THE POTENTIAL OF SOLAR ENERGY IN FOOD-INDUSTRY PROCESS HEAT APPLICATIONS

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THE POTENTIAL OF SOLAR ENERGY IN FOOD-INDUSTRY PROCESS HEAT
APPLICATIONS

Soteris Kalogirou
Department of Mechanical Engineering, Higher Technical Institute, P.O. Box 20423, Nicosia
2152, Cyprus, Tel. +357-2-306266, Fax: +357-2-494953, Email: skalogir@spidernet.com.cy

ABSTRACT: - In this paper an overview of the potential of solar industrial process heat, with
emphasis on the food industry, is presented. The temperature requirements of food industry
applications range from 60°C to 180°C. The characteristics of low to medium temperature solar
collectors that can be employed are given and an analysis of the efficiency and cost of the solar
systems is presented. Based on TRNSYS simulations, an estimation of the solar contribution of
solar process heat plants operating in Cyprus are given for different collector technologies. The
annual energy gains of such systems are from 610 to 910 kWh/m2-a and the resulting energy
costs obtained for solar heat are from 0.028 to 0.05 Euro/kWh depending on the collector type
applied. The costs will even be more favourable if the solar collectors become cheaper and
subsidisation of the fuel is removed.

Keywords: Food industry, process heat, stationary collectors, economic analysis


1. INTRODUCTION

Beyond the low temperature applications there are several potential fields of application for
solar thermal energy both at low to medium temperature level (60°C – 180°C). The most
important of them are: heat production for industrial processes, solar cooling and air
conditioning, solar drying and seawater desalination. Large amounts of energy are spent for
industrial heat generation in many countries. For example industrial process heat demand in the
southern European countries is about 15% of the overall demand of final energy requirements.
Stationary collectors have been developed with a good relation of cost and performance at
low to medium temperature. Recent developments in the field of medium and high temperature
solar collectors are summarised and an overview of efficiency and cost of existing technologies
is given.
The objective of this work is to investigate, based on TRNSYS simulations, the system
energy yield of solar process heat plants for different collector technologies. For this purpose
the climatic conditions of Cyprus will be employed by using the typical meteorological year
(TMY) for Nicosia, Cyprus. Finally by using the obtained simulated performance data, an
economic feasibility study is carried out in order to examine the viability of the systems
considered.


2. THE INDUSTRIAL PROCESS HEAT DEMAND

From a number of studies on industrial heat demand, several industrial sectors have been
identified with favourable conditions for the application of solar energy. The most important
industrial processes using heat at a medium temperature level are: sterilising, pasteurising,

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drying, hydrolysing, distillation, evaporation, washing and cleaning. Some of the most
important processes related to food industry are given in Table 1.
Large scale solar applications for process heat benefit from the effect of scale. Therefore the
investment costs should be comparatively low, even if the costs for the collector are higher. One
way to cause economically easy terms is to design systems without heat storage, i.e., the solar
heat is fed directly into suitable processes (fuel saver). In this case the maximum rate at which
the solar energy system delivers energy must not be appreciably larger than the rate at which the
process uses energy. This system however cannot be cost effective in cases where heat is needed
at the early or late hours of the day or at nighttimes when the industry operates on a double shift
basis.
Particular types of food industries, which can employ solar process heat, are the milk and
cooked pork meats (sausage, salami etc.) industries and breweries. The temperatures required in
these industries range from 60 to 180°C. Favourable conditions exist in food industry, because
food treatment and storage are processes with high energy consumption and high running time.

Table 1. Temperature ranges for different food industrial processes

Industry Process
Temperature
(°C)
80 100 120 140 160
DAIRY PRESSURISATION






STERILISATION
DRYING
CONCENTRATES
BOILER FEED WATER
TINNED FOOD
STERILIZATION






PASTEURISATION
COOKING
BLEACHING
MEAT WASHING,





STERILISATION
COOKING
BEVERAGES WASHING,






STERILISATION
PASTEURISATION
FLOURS & BY- STERILISATION

PRODUCTS

In a solar process heat system, interfacing of the collectors with conventional energy supplies
must be done in a way compatible with the process. The easiest way to accomplish this is by
using heat storage, which can also allow the system to work in periods of low irradiation and/or
nighttime. Where feasible, collectors can be mounted on the roof of a factory especially when
no land area is available. In this case shading between adjacent collector rows should be avoided
and considered. Some of the most important food processes are:

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2.1 Brewing and Malting

In the brewing process thermal energy accounts for about 80% of the total final energy
demand. The principal consumers are the wort boiling (25-50%), the bottle washers (25-40%),
and pasteurisation. Solar thermal energy can be used for low-pressure steam generation at 100–
110ºC and for refrigeration of the wort, which can be accomplished with absorption cooling.
In the malting process the principal energy consumption is for drying of the barley before
malting, and of the grains after germination to stop germination, for conservation where hot air
at 60 to 80ºC is used, and for cooling of air in the germination process required during the
summer months.

2.2 Milk industry

Dairies are very interesting applications for solar energy, because they often work seven days
a week, thus fully utilise the solar system compared to other industries, which allow the system
to be idle for two days per week.
In the milk industry thermal energy is used for pasteurisation (60-85ºC) and for sterilisation
(130-150ºC) processes. Drying of milk powder, due to the high constant energy demand, is
another important consumer. In the production, milk and whey are spray-dried in huge towers
with air, which is heated from 60°C to 180°C. The drying process can have a running time up to
about 8000 hours per annum [1].

2.3 Food preservation

Several processes were identified, where solar thermal energy could be used: scalding of
vegetables, sterilization (vegetables, fish, meat, baby food) with hot water or direct steam,
scalding, cleaning and pre-cooking of fish, sealing and cleaning of cans, cooking. In addition
cold demand can be covered by solar cooling and refrigeration.


3. CHARACTERISTICS OF SOLAR COLLECTORS

There are many types of collectors that can be applied for industrial process heat. The most
common and industrially matured systems are the flat-plate, compound parabolic and evacuated
tube collectors.
Due to the introduction of highly selective coatings, standard flat-plate collectors can reach
stagnation temperatures of more than 200°C. With these collectors good efficiency can be
obtained up to temperatures of 100°C.
Additional improvements in efficiency of flat-plate collectors and an extension of the range
of possible working temperatures up to 150ºC can be obtained by suppression of convection
heat transfer by evacuation (evacuated tubes, evacuated flat plate collector), gas fillings with
inert gases, convection barrier by an additional plastic foil or by honeycomb-type transparent
insulation (TI) materials [2].
Lately some modern manufacturing techniques have been introduced in the industry like the
use of ultrasonic welding machines, which improve both the speed and the quality of welds.
This is used for the welding of risers on fins in order to improve heat conduction. The greatest

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advantage of this method is that the welding is performed at room temperature therefore
deformation of the welded parts is avoided.
Compound parabolic collectors (CPC) are non-imaging concentrators and when a low
concentration ratio (up to about 2, corresponding to an acceptance half angle of 30º) is used
these collectors can be stationary. A CPC concentrator can be orientated with its long axis along
either the north-south or the east-west direction and its aperture is tilted directly towards the
equator. When orientated along the north-south direction the collector must track the sun by
turning its axis so as to face the sun continuously. As the acceptance angle of the concentrator
along its long axis is wide, seasonal tilt adjustment is not necessary. It can also be stationary but
radiation will only be received the hours when the sun is within the collector acceptance angle.
A large number of evacuated tube collectors are on the market. Evacuated tubes with CPC-
reflectors are also commercialised by several manufacturers. One manufacturer recently
presented an all-glass evacuated tube collector, which may be an important step to cost
reduction and increase of lifetime. Evacuated tube collectors have demonstrated that the
combination of a selective surface and an effective convection suppressor can result in good
performance at high temperatures. The vacuum envelope reduces convection and conduction
losses, so the cylinders can operate at higher temperatures than flat-plate collectors. Like flat-
plate collectors, they collect both direct and diffuse radiation. However, their efficiency is
higher at low incidence angles. This effect tends to give evacuated tube collectors an advantage
over flat-plate collectors in day-long performance.


4. COMBINATION OF SOLAR SYSTEM WITH CONVENTIONAL HEAT SUPPLY

The central system for heat supply in most factories uses steam at a pressure corresponding
to the highest temperature needed in the different processes. Typical maximum temperatures are
about 160–180ºC. Hot water or low pressure steam at medium temperatures (<150ºC) can be
used either for preheating of water (or other fluids) used for processes (washing, dyeing, etc.) or
for steam generation or by direct coupling of the solar system to an individual process working
at temperatures lower than that of the central steam supply. In the case of water preheating
higher efficiencies are obtained due to the low input temperature to the solar system, thus low-
technology collectors can work effectively and the required load supply temperature has no or
little effect on the performance of the solar system. The system may be pressurised in order to
allow storage at temperatures higher than 100°C.
The system shown schematically in Fig. 1 consists of an array of collectors, a circulating
pump and a storage tank. It includes also the necessary controls and thermal relief valve, which
relieves energy when storage tank temperature is above a preset value. The system is once
through, i.e., there is no hot water return to storage, which is what usually happens in food
industry applications. The used hot water is replaced by mains water. Mean monthly ground
temperature values are used for the mains water temperature in simulations. When the
temperature of the stored water is above the required process temperature, this is mixed with
mains water to obtain the required temperature.
If no water of adequate temperature is available in the storage tank its temperature is topped-
up with an auxiliary heater before use. For the modelling and simulation of the system the well-
known program TRNSYS is employed [3].

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5. ANNUAL ENERGY GAINS

To estimate the solar energy gains TRNSYS-simulations were carried out using the typical
meteorological year (TMY) for Nicosia, Cyprus [4]. Cyprus is located at the Eastern
Mediterranean at 35° north latitude. The climatic conditions of Cyprus are predominantly very
sunny with daily average solar radiation of about 5.4 kWh/m2 on a horizontal surface. In the
lowlands the daily sunshine duration varies from 5.5 hours in winter to about 12.5 hours in
summer. Mean daily global solar radiation varies from about 2.3 kWh/m2 in the cloudiest
months of the year, December and January, to about 7.2 kWh/m2 in July. The amount of global
radiation falling on a horizontal surface with average weather conditions is 1727 kWh/m2 per
year.

3-way valve

Auxiliary Load

Heater

Relief
valve

Collector

Array
Storage

Tank


Differential

thermostat



Make-up water


Solar pump

Fig. 1: Possibilities of the combining the solar system with the existing heat supply.

Four representative collector types were considered in this study:
• Flat-plate collector (FP).
• Advanced flat-plate collector (AFP). In this collector the risers are ultrasonically welded
to the absorbing plate, which is also electroplated with chromium selective coating.
• Compound parabolic collector (CPC) orientated with its long axis in the east-west
direction and tilted at local latitude (35°).
• Evacuated tube collector (ETC).
The collector characteristics are given in Table 2.

Table 2. Collector characteristics for the four collectors considered in this study.

Collector type
Optical efficiency (no)
Overall heat loss coefficient (UL) [W/m2K]
FP
0.79
6.67
AFP
0.80
4.78
CPC
0.72
1.51
ETC
0.82
2.19
²]

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The basic process considered is one where 2000 kg/hr of hot water are used at temperatures
between 60 to 180°C (load). The load is required for the first three quarters of each hour. The
industry is assumed to work on a single shift basis from 8.00 to 16.00. For the estimation of the
annual energy supply to the above process, the following assumptions were made:
• The collector field has a gross area of 400 m² and the inclination of FP, AFP and ETC
collectors is equal to the local latitude plus 5° (i.e., 40°), whereas the inclination of CPC
collectors is equal to 35°. Mutual shading of collectors is considered.
• Heat losses of the piping are considered. It is assumed that the collector field is
connected to the process with pipes 30 m long.
• The storage tank capacity is 25m3.
• Collector circuit flow rate is 6 kg/s.

The annual energy yield of the various collectors is shown in Fig. 2. As can be seen the
performance of the systems is insensitive to the load supply temperature. At low load
temperatures the water in the storage tank can satisfy most of the demand and as less water is
replaced in the storage tank by make-up water the storage tank remains at increased temperature
and thus the collectors are operating at higher temperature hence the collector is less effective
which is reflected as lower energy yield. The solar contribution F [i.e., (Qload-Qauxiliary)/Qload] is
also shown in Fig. 2. As can be seen the lower the load temperature the higher is the
contribution of the system and vice versa. This is because at higher load temperatures more
energy from the auxiliary is required to cover the load.

1000
1
900
0.9
a)
800
0.8
)
2
700
0.7
600
0.6
Nomenclature:
500
EY- Energy Yield
0.5
F- Solar contribution
400
0.4
Solar contribution (F
Energy yield (kWh/m
FP-EY
AFP-EY
300
CPC-EY
ETC-EY
0.3
FP-F
AFP-F
CPC-F
ETC-F
200
0.2
60
90
120
150
180
Load temperature (°C)

Fig. 2: Annual energy yield delivered to the process and solar contribution (F) of systems.

The collector with the higher energy yield and the higher contribution is the evacuated tube
and the lower is the typical flat plate collector. This result is in agreement with the performance
characteristics of the collectors shown in Table 2, i.e., a collector with better characteristics
gives more energy and thus has a higher contribution and vice versa. It should be noted that the
load is constant (same process) for all systems. This finding however has to be compared with
the economics of the system in order to select the best collector for this application. The critical
parameter for such an analysis is the cost of the collector.

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6. ECONOMIC ANALYSIS

A life cycle analysis is performed in order to obtain the total cost (or life cycle cost) and the
life cycle savings of the systems. Table 3 shows the estimated costs per square meter of the
collectors considered. The economic scenario used in this project is that 30% of the initial cost
of the solar system is paid at the beginning and the rest is paid in equal instalments in 10 years.
The period of economic analysis is taken as 20 years (life of the system), whereas the inflation
rates of fuel and electricity, used for pumps, are mean values of the last 10 years. Maintenance
and parasitic costs are also considered. Light fuel oil (LFO) is assumed to be used for a fuel-
only system. From the addition of fuel savings incurred because of the use of the system and the
tax savings the mortgage, maintenance and parasitic costs are subtracted and thus the annual
solar savings of the system are estimated which are converted into present worth values of the
system. These are added up to obtain the life cycle savings. A detailed description of the method
is given in [5].
It should be noted that in Cyprus the petrol subsidises LFO, and its normal price should be
60% more than the current market price. Both the current and the non-subsidised fuel price are
considered in the analysis.
The total system cost is estimated by adding up the initial payment the maintenance and
parasitic costs and the mortgage payments. From this figure the tax savings are subtracted and
the result is divided by the number of years the system is operational (life of the system) and the
total kWh produced by the system during its life, in order to obtain the heat price in Euros per
kWh for each collector technology considered, for comparison purposes.

Table 3. Investment cost parameters for collectors considered in this study.

Collector type
Collector price (Euro/m2)
FP
190
AFP
220
CPC
310
ETC
430
Prices include collector mountings and field piping.

The heat price obtained from the economic analysis for all the collectors considered here are
presented in Fig. 3 together with the current price of LFO and its non-subsidised value. As can
be seen the cheaper the collector the better is its economic viability, i.e., a lower heat price value
and bigger life cycle savings are obtained. As can be seen from Fig. 3 for the present application
and for the current price of LFO only the simple flat plate collector is viable. When a non-
subsidised value of LFO is considered however, all the collectors are viable.
The life cycle savings of the different systems considered by using both the normal and the
non-subsidised fuel prices are shown in Fig. 4. Life cycle savings represent the money saved by
installing the solar system instead of buying the fuel. As can be seen for the normal fuel price,
only the relatively cheap, FP and AFP collectors, are viable giving positive life cycle savings.
The other two types considered, i.e., CPC and ETC give negative life cycle savings, which
means that at the current fuel price it is more economic to use a conventional fuel system
instead of investing for a solar system.

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FP-HP
AFP-HP
CPC-HP
ETC-HP
LFO
LFO-N/S
0.055
0.05
h)
0.045
0.04
0.035
0.03
Heat price (Euro/kW
Nomenclature:
0.025
HP-Heat price
N/S-Non-subsidised
0.02
60
90
120
150
180
Load temperature (°C)


Fig. 3: Heat prices as a function of load temperature

It should be noted however that the situation is completely different for the case where the
non-subsidised fuel price is considered. The LCS of all cases examined are positive which
means that all systems are viable. The best collector giving the higher life cycle savings is the
AFP, which presents a good energy yield, compared to its cost. Much higher LCS are obtained
in this case as the solar system is replacing a more expensive fuel. It can also be concluded from
the results presented in Fig. 4 that it is more advantageous to apply solar energy to higher
temperature processes than to lower temperature ones as the savings incurred are much higher.
Additionally the load supply temperature, for values higher than about 100°C, has very little
effect on the resulting life cycle savings of the systems.

FP
AFP
CPC
ETC
FP-N/S
AFP-N/S
CPC-LCS N/S
ETC-LCS N/S
80000
)
60000
40000
20000
0
-20000
-40000
Nomenclature:
Life cycle savings (Euro -60000
N/S-Non-subsidised
-80000
60
90
120
150
180
Load temperature (°C)

Fig. 4 Life cycle savings as a function of load temperature for the various systems considered

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From the above discussion it can be concluded that the viability of the systems depend on
their initial cost and the fuel price. None of these costs are stable but are changing continuously
depending on international market trends and oil production rates. Finally it should be noted that
the systems considered here should be optimised by trying a range of collector areas and storage
volumes thus finding the best combination that gives the best life cycle savings.


7. CONCLUSIONS

An industrial process heat system for the food industry is analysed in this paper both with
respect to the energy yield and the resulting heat price for a number of collector technologies.
The annual energy gains of such systems are from 620 to 915 kWh/m2-a. The resulting energy
costs obtained for solar heat are from 0.028 to 0.05 Euro/kWh depending on the collector type
applied. These results are applicable to any country with similar weather conditions as Cyprus.
As is proved in the analysis presented in this paper the economic viability of the systems
depends on the initial cost of the solar systems and the fuel price. The costs will turn out to be
more favourable when the solar collectors become cheaper and subsidisation of the fuel is
removed. At the design stage the solar systems to be considered need to be simulated and their
economic benefits evaluated as indicated in this paper in order to select the best system for the
particular application at the collector cost and fuel price applicable. It is believed by the author
that solar energy should be given a chance even if the costs are at present may not be so
favourable. As the oil reserves are depleted the oil prices will certainly increase and thus solar
systems can provide real economic benefits.

REFERENCES

1. Benz N., Gut M. and Beikircher T., Solar Process Heat with Non-Concentrating Collectors for
Food Industry, Proc. of Solar World Congress’99, Jerusalem, Israel on CD ROM, 1999.
2. Benz N., Hasler W., Hetfleisch J., Tratzky S. and Klein B., Flat-Plate Solar Collector with
Glass TI. Proceedings of EuroSun 98, Portoroz, Slovenia, 1998.
3. Klein S.A. et al., TRNSYS 14.2, A Transient Simulation and Program. Solar Energy
Laboratory, University of Wisconsin, Madison, Wisconsin, WI. 1996.
4. Petrakis M. et al., Generation of a “Typical Meteorological Year” for Nicosia, Cyprus,
Renewable Energy, 13, 381-388, 1998.
5. Kalogirou S., Economic Analysis of Solar Energy Systems Using Spreadsheets, Proceedings
of the Forth World Renewable Energy Congress, Denver, Colorado, USA, pp. 1303-1307,
1996.

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