Solar Energy 78 (2005) 416–426 www.elsevier.com/locate/solener
Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit S.D. Sharma a
a,*
, Takeshi Iwata b, Hiroaki Kitano b, Kazunobu Sagara
a,1
Department of Architectural Engineering, Graduate School of Engineering, Osaka University, 2–1 Yamadaoka, Suita, Osaka 565-0871, Japan b Department of Architecture, Faculty of Engineering, Mie University, Tsu 514-8507, Japan Received 6 April 2004; received in revised form 20 July 2004; accepted 2 August 2004 Available online 11 September 2004 Communicated by: Associate Editor Michael Grupp
Abstract The thermal performance of a prototype solar cooker based on an evacuated tube solar collector with phase change material (PCM) storage unit is investigated. The design has separate parts for energy collection and cooking coupled by a PCM storage unit. Solar energy is stored in the PCM storage unit during sunshine hours and is utilized for cooking in late evening/night time. Commercial grade erythritol was used as a latent heat storage material. Noon and evening cooking experiments were conducted with different loads and loading times. Cooking experiments and PCM storage processes were carried out simultaneously. It was observed that noon cooking did not affect the evening cooking, and evening cooking using PCM heat storage was found to be faster than noon cooking. The cooker performance under a variety of operating and climatic conditions was studied at Mie, Japan. 2004 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Phase change material; Latent heat storage; Evening cooking; Evacuated tube solar collector
1. Introduction Domanski et al. (1995) and Buddhi and Sahoo (1997) have studied the use of a phase change material (PCM) as a storage medium for a box type solar cooker designed to cook in the evening or during non-sunshine hours. They used stearic acid (melting point 69 C) as * Corresponding author. Tel.: +81 6 6879 7643; fax: +81 6 6879 7646. E-mail addresses:
[email protected], sharma_
[email protected] (S.D. Sharma). 1 ISES member.
a PCM for heat storage. Buddhi and Sahoo filled the PCM below the absorbing plate of the cooker. In such type of design, the rate of heat transfer from the PCM to the cooking pot during the discharging mode of the PCM is slow, and more time is required for cooking an evening meal. Sharma et al. (2000) designed and developed a cylindrical PCM storage unit for a box type solar cooker to cook food in the late evening. Since this unit surrounds the cooking vessel, the rate of heat transfer between the PCM and the food is higher, and cooking can be faster. They reported that by using 2.0 kg of acetamide (melting point 82 C) as a latent heat storage material, a second
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.08.001
S.D. Sharma et al. / Solar Energy 78 (2005) 416–426
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Nomenclature net absorber area for the tubes (1.82 m2 · 2 = 3.64 m2) Cw specific heat of water (=4.18 kJ/kg C) G solar irradiance (MJ) Ii enthalpy of PCM (kJ/kg) Ih (ave) average solar irradiance at horizontal surface (W/m2) Is20 solar irradiance at 20 angle inclined from the horizontal surface (W/m2) Is20(ave) average solar irradiance at 20 angle inclined from the horizontal surface (W/m2) m mass flow rate (lmin1) mave average mass flow rate (lmin1) Mi mass of PCM (kg) Mload mass of loaded water (kg) QPCMStored total heat stored in the PCM (kJ) QwaterStored total heat gain by water (HTF) (kJ) Ta ambient air temperature (C) Ta(ave) average ambient air temperature (C) Tin water (HTF) temperature at the inlet of the ETSC (C) Tload water (loaded) temperature (C) Tout water (HTF) temperature at the outlet of the ETSC (C) Tmaxin maximum water (HTF) temperature at the inlet of the ETSC (C) Tmaxout maximum water (HTF) temperature at the outlet of the ETSC (C) TPCMmax PCM maximum temperature inside the storage unit (C) TPCM(L,T,3 cm) PCM temperature at 3 cm radial distance from left top side (30 cm height) of the PCM storage unit (C) TPCM(L,C,3 cm) PCM temperature at 3 cm radial distance from left center side (15 cm height) of the PCM storage unit (C) A
batch of food could be cooked if it is loaded before 3:30 PM during winter. They recommended that the melting temperature of a PCM should be between 105 and 110 C for evening cooking. Buddhi et al. (2003) tested acetanilide as a PCM with a melting point of 118 C for night cooking in a box type cooker with three reflectors. Acetanilide was filled in the cylindrical storage unit and it was reported that by using 4.0 kg of acetanilide, food could be cooked up to 8:00 PM. Morrison and Mills (1987), Balzar et al. (1996) and Kumar et al. (2001) have used evacuated tube solar collectors (ETSC) for cooking. Schwarzer et al. (2003) tested a design with a collector having reflectors and pebbles as thermal storage for cooking. This design
TPCM(L,B,3 cm) PCM temperature at 3 cm radial distance from left bottom side (0 cm height) of the PCM storage unit (C) TPCM(R,T,3 cm) PCM temperature at 3 cm radial distance from right top side (30 cm height) of the PCM storage unit (C) TPCM(R,C,3 cm) PCM temperature at 3 cm radial distance from right center side (15 cm height) of the PCM storage unit (C) TPCM(R,B,3 cm) PCM temperature at 3 cm radial distance from right bottom side (0 cm height) of the PCM storage unit (C) TPCM(B,C,3 cm) PCM temperature at 3 cm radial distance from bottom center of the PCM storage unit (C) t total time (from Pump ON to Pump OFF) (minutes) Vi volume for each point inside PCM storage unit (m3) Greek symbols qs density of PCM at solid phase (=1.48 kg/m3) ql density of PCM at liquid phase (=1.30 kg/m3) qpcm density of PCM (qpcm = (qs + ql)/2 = 1.39 kg/m3) as absorptivity transmittivity product for the collector (0.81) Subscript i The number of thermocouples inside the PCM Abbreviations PCM phase change material HTF heat transfer fluid (water) ETSC evacuated tube solar collector
has the possibility of indoor cooking and can incorporate a baking oven. No work has been performed on solar cookers with latent heat storage using ETSC. We tried to develop a solar cooker with PCM storage based on ETSC. For this purpose, there is a need to identify a latent heat storage material with appropriate melting point (>110 C) for cooking (Sharma et al., 2000). Erythritol (melting point 118 C, latent heat of fusion 339.8 kJ/kg) was used for the present set-up. The prototype was fabricated by a local manufacturer and installed on the roof of the Satellite Venture Business Laboratory, Mie University, Tsu, Japan (Longitude 136 31 0 and Latitude 3444 0 ) for testing thermal performance. Experiments were conducted for different loads and second batch loading times.
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Experimental results are presented for a system load of 5 kg, 7 kg, 8 kg and 10 kg water, respectively. The performance results are also presented for a range of operating and climatic conditions.
Table 1 Specification for the evacuated tube solar collector (single panel)
2. Experimental set-up and measurements
Item
Specificationa
Manufacturer name
Nippon Electric Glass Co., Japan DP6-2800 6 L2972 · W930 · H185 2.76 1.82 64.0 20
