Table 1 The results of the variance analysis (ANOVA) Table 1
Related Figures (8)
Fig. 1. The arrangement of the heat storage and greenhouse heating system. In the following work, seasonal thermal energy storage using paraffin wax as a PCM with the latent heat storage technique was attempted to heat a greenhouse of 180 m? floor area. The sche- matic arrangement of the LHS system for greenhouse heating is given in Fig. 1. The system con- sists mainly of five units: (1) flat plate solar air collectors (as heat collection unit), (2) LHS unit, (3) experimental greenhouse, (4) heat transfer unit and (5) data acquisition unit. The greenhouse heating system utilizes seasonal latent heat storage located in the Cukurova region of Turkey. The constructional features of the LHS system are described in detail in the following sections. Fig. 2. The latent heat storage unit and sensor placement; all dimensions in mm. A cylindrical steel tank was used as the seasonal LHS unit. The dimensions of the LHS unit and the placement of the heat exchanger in the LHS unit are given in Fig. 2. The diameter and the total volume of the steel tank used as the LHS unit were 1.7 m and 11.6 m’, respectively. The whole surface of the LHS unit was insulated with 0.05 m of glass wool and 0.001 m of steel plate, respectively. The LHS unit was placed horizontally on the ground surface at a distance of 2 m from the experimental greenhouse. The LHS unit volume per square meter of greenhouse ground surface area was 0.066 m*/m?, while the storage volume per square meter of the solar air collector was about 0.44 m*/m?. The LHS unit was attached to the experimental greenhouse by means of polyethylene (PE) pipes. Solar energy, collected by the solar air collectors, was transferred to the LHS unit by circulating air through the PE pipes. Perforated PE pipe as the heat exchanger was installed in the LHS unit (Fig. 2). Perforated PE pipe was used to ensure direct contact between the heat transfer fluid and the PCM in the LHS unit. The total length and diameter of the perfo- rated PE pipe were 97 m and 0.1 m, respectively. Fig. 3. The rates of heat and thermal exergy stored in the heat storage unit during charging. Fig. 4. The change of efficiency of the latent heat storage system during the charging period: (a) energy efficiency; (b) exergy efficiency. RLOALAOLWE £40FANG LEEW WOOL. The total energy efficiency ranged from 53.7% to 79.6%, while the net energy efficiency was ir the range of 22.1-47.9% during the first charging period (Fig. 4a). While the net exergy efficiency} at 11:00 h was only 22.1%, it reached its maximum value (47.9%) at 14:00 h. During the experi mental period, it was found that the average daily total and net energy efficiencies were 74.3% anc 40.4%, respectively. On t whereas the net exergy et he other hand, the total exergy efficiency ranged from 10.5% to 80.6% ficiency was in the range of 0.2—6.2% during the same charging perioc (Fig. 4b). The average daily total and net exergy efficiencies were 65.2% and 4.2%, respectively. When the net energy et iod, the following results net exergy efficiency. This is expected because the total energy content of the hot air used as hea transfer fluid is taken into account in order to calculate the net energy efficiency. In other words to calculate the net energy efficiency, the quantity of the energy transferred is taken into account and the quality of the energy transferred is neglected. (2) For all charging periods, similar result: were obtained in terms of the net energy efficiency of the LHS system. The average daily ne energy efficiency of the LHS system remained nearly constant (approximately 75%) during th ficiency is compared with net exergy efficiency during the charging per. can be drawn: (1) The net energy efficiency was always higher than th charging periods. The net energy efficiency of the heat storage system did not change much durin Fig. 5. Effect of temperature difference on efficiency of the latent heat storage system. The linear regression coefficient of determination (R*) for the net energy and exergy efficiencies were 98% and 96%, respectively. The results of the variance analysis (ANOVA) for the relation- ships between the net energy/exergy efficiency and temperature difference are given in Table |. The relationships between the net energy/exergy efficiency and temperature difference are statistically important (P = 2.59 x 107° and P= 1.29 x 10~“) at the 95% probability level. For that reason, the obtained regression equations (Eqs. (13) and (14)) represent the relationships between the The results of the regression analysis Table 2 Equations for the heat transfer coefficients for bottom and horizontal surfaces of the LHS unit
