Experimental investigation of energy storage for an evacuated solar collector Saffa Riffat, Liben Jiang, Jie Zhu and Guohui Gan School of the Built Environment, The University of Nottingham, University Park, NG7 2RD, UK E-mail:
[email protected] Abstract This paper presents the results of an experimental study involving energy storage materials (paraffin wax and water) placed inside a new type of cylindrical evacuated solar collector system. The potential of this system to provide continuous hot water, even during short periods of low incident solar radiation, was investigated. Five cases, including a reference case, were studied experimentally and the performance of each was analysed. Of the materials studied, water was found to give best performance in terms of thermal storage and subsequent release of stored energy. Partial filling of the evacuated solar collector tubes with water gave greatest overall thermal efficiency for a cycle including two periods of no input energy under the test conditions employed. Keywords heat pipe; evacuated solar collector; paraffin wax; water
1. Introduction Increased awareness of the finite nature of the world’s fossil fuel resources and the negative impact on global climate due to their combustion has encouraged the development of technologies that exploit renewable energy, e.g., solar energy. However, the intermittent nature of renewable energy sources, and their limited ability to meet fluctuations in demand mean that some form of energy storage is essential to provide continuous and efficient supply. Phase change materials (PCMs) can be used for thermal energy storage owing to their large latent heat capacity. A number of thin flat containers or tubes containing PCM may be used as a plate heat exchanger [1] or shell/tube heat exchanger [2]. To overcome the low thermal conductivity of PCMs, structures such as a honeycomb have been used to improve the rate of heat transfer [3]. Alternatively, materials with high thermal conductivity have been mixed with the PCM to increase overall thermal conductivity [4, 5]. Finned tubes have also been tested for this purpose. Some integrated technologies have been investigated and PCMs have been impregnated into wallboard or plasterboard [6, 7, 8], concrete blocks [9, 10] and underfloor boards [11]. The large heat transfer area of these building elements assists heat transfer between the structure and the enclosed space [12]. Sensible thermal energy storage has also been modelled [13] for space heating using a unit enclosing parallel circular tubes, and water as the storage medium. With the exception of integration with fixed building elements, all previously reported work has involved a separate unit for energy storage, which increases the cost of manufacture and the space required for installation. This paper presents International Journal of Low Carbon Technologies 1/2
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Heat pipe Shaped aluminum sheet Inner wall Outer wall
Figure 1.
Aluminum sheet Energy storage material Wooden insert Evacuated double walled glass tube
Cross section of solar collector used for experimental work.
preliminary experimental results for a system comprising a compact evacuated solar collector integrated with an energy storage material. 2. Description of the principle and experimental set up Fig. 1 shows the simplified cross-section of a new type of evacuated solar collector. The evacuated tube has two layers. The outer tube is made of extremely strong transparent borosilicate glass that is able to resist impact. The inner tube is also made of borosilicate glass, but is coated with a selective coating (Al-N/Al), which has excellent solar heat absorption and low heat reflection properties. A shaped aluminum sheet is attached to the inner layer of tube, at the same time embracing the conventional cylindrical gravity-assisted wickless heat pipe to ensure effective heat transfer from the aluminum sheet to the heat pipe. When solar radiation is incident on the evacuated tube, a large proportion passes through the outer layer and is absorbed by the coating applied to the inner layer. The absorbed thermal energy is then transferred by conduction to the aluminum sheet and the heat pipe. Within the heat pipe, heat is transferred to the working fluid (cooling water) via a manifold. There are two major advantages of this type of solar collector. Firstly, the evacuated part of the tube greatly reduces convection and conduction losses. Radiative losses will be small as the temperature of the inner layer is low. The system will therefore have high thermal efficiency. The second advantage is ease of maintenance. A shaped wooden insert was placed inside the tube to ensure most of the energy storage material would be in close proximity to the evaporator section of the heat pipe. Aluminium sheet was used to hold the wood piece in position and enlarge the heat transfer area of the energy storage material and the heat pipe, hence improving the heat transfer efficiency between them two. Four collector tubes, each with an absorber section of 0.42 m long, connected at the top to a copper manifold protected by fibre glass insulation were assembled. For laboratory trials, water flow rate through the manifold was set at 0.15 l/min, and artificial radiation was provided at 930.4 W/m2. A DataTaker of DT500 was used to record the temperatures of inlet and outlet cooling water, fill material inside tube, and heat pipe. International Journal of Low Carbon Technologies 1/2
Experimental investigation of energy storage for an evacuated solar collector
Table 1.
Case 1 2 3 4 5
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The materials and percentage fill by volume for test cases Material
Percentage fill by volume
Empty tube Paraffin wax Paraffin wax Water Water
22.43% 56.7% 25.28% 100%
3. Case studies The performance of the solar collector was examined for five cases involving two types of fill materials. Weather data from the software package (Ecotect v5.20) were used to inform the pattern of simulated solar radiation to which the collector was subject. This suggested that the solar insolation was higher than 930.4 W/m2 for a total of 280 minutes on the brightest day (29th July, 2001) during the period 1.1.2001–1.1.2002 in Nottingham (Lat: 53.0°N, Long: −1.2°), with intermittent interruption by cloud. To simulate these weather conditions in each experiment, the light source was switched on for an initial period of 200 minutes at a level of 930.4 W/m2. The light source was then switched off for 40 minutes before being switched on again for 40 minutes to make one period. The on/off-period, 40 minutes on and 40 minutes off, was chosen so the temperature difference between the inlet and outlet cooling water would be larger (1°C) than possible errors. In each test, the initial 200 minutes was followed by two periods of lights being off and on for total 160 minutes. The solar collector was first tested without any energy storage material in the tubes to provide a reference case. Four more cases were then investigated with various proportions of energy storage material in the solar collector. The materials and percentage fill by volume are listed in Table 1, which are based on the real test. Case 1: Empty tube (reference case) As shown by Fig. 2, after 75 minutes the solar system achieved steady state operation. The temperature of the inside tube was about 87°C, much higher than the temperature of the evaporator section of heat pipe, which was approximately 55°C. However, when the light source was switched off, the temperature of the inside tube fell rapidly to a minimum of 35°C. The temperature of the condenser section of the heat pipe fell to a value only 4°C above the inlet temperature of cooling water and so there was negligible thermal energy output during this period. Case 2: 22.43% paraffin wax For Case 2, 221.75 g of liquid paraffin was introduced into the tube and occupied 22.43% of the volume of the solar collector. Fig. 3 shows the temperature changes against time for Case 2. It can be seen that there was no steady state condition as the temperature increased continuously and exceeded the melting point of the wax International Journal of Low Carbon Technologies 1/2
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S. B. Riffat, L. Jiang, J. Zhu and G. Gan 100 90
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Temperature against time for Case 1: Empty tube (Reference case).
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Temperature against time for Case 2: 22.43% paraffin wax.
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Temperature against time for Case 3: 57.6% paraffin wax.
(49°C). This was because paraffin wax has a very low thermal conductivity, which reduced heat transfer from the interior of the wax to the exterior surface. When the light source was switched off, there was still heat output owing to the gradual release of heat from the paraffin wax. Even 40 minutes after switching off the light source, the temperature of the condenser section of the heat pipe was still above 40°C, and at the end of the period, the temperature difference between inlet and outlet cooling water was approximately 2.5°C. Case 3: 57.6% paraffin wax Case 3 involved a greater mass (308.82 g) and percentage volume (57.6%) of paraffin wax inside each tube. The experimental results plotted in Fig. 4 show temperatures profiles against time similar to those of Case 2. The maximum temperature of the paraffin wax was no more than 50°C for this larger mass (52°C for Case 2). Analysis of the energy balance of the system suggests that not all the wax completed the phase change. When the light source was switched off, the temperature of the condenser section of the heat pipe decreased to 36°C after 40 minutes (41°C for Case 2), and the sensible heat of the wax made a large contribution to the heat recovery. Case 4: 25.28% water Water was also investigated as a fill material because it is cheap and readily available. Furthermore, it is environmentally friendly and has good fluidity, which is International Journal of Low Carbon Technologies 1/2
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Temperature against time for Case 4: 25.28% water.
capable of sustaining effective convection cells which will assist heat transfer to the heat pipe. Each tube was filled with 25.28% by volume of water and the experiments were carried out under the same conditions as previously described for paraffin wax. The results (Fig. 5) show that the temperature of the water reached a maximum of 61.9°C after the initial period. The sensible heat stored in the water was then easily transferred to the heat pipe while the light source was switched off and the temperature of the condenser section of the heat pipe never fell below 48°C. Compared with the Case 2 and Case 3, higher condenser temperature for Case 4 means larger temperature difference between the condenser and inlet cooling water, resulting in the higher efficiency of heat recovery during the period of no incident radiation. Case 5: 100% water For Case 5, the tubes were filled completely with water. The temperature of the water was lower than in Case 4 after the initial 200 minutes, owing to the greater mass of water used. The water temperature after the start up period was 48.5°C for Case 5 (61.9°C for Case 4). The temperature of the condenser section of the heat pipe reached a maximum of 43.9°C and fell to 37°C, 40 minutes after switching off the light source. The energy absorbed after switching on again for 40 minutes was insufficient to compensate for the heat lost during the off period and the condenser temperature fell to 33.3°C after one more period of 80 minutes. Small temperature difference between the condenser and inlet cooling water in this case gave a low efficiency of the thermal energy recovery under these test conditions. International Journal of Low Carbon Technologies 1/2
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Temperature against time for Case 5: 100% water.
4. Energy analysis As an indicator of performance, the thermal efficiency of the system may be expressed as:
h=
Ú
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c ◊ m ◊ dT
I ◊ A ◊ (t2 - t1 )
¥ 100%
where, c is the specific heat of cooling water (J/kg·K); m is the mass flow rate of cooling water (kg/s); dT is the instantaneous temperature difference between the inlet and outlet cooling water (K); I is the power per unit area of the incident radiation (W/m2); A is the real absorber area of solar collector (m2); and t1 and t2 are the start and end time, respectively (s). From the experimental work, the efficiency of system is as follows: h = 0.839 - 13.84
Tmean - Ta I
where Tmean is the mean temperature of the inlet and outlet cooling water, and Ta is the ambient temperature. It can be seen that the solar collector has a high initial thermal efficiency of 83.9%. The energy balances for five cases are shown in Fig. 7. When the tube contained energy storage material, no stable condition was reached during the initial 200 International Journal of Low Carbon Technologies 1/2
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Figure 7.
Table 2. Test Cases Empty tube/reference case 22.43% paraffin wax 56.7% paraffin wax 25.28% water 100% water
Energy balance for the five cases.
Average thermal efficiency for each stage
First 200 minutes
1st period of 80 minutes afterwards
2nd period of 80 minutes afterwards
Whole 360 minute cycle
0.73 0.50 0.40 0.51 0.37
0.77 1.00 0.92 1.21 0.98
0.75 0.93 0.77 1.09 0.90
0.57 0.49 0.41 0.54 0.41
minutes of incident radiation. For the reference case, very little energy could be recovery when the light source was switched off. Water and paraffin wax were effective materials for energy storage, as during the off period a significant amount of energy could be recovered. An average thermal efficiency for each period, defined as the ratio of the energy transferred to the cooling water compared to the solar energy input within the same period, is listed in Table 2. The table shows that during the first stage of 200 minutes, the reference case had the highest efficiency. This was because most of the input energy was removed by the cooling water, and only a small proportion was used to increase the air temperature inside the tube up to 86.5°C. For instant use of solar energy, this case was superior to others. However, during the following 2 off/on periods, the average thermal efficiencies for Case 1 changed very little, as only a very small amount of energy could be recovered when the light source was switched off. Use of paraffin wax and water inside the tubes allowed some energy to be stored. This was released during the off periods to produce a smoother profile of energy output, and contributed to higher thermal efficiencies in the 1st and 2nd periods International Journal of Low Carbon Technologies 1/2
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(i.e. more than 100% for Case 4). However, compared with the reference case, the use of a thermal energy storage material resulted in a lower instant thermal efficiency for the initial 200 minutes and a lower overall thermal efficiency for the whole testing cycle. Fig. 3 to Fig. 6 show that an energy balance could not obtained for case 2 to case 4 during either the 1st or 2nd period because some portion of energy was taken away by the cooling water. Under the same operating conditions, tubes containing a greater mass of energy storage material warmed up more slowly when the light source was switched on for subsequent periods, which resulted in a lower peak temperature. This caused the average thermal efficiency within the 2nd period to be lower than during the 1st period. If the same mass of paraffin wax and water were used, the former would store more thermal energy owing to its phase change when the working temperature is less than 100°C. However, the very low conductivity and high viscosity of paraffin wax restrict its suitability as an energy storage material. Of the cases studied, the experimental results showed that 25.28% by volume of water gave best performance in terms of continuous operation containing periods of no insolation. 5. Conclusions The following conclusions can be made from the experimental work. • There was no obvious steady state during the initial heating period for the cases involving paraffin wax inside the tubes when the temperature rose up through the melting point. This was due to the very low conductivity and high viscosity of paraffin wax, which inhibit heat transfer. • Water showed higher heat transfer efficiency with heat pipe and so good performance as energy storage material during the two off-on periods after initial 200 minutes heating period. • Under the test conditions used, the case with higher masses of energy storage material (56.7% paraffin wax and 100% water) had lower thermal efficiencies. These thermal efficiencies decreased with time, as subsequent periods of energy input were insufficient to replace the energy dissipated. • The solar collector filled with 25.28% water had highest average thermal efficiency under the test conditions.
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