CONSTRUCTION AND OUT-DOOR TESTING OF A LOW COST,
SOLAR COLLECTOR FABRICATED FROM PLASTIC “TIM”
MATERIAL
L. Tóth, L. Horváth*, G. Mink *
Institute of Mechanical Engineering, Polytechnic of Dunaújváros, 2401 Dunaújváros,
POBox 152, HUNGARY Tel: +36 25 551 100; Fax: +36 25 551 100; e-mail:
totlaci@mail.poliod.hu
*Research Laboratory of Materials and Environmental Chemistry, Chemical Research
Center, Hungarian Academy of Sciences, 1525 Budapest POBox 17, HUNGARY
Tel: +36 1 325 5992, Fax: +36 1 335 7892, e-mail: mink@chemres.hu
Abstract: The presently used, high quality solar collectors are designed to 6 bar pressure and
for the use of antifreezing fluids as heat carriers. Their wide-spread application, however, is
hindered in many countries, because their price is relatively high, it is about 200 euro/m2. It
was assumed that from solar grade transparent insulation materials (TIMs) such atmospheric
collectors can be fabricated that and do not need antifreezing material and might result in
significant cost reduction in solar water heating.
In the present paper the results of the out-door testing of a plastic TIM collector of 1.35 m2
area will be presented. The collector body is fabricated from 18 mm thick, three-walled
polycarbonate structure with longitudinal channels between the walls. The two upper walls
serve as double glazing. In the lower channels black textile strips are layered that serve as
solar absorbers. The unit is tilted. Pure water, used as a heat exchange medium is pumped
from an atmospheric hot water storage tank to the upper extremity of the collector where it is
distributed to each channel. Inside the collector water flows by gravity, wetting continuously
the black textile and then, it enters the water storage tank.
Outdoor tests have shown that the collector efficiency (in efficiency vs. T/G representation,
up to T/G = 0.06) is only about 10 % lower than that of a state-of-the-art collectors with
selective absorber. Economic estimations for a domestic hot water system consisting of TIM
collectors and an atmospheric water storage tank are also given.
Key words: plastic collector, performance test, economic estimation
1. INTRODUCTION
Solar collectors are widely used for process heat, space heating and/or producing domestic hot
water by solar energy. It is recognized in many countries that the large scale application of
this clean and environmentally safe solar technology decreases significantly the use of fossil
energy resources and the impact on the environment. Therefore, the propagation of the solar
collectors has become one of the main points of the energy policy in many countries.
For water-heating the most practical collectors are the flat-plate collectors which consist of a
transparent glazing, preferably 4 mm thick glass pane of low iron content; a metal absorber,
generally a 0.5 – 1.0 mm thick copper plate in metallic contact with the metal tubes that
transport the heat exchange fluid, and proper insulation made by inexpensive insulation
materials to decrease back and edge losses. To decrease the top losses of the collector, in most
cases the absorber plate is covered with a selective layer which has high absorptivity for solar
radiation and a very low emissivity for longwave radiation.
Generally, all the above parts are placed in a metal housing, most likely in a stainless steel
one. To achieve better performance, the collectors are tilted towards South, ideally with a tilt
angle that results closely perpendicular incident angle in the peak hours. The heat exchanging
medium enters the collector from below and exits the collector at its upper extremity.
Therefore, the transportation of the fluid is mainly based on auto circulation, but in most cases
liquid pumps are also used. The flat plate collectors are designed to 6 bar pressure, since their
stagnation temperature is much above 100 oC. Due to the local climatic conditions, in many
countries antifreezing liquid, a poisoning material should be used as heat carrier. This means
extra costs when installing a solar collector, and also, when – generally after 5-8 year use - the
liquid has to be replaced by a new charge. To avoid the use of antifreezing materials and
make simpler the system installation, specially designed elastomer-metal absorber has been
developed and analyzed by Bartelsen et al. [1]. In this system plastic tubes of improved
thermal conductivity and improved mechanical strength were integrated to a copper plate
which was supplied with a selective layer.
Solar grade transparent insulation materials (TIMs) are produced in large quantities for solar
architecture. Most of the TIM materials are produced from thin wall structured polycarbonate
structures and therefore, they have excellent thermal (up to 120 oC) mechanical, light
transmission and heat insulation properties. Because of their thin walls, another advantage of
these structures is that they are as light as 1-3 kg per square metre. Till now a wide variety of
structured polycarbonate TIM materials have been developed. At present they are produced in
large quantities and marketed at reasonable price ranging from 10 to about 30 euro per square
metre. To avoid polymer degradation, these glazings are supplied with factory made UV
absorption layer(s), optionally on one or both sides. To reduce the price of the collectors,
several new, all-plastic solar collector prototypes, utilizing both coaxial tubular plastic as
collector or using double-walled, and selectively coated TIM as absorber plate have been
recently developed and analyzed by Kudish et al. [2]. It is worthwhile to mention here that in
these systems the water flow is cross-sectional both in the inner, coaxial tubes and also in the
channels of the TIM. In other words, such systems can be operated only in such areas where
the temperature never (or only very seldom) falls below zero.
Another risk of using plastic materials is that their mechanical strength of plastics decreases
significantly with increasing temperature. Therefore, plastic units can not be operated under
pressure at temperatures close to 100 oC.
The aim of the present work was to develop, test and analyze a prototype domestic hot water
system which consists of a solar collector made of plastic TIM material and a compatible
atmospheric water storage and process control system that fulfills the following requirements:
- The whole system is atmospheric and process controlled to avoid temperatures higher
than 100 oC and the deformation or the damage of the TIM at high temperatures.
- The collector is designed so that its thermal efficiency approaches that of the present
solar collectors.
- The system allows the use of pure water without the risk of frost-crack in cold climates or
in cold seasons, and
- The system offers price reduction in solar hot water production.
2.
SYSTEM DESCRIPTION
2.1. Collector body from 3-walled plastic TIM material
The width of the structured PC materials is up to about 2 m, and their length is up to about 9
m. Their ultimate advantage is that easy to cut them at either direction. Much care has to be
paid, however, for their installation, following strictly the instruction of the producer,
because of the high thermal expansion of plastics. According to the producer’s instructions,
the module can be tilted or curved, but only in the direction of the longitudinal channels.
The starting material of the collector body is shown in Fig. 1. It is a 18 mm thick, threewalled polycarbonate structure with longitudinal channels between the walls. The spacing
between the channels is 20 mm.
Figure 1. Front view (A) and side view (B) of a 3-walled plastic TIM used for collector body.
The schematic diagram of the collector prototype is shown in Fig. 2. The two upper walls
serve as double glazing. In the lower channels black textile strips are layered on wall 3 that
serve as solar absorbers. When the pump is on, pure water, the heat exchange medium is
pumped from the hot water storage tank to the upper extremity of the collector where it is
distributed to each channel. In the present case the distribution of water was made with a
multi-bored cross-sectional feeding tube, but a method based on capillary activity [4] also
proved to be reliable.
The unit is tilted. In contrast with the models reported by Kudish et al. [2], inside the collector
water flows by gravity and then enters the upper part of the hot water storage tank, wetting
continuously the black textile. This water flow is not cross-sectional since it flows in the form
Sunshine
Black textile
bands
wall 1
wall 2
Water in
wall 3
Water
out
Insulation
Figure 2. Schematic diagram of the cross-sectional view of the collector fabricated from 3walled plastic TIM.
of a thin descending water film. In this case the air space above the film also behaves, to some
or more extent, as an insulating layer that decreases the top losses of the collector. The pump
that transports the water to the upper part of the collector is temperature controlled: It is
running only in that case when the inside temperature of the collector is higher than the
temperature of the hot water storage tank. Since the thermal mass of such a collector is very
low, its response to solar radiation is fast. There is no risk of frost-crack, since in the periods
when the pump is off, the amount of the residual water that wets the textile is negligible.
Therefore, irrespective of the climate or the season, there is no need to use antifreezing fluids.
The collector might be fixed directly above a roof insulation, or, as shown in Fig. 2, it might
supplied with own back insulation. The optical efficiency of this model is approximated as 76
%. At low temperatures the overall top loss coefficient UT is similar to that of a double glazed
collector. At high temperature, because of the strong temperature dependence of the
evaporative heat transfer between the wet wick and wall 2, UT gradually approaches such
values which are more or less characteristic to a single glazed collector. Nevertheless, in
Winter when the system operates at lower temperatures, its performance is very similar to that
of a double-glazed collector with non-selective absorber.
2.2.
The atmospheric solar hot water system
Fig. 3 shows the scheme of the atmospheric hot water system. It consists of a plastic TIM
collector of 1.35 m2 area, a 50 litre volume hot water storage tank which is supplied with a
heat exchanger, a variable speed water pump, temperature sensors to measure the collector
inlet and outlet temperatures and the inside temperatures of the collector and the storage tank,
and a process control system to govern the operation of the pump and to avoid the overheating
of the collector in such cases when either the pump or the electric network goes wrong.
The system operates in the following way: When the inside temperature of the collector is
higher than the temperature of the storage tank, the pump is switched on automatically, with
the help of the process control system. The water is pumped to the collector from the lower
part of the tank. In normal operational mode the mass flow rate of water is 60 kg/h. As shown
in the Figure, the tank is open to the atmosphere with the help of an atmospheric pipe. The hot
water exiting the collector passes through a 2-way magnetic valve, which is always open in
the direction of the tank when the pump is running.
Fig. 3. The schematic diagram of the atmospheric hot water system
To avoid the risk of the overheating of the collector in such cases when either the pump or the
electricity go is made as follows: In both cases the 1-way magnetic valve which is connected
to the mains opens, and a weak current of cold water (a few kg/h) starts to flow from the
mains into the collector. At the same time the direction of flow in the 2-way magnetic valve
also changes, and hot water and vapour that exit the collector are dumped to the atmosphere.
3. RESULTS AND DISCUSSION
3.1.
Experimental set-up and procedure
The experiments were carried out in June, in the area of the Polytechnic of Dunaújváros. The
tilt angle of the collector was not ideal for Summer seasons, for practical reasons it was set at
60° throughout the experiments. Due to this, and also to the fact that Dunaújváros is an
industrial centre, the solar radiation incident on the collector never exceeded 600 W m-2.
The radiation intensity incident on the outer glazing surface was measured by a Kipp and
Zonen solarimeter which was fixed onto the plane of the collector. The temperatures were
measured with calibrated temperature sensors. Ambient temperature and wind velocity was
measured by a meteorological set. For back insulation 50 mm thick polyurethane foam with k
= 0.035 Wm-1K-1 thermal conductivity was used, and the insulated collector was placed into a
wooden housing. The experimental set-up allowed us to study the development of the (quasi)
steady state and the collector performance (the energy balance on the collector under steady
state condition; the collector efficiency and the overall heat loss coefficient of the collector as
a function of the operational temperature).
3.2. Collector performance
At low temperatures the prototype TIM collector behaved as a double glazed collector with
non-selective absorber. Increasing temperature, due to the increasing role of the evaporative
heat transfer, resulted in a monotonous increase of the top loss coefficient U T: At wind
velocities 2 - 5 m/s, which correspond to the yearly average in Hungary, the calculated top
loss coefficient UT was around 4.4 W m-2 K-1 at 40 oC, and 6.8 W m-2 K-1 at 80 oC. Data
obtained from the experimental results by interpolation are given in Table 1.
Table 1. Variation of the back loss coefficient UB, the top loss coefficient UT and the overall
heat loss coefficient U with the average collector temperature Tav. Wind velocity = 2-5 ms-1.
Wind speed
ms-1
2–5
2–5
2–5
2–5
2–5
Tav
o
C
40
50
60
70
80
UB
W m-2 K-1
0.7
0.7
0.7
0.7
0.7
UT
W m-2 K-1
4.4
5.0
5.6
6.3
6.8
U
W m-2 K-1
5.1
5.7
6.3
7.0
7.5
3.2. Comparison with state-of-the-art solar collectors
Efficiency
MacGregor [3] made comparison between a typical commercial flat-plate collector with
selective absorber and a vacuum-tube collector on a unit “collector aperture area” basis. Fig.
4. shows the performance of these collectors, in comparison with the plastic TIM collector, in
efficiency versus (Tav - Tamb)/G representation, where Tav is the average collector temperature,
Tamb is the ambient temperature and G is the solar radiation incident to the collector area.
0,9
Flat-Plate [3]
0,8
TIM
0,7
Evacuated [3]
0,6
0,5
0,4
0,3
0,2
0,1
0
0
0,02
0,04
0,06
0,08
0,1
0,12
(Tav - Tamb) / G
Figure 4. Efficiency vs. (Tav - Tamb)/G curves for a typical flat plate collector with selective
absorber [3] (diamonds); TIM collector (squares); and evacuated tube collector [3] (triangles).
It is seen that for (Tav - Tamb)/G values lower than 0.06, the efficiency of the plastic TIM
collector is only about 10 % lower than that of the flat plate collector with selective absorber.
It is also observed that at (Tav - Tamb)/G values higher than 0.6 the efficiency of the plastic
TIM collector decreases more rapidly, due to the increasing role of the evaporative heat
transfer between the wet wick and wall 2 of the plastic structure, cf., Figs. 1 and 2.
3.3. Economic estimation
The estimated material, labour and replacement costs of solar hot water systems using state–
of-the-art selective collector and plastic TIM collectors are given in Table 2. Fig. 4 shows that
up to (Tav - Tamb)/G values of 0.06, which is the usual operational range of the collectors, the
efficiency of the TIM collector is about 10 % lower than that of the high quality flat plate
collector. The relative difference is about 20%. Therefore, to achieve a similar heating power,
for plastic TIM collectors 20 % higher collector area is needed. Since the lifetime of the TIM
collector is only 10 years, during the 20 y lifecycle of the whole system, the collector has to
be replaced in year 11. Data given in Table 2 suggest that atmospheric solar hot water systems
using plastic TIM collectors offer not negligible cost reduction in hot water production.
Table 2. Estimated material, labour and replacement costs of solar hot water systems using
state–of-the-art selective collector and plastic TIM collectors, in both cases for 20 y lifetime.
ITEMS
Water heating system with Water heating system with
Flat Plate Collector
Plastic TIM Collector
Size/Quantity & Price
lifetime
Euro
2
Collector
5 m , 20 y
1000
Collector replacement in year 11.
Additional back insulation Water pump+mounting set 1 piece, 10 y
136
Water pump replacement
136
in year 11.
Control unit with sensors
1 piece, 20 y
40
6 bar pressure storage tank 1 piece, 20 y
640
with heat exchanger
Atmospheric storage tank with heat exchanger.
Dilatation tank
10 litre, 20 y
40
Absorption
type
gas 1 piece, 20 y
80
separator
Antifreezing fluid
20 litre, 7 y
80
Replacement of the anti- 40 litre
160
freezing fluid in y 7 & 14.
Control unit & magnetic valves for water feed.
Thermometers, auxiliary
400
materials, mounting
TOTAL
2712
Size/Quantity & Price, euro
lifetime
6 m2, 10 y
264
6 m2, 10 y
264
6 m2, 20 y
1 piece, 10 y
1 piece, 10 y
216
136
136
1 piece, 20 y
-
40
-
1 piece, 20 y
400
-
-
-
-
1 piece, 20 y
100
400
1956
CONCLUSIONS
A prototype plastic TIM collector, designed to operate at atmospheric pressure and at
temperatures lower than 100 oC has been tested and analysed.
Though the new prototype is less efficient than the presently used flat plate collectors,
domestic water heating systems based upon such collectors and atmospheric hot water storage
tank offer not negligible cost reduction in solar water heating.
One of the main advantages of the water heating system developed in this study is that it uses
pure water as heat carrier instead of antifreezing material, irrespective of the climatic
condition.
LIST OF SYMBOLS
G
k
T amb
T av
UB
UT
U
= solar radiation incident to the outer surface of the collector, Wm-2.
= thermal conductivity, Wm-1K-1.
= ambient temperature, oC
= avarage collector temperature, oC
= back loss coefficient of the collector, Wm-2K-1.
= top loss coefficient of the collector, Wm-2K-1.
= overall heat loss coefficient of the collector, Wm-2K-1.
REFERENCES
[1]
Bartelsen B, Rockendorf G, Vennermann N: Development of an elastomer-metalabsorber for thermal solar collectors. Proceedings of the EuroSun’96 Conference
Freiburg, Germany 1996, Vol. 1, pp. 495-499.
[2] Kudish AI, Evseev EG, Rommel M, Köhl M, Walter G and Leukefeld T: Research and
development of solar collectors fabricated from polymeric materials. Proceedings of the
ISES Solar World Congress Jerusalem, Israel, 1999.
[3] MacGregor K: A comparison between flat-plate and vacuum-tube solar collectors. Book
of Abstracts of the EuroSun’96 Conference Freiburg, Germany 1996, Vol. II, pp. 67-68.
[4] G Mink, L Horvath, L Toth and E Karmazsin: Design parameters and performance
analysis of low cost, all plastic solar collectors fabricated from plastic „ TIM” materials.
Energy and the Environment 2000, Ed. B. Frankovich; Croatian Solar Energy
Association, 2000; 71-79.