fundamental study for the power tower's high concentrated

1 Current affiliation: DEC Dynamic Engineering Consultants

P.O.Box 79593, Dubai, United Arab Emirates

FUNDAMENTAL STUDY FOR THE POWER TOWER’S HIGH CONCENTRATED PHOTOVOLTAIC/THERMAL-COMBINED THERMAL RECEIVER

Ayman Hagfarah1 School of Engineering and Physical Sciences

Heriot-Watt University Dubai, United Arab of Emirates

Mehdi Nazarinia School of Engineering and Physical Sciences

Heriot-Watt University Dubai, United Arab of Emirates

ABSTRACT The present study introduces fundamental aspects of a

novel concentrated photovoltaics (CPV) technology. The

technology is based on combining CPV/T receiver along with a

solar thermal receiver. The combination is referred to as a

High Concentrated Photovoltaic/Thermal – Combined receiver

or HCPV/T-CT. The receiver is allocated in lieu of the

conventional solar thermal receivers in the solar tower power

plant schemes. The plant is designed to generate electricity and

thermal energy simultaneously prior to integration with the

conventional water desalination plant. The centralized

generation in the CPV/T- CT receiver will remarkably simplify

the complexity of the conventional solar power plants, and

eliminate the piping networks’ energy losses in the CPV/T

Dish tracking plants. The viability of the HCPV/T-CT power

tower plant has also been investigated by; firstly, designing and

simulating the plant performance using the System advisor

model (SAM) software, and secondly, designing a prototype

receiver and then deriving a mathematical model. The

Levelised Cost Of Electricity/Energy (LCOE) was found to be

0.119 $/kWhe and 0.021 $/kWhe for electricity and energy

generation, respectively, while the photovoltaic cells

temperature maintained below the 90 °C.

INTRODUCTION

The importance of the clean energy was emphasized

recently by the issues related to the fossil fuel impact on the

environment, energy security, the growth of the population and

the economic development. These factors brought the attention

to the solar energy as an alternative energy resource. The main

challenge for solar energy technologies is the capability to

provide a competitive low Levelised Cost Of Electricity

(LCOE) [1]. On the utility power generation scale the

conventional Concentrated Solar Power (CSP) technologies,

such as Thermal power tower solar plants and the Parabolic

Trough power plant, are comprised of many complex parts

(e.g. Steam turbines, generators, working fluid transportation

system, etc. …) and of these parts incur reduction in the

plant’s efficiency, another example of conventional

Concentrated Solar Power (CSP) technologies is the CPV/T

tracking based system plants where the efficiency is reduced

further due to the pumping and heat losses in the coolant’s

piping network [2]. Another factor that decreases the

concentrated photo-voltaic performance is the hot spot

formation which caused by the inhomogeneity of the

illumination on the cells’ surface and/or by the ineffective

cooling system [3]. The present study focuses on coupling two

of the emerging solar energy technologies; the concentrated

photovoltaic and the Solar power tower. The purpose of the

study is to investigate the viability of a novel type of the CSP

plant technology based on combining of the CPV/T receiver

along with the solar thermal receiver. This combination will be

referred to as a High Concentrated Photovoltaic/Thermal –

Combined Thermal receiver (HCPV/T-CT). The HCPV/T-CT

receiver is allocated in lieu of the conventional solar thermal

receiver in the solar tower power plant scheme. The

centralized concept of the CPV/T- CT plant is developed to

replace the complexity of the conventional CSP plants, and to

reduce the operation cost of the CPV/T Dish tracking and the

parabolic trough plants as it will eliminate the pumping and

the heat losses in the piping network. Moreover the central

fixed receiver is expected to significantly reduce the power

required to track the sun, as in the HCPV/T-CT technology the

sun is tracked only by the heliostats (mirrors) while in the

conventional photovoltaic (CPV/T) or the Stirling system it

required to move the whole system to track the sun [2].

Proceedings of the ASME 2016 10th International Conference on Energy Sustainability ES2016

June 26-30, 2016, Charlotte, North Carolina

ES2016-59051

1 Copyright © 2016 by ASME

II. PREVIOUS STUDIES: The high Concentrating Photovoltaic Power Towers

technology was disclosed by Frohberger [4], Dirk et al in 2010

when they conducted a feasibility study to evaluate the future

potential of the CPV Power Towers in Seville, Spain. In order

to calculate the reflected solar radiation on the receiver, they

used a tray tracing software called OPTIMTSA .The receiver

was a photovoltaic cells allocated on the tower in the lieu of

the conventional solar thermal receiver. They assumed that the

operating temperature of the PV cells was maintained at 25 °C

in their 1 MW prototype plant by means of a water cooling

system in an attempt to obtain a low value of the Levelised

Cost Of Electricity generation (LCOE), the study showed an

overall efficiency of 18.7% for the plant, and a LCOE of 0.29

$/kWh.

III. HCPV/T COMBINED THERMAL RECEIVER

The proposed receiver in this study is a combination of the

solar thermal receiver and a photovoltaic thermal receiver

CPV/T. The two receivers are mechanically coupled to each

other to form a one surface which its axis is parallel to the

tower axis. Each receiver comprises of a surface facing

outward from the power tower to continually receive the

reflected solar irradiance. The resulted apparatus is referred to

as Concentrated Photovoltaic Thermal Combined receiver. A

heat transfer fluid, water, will be pumped to be fluidly coupled

with inward surface of the photovoltaic’s substrate absorbing

the generated heat and controlling the PV cells temperature,

the coolant will flow through small designed passages to insure

a maximum heat transfer rate with minimum pumping losses.

To increase the exergy of the heat transfer fluid while

maintaining the operating temperature of the PV cells below

90 °C, as shown in Fig. (1).

Figure 1. Schematic drawing of the CPV/T- CT receiver, the

drawing shows the front view (aperture surface) of the receiver.

The coolant fluid will continue flowing upward

transferring the absorbed heat from the PV panels’ portion to

enter the solar thermal receiver portion where it will be heated

up to the desired temperature.

The combined solar thermal receiver comprises of

plurality of pipes coated with black paint and directly exposed

to the solar irradiation, a thick layer of insulation material will

be utilized to cover the backward surface of the whole receiver

in order to minimize the convection heat losses. The receiver

will be designed in a way to raise the water temperature up to

90 °C. The harvested thermal energy can be transferred by

means of heat exchanger to the conventional desalination plant

such as conventional Multi-Stage Flash or Multi-Effect

Distillation process [5].

A. Materials selection and evaluation The proposed cells for the receiver are triple junction

InGaP/InGaAs/Ge manufactured and provided by CESI in

Italy. The efficiency of the Cell was confirmed and tested in the

laboratory to be 39% tested at 512 Sun, 25 °C and 1.5 AM.

The Triple Junction PV cells were designed to be fixed on a

Direct Bonded Copper Ceramic Substrate, DBCu Nitride

(AIN), as shown in the Fig. (2).The substrate copper layer was

coated by Cu 70 ± 20 um, Ni 100-300uʺ and Au 20 ±5uʺ. The

gold wire bonding process was used to interconnect the

negative poles of the PV cells with the positive copper layer of

the DBC substrate, Direct Bonded Copper Ceramic Substrate,

DBCu Nitride (AIN), as shown in the Fig. (2).The substrate

copper layer was coated by Cu 70 ± 20 um, Ni 100-300uʺ and

Au 20 ±5uʺ. The gold wire bonding process was used to

interconnect the negative poles of the PV cells with the

positive copper layer of the DBC substrate.

In order to protect the cells without compromising the

quality of the solar irradiation a Low iron clear float glass with

anti-reflective coating is selected to cover the CPV assembly.

The selected glass for the receiver was manufactured by

Pilkington plc, the glass has a transmittance value of 93.9%

for the direct solar light for a 3 mm glass thickness as shown

on the fig (3). A secondary concentrator is used to reduce the

required number of the cells (cells raw material) and to

improve the solar illumination on the Cells surface by

redistributing the solar irradiance on the PV cells surface. Fig.

(2) shows the bypass diodes that were connected in parallel

with the module so to protect the cell from the hot spot

destructive effects in case of the mismatch current. In this

design the solder paste is used to attach the PV cells on the

DBC substrate. The paste contains 62% tin, and its thermal

conductivity is 50 W/K m. The thickness of the solder paste is

0.1 mm.


Figure 2. Front view of the HCPV receiver module aperture surface shows the 16 Triple junction cells and the bypass diodes soldered to the AIN substrate.

Figure 3. Cross section of the CPV/T receiver module shows the assembly of the secondary concentrator, cover glass, heat

exchanger, CPV cells and the Substrate.

B. The Heat Exchanger The design of the heat exchanger must assure the

capability to dissipate the generated heat (100 W/cm2) in order

to maintain the cell operating temperature below 90 °C. Micro

passages with 500 µm width confined by longitudinal extruded

fins with 500 µm width were designed to increase the heat

transfer rate. The fins are extruded from the heat exchanger’s

base plate as shown in the fig. (3), the base plate material must

have a high thermal conductivity and low cost as well. Since

the usage of the dissimilar metals causes the galvanic

corrosion, the heat exchanger base plate was made of copper to

match with the substrate copper bottom layer. The thickness of


the base plate was sized 3mm to assure proper diffusion for the

heat flux from the solar cells up to the coolant streams by

means of attempting to reduce the impact of the temperature

gradient of the coolant’s flow stream on the uniformity of PV

cells surface’s temperature. This factor was carefully

considered in this study not only for the mentioned reason but

also to avoid the possibility of the PV cells to work above the

90 °C as a result of the high thermal resistance caused by the

plate thickness. Performing the hot curing process on the

solder paste the 16 triple junction solar cells and the 6 Amp

Schottky diodes were fixed on the substrate to form the HCPV

receiver as shown in the Fig (4).

Figure 4 The assembled parts of the HCPV Receiver prototype.

IV. THE METHODOLOGY

A. System Advisor Model In order to investigate the plant potential and

reliability, as discussed System Advisor Model (SAM, version

2014.1.14) [6] software is used to design, optimize and

simulate the heliostats field and the performance of the

receiver along the day hours and for the whole year

considering Abu Dhabi city weather data. SAM considers the

heliostat’s cosine losses, blocking and shading, when

calculates the direct incident energy on the receiver aperture

area. The pumping requirements, convective and radiative

energy losses were also considered. SAM and the other solar

energy software do not exactly support the CPV/T receivers in

the power tower plant, thus at first the design of the Molten-

salt power tower is considered in lieu of the CPV/T receiver in

the simulation process, also the programme conducts the

financial analysis for the plant and generates the required

number of the heliostats. Then the hourly incident energy,

convective heat loss and the radiation losses of the simulated

conventional Molten-salt receiver for the 8760 hours of the

year is extracted and coupled with CPV/T receiver derived

formula in an excel sheet to find the exact electrical and

thermal energy annual yields.

A comparative study was conduct between the

CPV/T-CT power tower plant and the conventional solar

thermal power tower plant, the purpose of the study is to

illustrate the power output of each plant for the same aperture

area (incident solar energy) the receiver’s aperture area was

fixed at 578 m2 for the both plants. The first plant is the

conventional solar thermal power tower plant simulated

directly by SAM, the second one is the CPV/T-CT power tower

plant semi-simulated by SAM, the incident solar energy were

completely identical set at 500 Suns for both of the simulated

plants. The plants showed capacity of 72 MWe and 60 MWe,

respectively.

B. The Mathematical Model The mathematical model thoroughly analyzed the

heat and mass transfer in the assembly prior to design the

receiver and optimize its performance, the analysis included

the following process; firstly the fluid flow and the forced

convection heat transfer in the heat exchanger, Secondly the

conduction heat transfer through the CPV assembly, Thirdly

the free convection heat and the mass transfer in the secondary

concentrator void generated by the temperature difference,

finally the natural convection heat transfer and radiation heat

transfer between the CPV assembly and the surrounding

environment.

The amount of the generated thermal energy in the

solar cell is function in the cell efficiency and the efficiency in

turn depends on the cell’s temperature. So in order to calculate

the PV operating temperature, it is required first to know the

generated thermal energy on the cell, the complexity of the

analysis stemmed from the linkage of the all values with each

other. A logarithm was developed prior to generate the

mathematical model. The logarithm presume the cell operating

temperature ( initial value) and accordingly find the other

performance characteristics, the resulted values is set again as

initial values for the model, and consequently a series of

iterations processes were conducted to find the correct

operating values. The iteration process numbers were governed

be obtaining the desired residual error value.

V. THE RESULTS AND DISCUSSION

A. The Mathematical Model The solar concentration values (Cr) are plotted with

respect to the other cell’s performance parameters (Cells

operating temperature, Reynolds number, Combined Thermal

Receiver’s efficiency, PV cell’s efficiency and the coefficient of

performance) which based on the mathematical model results.


The selection of the curves were focused on the non-

dimensional parameters were they can be projected on the

actual receiver size.

The variation of Reynolds number is shown on Fig. 5, it

can be noticed that Reynolds number in the CPV/T receiver

portion was increased in a linear trend with respect to the solar

concentration, this attribute to the increase in the demand for

the water flow rate to carry the generated thermal energy on

the cells as a result of the increase in solar concentration.

Figure 5. Performance curve shows the Reynolds Number for the CPV/T prototype receiver vs. the solar

concentration.

The efficiency of the cells was simulated for each of the

corresponding PV operating temperature; each solar

concentration value requires to adjust the coolant flow rate at

certain value prior to maintain the cooling water-outlet form

the CPV/T portion at 60 °C, although the operating

temperature for the PV cells should be constant since the mass

flow rate keep increasing along with the solar concentration to

overcome the generated heat, instead as shown in the Fig. (6)

it is noticed that the temperature slightly increased along with

the solar concentration, the explanation is accounted for the

CPV assembly materials properties. The minimum allowable

thickness for these materials and its thermal conductivity form

a thermal resistance, the resistance of the CPV structure in

turn creates temperature gradients between the cooling water

and the cells surface.

The second portion of the CPV/T CT receiver is the solar

thermal receiver, it important to consider its performance

characteristic in the study, the efficiency of the receiver was

simulated by SAM programme for the 8761 hours per year

considering Abu Dhabi city weather data. The performed

efficiency calculation was based on the radiation and

convection losses which were simulated by SAM.

Figure 6. Performance curve shows the PV cell surface’s

operating temperature for the CPV/T prototype receiver vs. the solar concentration.

As shown on the Fig. (7) the best operating

conditions for the thermal receiver is found at the higher solar

concentrations, it obvious that the increase in the rate of the

absorbed heat gain is higher than the increase of the radiation

and convection heat losses rate from the aperture surface.

Figure 7. Performance curve shows the thermal efficiency

of the solar thermal receiver portion vs. the solar concentration, the receiver design for maximum

concentration of 1000 sun.

Also the Coefficient Of the Performance (COP) of the

CPV/T receiver where calculated for the solar concentration

range from 300 to 1000 Suns considering the generated

electricity power against the pumping losses in the heat

exchanger , the results showed a slight decrease in the C.O.P

along with the increase of the sun concentration. The value of


the C.O.P. can be approximated to 99.99% throughout the sun

concentration rang (300 – 1000 sun), the pumping losses in

the heat exchanger is almost negligible compared the

generated power, the low pumping losses is accounted to the

low Reynolds number as shown on the fig. (5).

B. System Advisor Model Using the System Advisor Model (SAM) to design and

simulate the heliostats and the receiver size, it was found that

the tower will require 3767 heliostats to generate 500 Suns on

the receiver aperture area. It also noticed that the CPV/T-CT

plant produce thermal energy 545 GWh/year equivalents to

almost times as much as its electrical power production 95

GWh/year. The Packing factor of the PV cells converts as

much as 27 % of the incident solar irradiance directly to

thermal energy; although this energy will be exploited in the

desalination process downstream, yet it incurred a heat

exchanger design with higher heat transfer coefficient and

higher coolant flow rate as well. This result emphasize the

importance of the packing factor and the necessity of

researches for technologies that can increase the packing factor

by develop a wire bonding technique require less gap area

between the cells.

Table (1) below shows the calculated break down cost for

the CPV/T-CT plant, the break down showed that the

heliostats cost 94$ Million which dominates the plant cost by

almost 50 %, While the CPV cells contribute by only 5%, this

figures outlined the importance of researches which focus on

the heliostats cost reduction rather than the Multijunction

cells. The main result of the feasibility study is LCOE and as

shown in the Table (1) the LCOE for the CPV/T-CT Power

Tower Plant was found to be 0.1186 $USD/kWh, this value is

considered low and competitive compared to the present LOCE

values in the PV-utility scale market, the minimum LOCE

(Electricity) value was 0.119 $USD/kWh in 2013 [1]. .

Table 1. The installation and Operation costs of the CPV/T-CT Power Tower Plant. Description Value Units

Payback period 8.7 Year

Net Present value 25,427,852 $USD

Installation Cost Rate 3,212 $USD/kWh

LCOE (Electricity) 0.11862 $USD/kWh

LCOE (Energy) 0.02074 $USD/kWh

VI. CONCLUSION The study showed that the utilization of CPV/T Combined

Thermal receiver technology in lieu of the conventional solar

thermal receiver is reliable and economically viable. the

simulation results showed that the CPV/T CT power plant has

a Levelised Cost of electricity generation equal to 0.119

$/KWhe, while the equivalent conventional power tower plant

with the thermal receiver has a LCOE 0.165 $/KWhe. Since

the CPV/T CT plant produce thermal energy and electricity

simultaneously, the Levelised Cost of energy generation is so

significant for the evaluation of the plant performance, the

calculation and simulation results showed 0.021 $/KWh of

Levelised Cost of energy generation. The generated

mathematical model proved the ability of the CPV/T receiver

to raise the water temperature up to 65 C ° while maintaining

the PV cells temperature below the 90 C° when the inlet water

temperature is 25 C°. In the CPV/T CT receiver, the

percentage of aperture area of the thermal receiver to the

CPV/T portion was 43.3 %, this ratio of the surface area

APV/Asolar can be projected to any size of the CPV/T CT

power tower plant along with the associated operating

conditions. The design of the HEX showed that it requires a

laminar water flow pattern through the fins’ passages with

maximum 81.16 Reynolds number to maintain the PV cell

temperature below 90 C° under 1000 sun concentration. The

remarkably low Reynolds number for the cooling water flow

reduced the pumping power losses and increased the

Coefficient of performance for the CPV/T receiver to 99.99%.

ACKNOWLEDGMENTS The authors wish to thank CESI S.p.A., Milan, Italy for

their collaboration by manufacturing and providing the Triple

junction solar cells.

REFERENCES [1] Hoeven M., 2014, “Technology Roadmap, Solar

Photovoltaic Energy” International Energy Agency, Paris,

France.

[2] Lovegrove K., Watt M., Passey R., Pollock G., Wyder J.,

and Dowse J., 2012, “Realising the potential of Concentrating

Solar Power in Australia” Australian Solar Institute, Australia.

[3] Baig H., Heasman K., and Mallick T. M., 2012 “Non-

uniform illumination in concentrating solar cells”, Renewable

and Sustainable Energy Reviews, Vol. 16, 8, pp. 5890-5909.

[4] Frohberger D., Jaus J., Wiesenfarth M., Schramek P. and

Bett A., 2010, “Feasibility Study on High Concentrating

Photovoltaic Power Tower” Proc. 6th international conference

on concentrating photovoltaic systems, A. Bett and F. Dimroth,

eds., Freiburg, Germany, pp. 194–198.

[5] Al-Karaghouli A., and Kazmerski L., 2011 “Desalination,

Trends and Technologies”, M. Schorr, eds., National InTech,

Colorado, pp.150.

[6] NREL of the U.S. Department of Energy, Office of Energy

Efficiency & Renewable Energy, Forrestal Building, 1000

Independence Avenue, SW, Washington, DC 20585, U.S.A.


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