net electricity output (kW h/yr) 12.01 3.27 36.15
Gas water heater (efficiency 80%)
Required gas energy (kW h/yr) 649.88 538.91 410.05
Operational cost (€/yr) 37.50 15.00 77.50
Electric water heater (efficiency 90%)
Required electricity energy (kW h/yr) 577.68 479.03 364.49
Operational cost (€/yr) 98.75 37.50 55.00
PV/LHP heat pump system + auxiliary gas boiler
Gas required from auxiliary gas boiler (kW h/yr) 464.86 141.94 0.00
Operational cost (€/yr) 24.76 4.34 5.43
Annual saving by replacing gas boiler (€/yr) 13.00 11.16 71.46
Annual saving ratio by replacing gas boiler (%) 34.43 72.03 92.95
Annual saving by replacing electric heater (€/yr) 73.44 32.61 49.25
Annual saving ratio by replacing electric heater (%) 74.79 88.27 90.08
PV/LHP heat pump system + auxiliary electric heater
Electricity required from auxiliary electric heater (kW h/yr) 413.21 126.17 0.00
Operational cost (€/yr) 67.99 9.99 5.43
Annual saving by replacing gas boiler (€/yr) 30.23 5.51 71.46
Annual saving ratio by replacing gas boiler (%) 0.00 35.57 92.95
Annual saving by replacing electric heater (€/yr) 30.21 26.96 49.25
Annual saving ratio by replacing electric heater (%) 30.76 72.98 90.08
Fig. 13. Annual operating costs of different water heating systems.
32 X. Zhang et al. / Applied Energy 135 (2014) 20–34

the three regions due to the different efficiencies of the national
power plants, given in Table 9.
Shanghai was found to have the highest life-cycle CO
2
emission
saving of about 4.08 ton/m
2
and 17.87 ton/m
2
when replacing gas
and electric water heaters with the prototype system respectively,
which is mainly owing to the lowest efficiency of its national
power plant. In London and Hong Kong, the prototype system
had a relatively lower carbon emission reduction of around
1.97 ton/m
2
& 7.92 ton/m
2
and 3.11 ton/m
2
& 11.27 ton/m
2
to
replace gas and electric water heaters, respectively. The analytical
results illustrate that the maximum environmental benefits would
be achieved by operating the PV/LHP heat pump prototype system
in the Shanghai area at the present time.
6. Conclusions
This paper reported a dedicated socio-economic performance
study of a novel LHP based PV/T heat pump hot-water generation
system for application in three different climatic regions: namely,
cold area (represented by London), warm area (represented by
Shanghai), and hot area (represented by Hong Kong). This involved
the prediction of the fossil fuel energy saving, return time on
investment and life cycle carbon emission reduction of the new
system, relative to the traditional gas-fired and electrical boilers
based water heating systems.
The energy performance of the proposed system was simulated
using the established dynamic model that delivered the monthly
thermal and power performance of the system over a typical oper-
ational year. Summary of the annual heat & power outputs from the
prototype panel in three typical climatic regions were 741.85 kW h/
m
2
yr & 120.74 kW h/m
2
yr in Hong Kong, 616.78 kW h/m
2
yr &
129.14 kW h/m
2
yr in Shanghai and 197.23 kW h/m
2
yr &
65.11 kW h/m
2
yr in London, respectively. It was seen that this pro-
totype system obtained the highest heat output in Hong Kong,
which could provide sufficiently high water temperatures (above
45 °C) throughout a year. In Shanghai, the prototype system
obtained a higher volume of electricity but a lower volume of heat
compared to Hong Kong, which could provide hot water service
up to 7 months. In London, the prototype module provided the low-
est volume of electricity and heat, which could only deliver hot
water service for 1 month independently. As a result, the system,
if operated in London, would consume a higher volume of additional
energy provided by the backup heaters. From energy efficiency
point of view, the system is most applicable to the hot climatic
region.
Instead of the energy outputs, the local energy charging rates
and renewable incentives were found to be the critical factors that
impacted on the investment return time. To replace a typical gas-
fired boiler, this prototype system (with a backup gas boiler) has
the shortest cost payback period of 7 years and the highest life-
cycle net cost saving of nearly €2174 per m
2
in Hong Kong. In Lon-
don, the cost payback period would be 8 years with a life-cycle net
cost saving of about €985 per m
2
after considering the renewable
award. It was found to be uneconomical to invest in the prototype
system in Shanghai, with a payback period of 41 years (more than
its life span) due to the low gas charging tariff. To replace a conven-
tional electric water heater, the system’s payback periods were
estimated at nearly 5, 20 and 14 years, respectively, in London,
Shanghai and Hong Kong. The life-cycle net cost saving would,
accordingly, be around €2151, €184 and €756 per m
2
. From the
economic point of view, this system seems most applicable to Lon-
don or Hong Kong where either the energy charging rates are high
or the governmental financial support is positive.
Apart from the energy output, the local carbon conversion fac-
tors of gas and electricity were found to be the most critical param-
eters that affected on the environmental benefits. The system
could obtain the highest life-cycle carbon reduction volume at
4.08 tons/m
2
and 17.87 tons/m
2
of in Shanghai when using it to
replace gas and electric water heaters respectively. In London
and Hong Kong, this system would have relatively lower life-cycle
carbon emission reduction of around 1.97 ton/m
2
& 7.92 ton/m
2
and 3.11 ton/m
2
& 11.27 ton/m
2
by replacing gas and electric water
heaters with it, respectively. This phenomenon could be inter-
preted in such a way: as the higher carbon conversion factor
implies the poorer energy quality (more carbon emission volume
of per kW h energy generation), the system appeared to be more
environmentally benefiting in a place where the energy quality is
lower, e.g. Shanghai.
A question may arise from the research: which area is most
suitable for the system application? Answer to this question is
highly dependent upon the priority order among the three factors:
(1) energy efficiency, (2) economic revenue, and (3) environmental
benefit. This is thought to vary with the users, local concerns and
policy influence, etc.
The research outcomes will be able to assist in decision making
in implementation of the new PV/thermal technology and analyses
of the associated economic and environmental benefits, thus con-
tributing to realisation of the regional and global targets on fossil
fuel energy saving and environmental sustainability.
Acknowledgements
The authors would acknowledge our sincere appreciation to the
financial supports from the University of Hull, Shanghai Pacific
Energy Centre, and EU Marie Curie International Research Staff
Exchange Scheme (R-D-SBES-R-269205).
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