Comparative performance of a solar assisted heat pump dryer with a heat pump dryer for Curcuma

Received Nov 3, 2019 Revised Feb 4, 2020 Accepted Apr 25, 2020 This study evaluated the performances of solar assisted heat pump dryer (SAHPD) and heat pump dryer (HPD) for drying of Curcuma xanthorrhiza Roxb. The HPD and SAHPD reduced mass of Curcuma from 30.70 kg to 7.85 kg needed 10.5 hours and 8 hours with average temperature and relative humidity 49.2C and 26.5%, and 57.7C and 19.8%, for SD and SAHPD respectively. The moisture of Curcuma dried from 3.167 db to 0.065 db with an air mass flow rate of 0.121 kg/s. The SAHPD reduced the drying time about 24% compared to HPD. The drying rate and the specific energy consumption were calculated in an average 1.05 kg/h and 1.36kg/h, and 1.17kWh/kg and 2.07kWh/kg for HPD and SAHPD, respectively. The specific moisture extraction rate and the dryer thermal efficiency were calculated in an average 0.931 kg/kWh and 0.521 kg/kWh, and 61.0% and 34.3% for HPD and SAHPD, respectively. Whereas, the pickup efficiency and the coefficient of performance of the heat pump were calculated in an average 57.5% and 59.2%, and 4.03and 4.35 for HPD and SAHPD, respectively. The SAHPD is capable of drying Curcuma quickly because of the high pickup efficiency and high drying rate.

system consists of compressor, condenser, evaporator, and expansion valve. The working fluid of the heat pump is R-22. Compressor use of electrical capacity of 1 HP. The solar collector equipped with finned double-pass solar collector with black absorber, transparent cover glass material, inside and outside the collector coated with aluminum 1mm thick, angle iron frame, and insulation. Two solar collectors are connected in series with an area of 1.8 m 2 each. The drying chamber uses of the cabinet type and contain the drying trays with adjustable racks to place the Curcuma xanthorrhiza Roxb. It walls consist of triple layers, an outside layer uses aluminum sheet, a middle insulated with glass fiber materials and inner layer of uses of aluminum sheet. The drying air is circulated by using blower with electrical capacity of 2 HP.

Experimental procedure
The sample is fresh Curcuma xanthorrhiza Roxb was purchased at the local market in Padang, West Sumatra, Indonesia. The experiments were carried out at Padang Institute of Technology, West Sumatra, Indonesia. After washing, the Curcuma xanthorrhiza Roxb was cut into chips of 2-3 mm. As much as 30.7 kg put into the drying chamber for the drying process, shown in Figure 2.
Inlet and outlet air temperature of solar collector, heat pump, and drying chamber during the operation of the drying system were measured by using T type copper-constantan thermocouples with an accuracy of ± 0.1 o C, and operating temperature range (-200 o C to 400 o C). The solar radiation was measured by an LI-200 pyranometer in ± 0.1Wm -2 accuracy, and with maximum solar radiation of 2000 Wm -2 , operating temperature range (-40 o C to 400 o C) and operating relative humidity range (0% to 100%). The air velocity was measured with 0-30 ms -1 range an HT-383 anemometer, an accuracy of ± 0.1 ms -1, and with operation temperature range (-10 o C to 45 o C). The solar radiation and air temperature were recorded by an AH4000 data logger with reading accuracy of ± 0.1 o C. The weight change of the Curcuma xanthorrhiza Roxb was measured by 0-15 kg range a TKB-0.15 weighing, an accuracy ± 0.05kg. The Curcuma xanthorrhiza Roxb was weighed every 30 minutes and temperature was measured every 30 minutes.
The drying experiments were carried out for drying of Curcuma xanthorrhiza Roxb chips to study the dryer performance under two different operating modes: (1) heat pump dryer (HPD); (2) combination between heat pump dryer with solar collector (solar assisted heat pump dryer: SAHPD). For the heat pump dryer mode of operation as shown in Figure 3, the solar collector is not operated. For the combination mode as shown in Figure 4 the solar collector and the heat pump operated both.

Experimental data analysis
The performances of the HPD and the SAHPD for drying of Curcuma xanthorrhiza Roxb. are characterized by drying rate, specific moisture extraction rate, specific energy consumption, dryer thermal efficiency, and pickup efficiency. It is highly depending on the performance of each of the drying system components such as solar collector and heat pump.
The thermal efficiency of a solar collector is the ratio of useful heat gain by solar collector to the energy incident in the plane of the collector. It was determined using the following equation [21], coll =̇a ir Pair ( out, SC − in,SC ) Where I T is solar radiation incident in the collector, A SC is an area of collector, T in, SC and T out, SC are inlet and outlet air temperatures of solar collector, respectively. ṁ air is air mass flow rate, C pair is specific heat of air. The coefficient of performance of a heat pump (COP) is the ratio of useful heat or heat energy released by the refrigerant in the condenser to the electrical energy consumed by compressor. It was determined using the following equation [18], Where in, Cond and out, Cond are inlet and outlet air temperatures of condenser, respectively, and Comp is the electrical energy consumed by the compressor. The moisture content of the Curcuma was calculated by two methods such as wet and dry basis as [22], The moisture content wet basis was calculated as The moisture content dry basis was calculated as where m d the mass of bone is dry of the Curcuma, and m wtc is mass of wet Curcuma. The drying rate is the mass of water evaporated from the wet Curcuma per unit time. It was determined as [6], Where M db,t is moisture content dry basis of curcuma at the time "t", db,t+Δt is moisture content dry basis of curcuma at the time " + Δt", m water is the mass of water evaporated, t is drying time, and is drying time interval.
The mass of the water evaporated (m water ) from the wet Curcuma was calculated as [21], Where wetc is initial mass of wet curcuma, M wb,f is final moisture content on the wet basis, and M wb,i is initial moisture content on wet basis. Specific moisture extraction rate (SMER) is ratio of the moisture evaporated from wet product to the energy input to drying system. The specific moisture extraction rate of the HPD and the SAHPD were calculated using the following equations [23], for the HPD, for the SAHPD, where W b is the electrical energy consumed by blower Specific energy consumption (SEC) is the measure of the energy used to remove 1 kg of water in the drying process. The specific energy consumption of the HPD and the SAHPD were calculated using the following equations, for the HPD, for the SAHPD, Thermal efficiency of drying system is ratio of the energy used for moisture evaporation to the energy input to drying system. The thermal efficiency of the HPD and the SAHPD were calculated using the following equations [24], for the HPD, where fg is latent heat of vaporization of water (kJ/kg).
Pick-up efficiency as the ratio of the moisture evaporated from wet product or the moisture pickedup by the air in the drying chamber to the theoretical capacity of the air to absorb moisture. The pickup efficiency of the HPD and the SAHPD were calculated using the following equation [25], Where da m  is mass flow rate of dry air (kg dry air /s), i Y is absolute humidity of air entering drying chamber (kg water /kg dryair ) and as adiabatic saturation humidity of air entering drying chamber (kg water /kg dryair ).

Experimental uncertainty
In the drying experiments of the Curcuma the data was obtained by appropriate instrument, however, errors and uncertainties can arise because of the situations such as instrument selection, condition, environment, observation, reading, and test planning. Uncertainty was calculated using the following equation [20,26]

RESULTS AND ANALYSIS
The variations of solar radiation and solar collector efficiency with time of the day are shown in Figure 5. As seen from the figure the weather is quite sunny, the solar radiation is varied from 607.4 Wm -2 to 962.7 Wm -2 and in average is 801.1 Wm -2 was recorded. The solar collector efficiency is varied from 31.1% to 49.2%, and in average is 39.2%, with an air mass flow rate of 0.121 kgs -1 . As observed from the Figure 5 the efficiency of solar collector is depending on the solar radiation. The solar radiation fluctuates, then the solar collector efficiency also fluctuates. The evaluation of the uncertainty of experiment as shown in Table 1. The variation of inlet and outlet air temperature of condenser and COP of the heat pump with time of the day for SAHPD and HPD are shown in Figure 6. The average inlet and outlet temperature of the condenser are 27.5oC and 51.6oC, and 24.9oC and 50.9oC for HPD and SAHPD were recorded, respectively. Whereas, the averages of the COP of the heat pump were calculated of about 4.03and 4.35 for HPD and SAHPD, respectively, with an air mass flow rate is about 0.121 kgs-1. The variation of ambient temperature, and air temperature inlet and outlet of drying chamber with drying time for SAHPD and HPD are shown in Figure 7. For the HPD, the ambient temperature, and air temperature inlet and outlet of the drying chamber are varied from 29.0oC to 34.3oC, 43.5oC to 51.3oC, and 30.1oC to 39.6oC, respectively, with corresponding average values of 32.3oC, 49.2oC and 32.8oC. Whereas, for the SAHPD, the ambient temperature, and air temperature inlet and outlet of the drying chamber are varied from 33.1oC to 35.1oC, 53.4oC to 61.7oC, and 32.0oC to 47.2oC, respectively, with corresponding average values of 34.3oC, 57.7oC and 36.9oC. The results indicated that the air-drying temperature in the SAHPD was higher than the HPD. The difference value is 8.5°C, this can be stated that the drying rate in the SAHPD is higher than the in HPD. As seen from figure that the drying chamber outlet air temperature increased with increasing in drying time. This due to, the heat transfer coefficient decreased in the drying time.
The variation of ambient relative humidity, and air relative humidity inlet and outlet of the drying chamber with drying time for SAHPD and HPD are shown in Figure 8. For the HPD, the ambient relative humidity, and air relative humidity inlet and outlet of the drying chamber are varied from 58.9% to 80.4%, 24.7% to 28.8%, and 48.2% to 92.3%, respectively, with corresponding average values of 68.9%, 26.5% and 72.7%. Whereas, for SAHPD, the ambient relative humidity, and air relative humidity inlet and outlet of the drying chamber are varied from 56.2% to 70.9%, 17.7% to 23.9%, and 37.1% to 88.3%, respectively, with corresponding average values of 64.0%, 19.8%, and 68.6%. The results indicated that the air-drying relative humidity in the SAHPD was lower than the HPD. The difference value is 6.7%. As seen from figure that the drying chamber outlet air relative humidity decreased with increasing in drying time. This due to, the mass transfer coefficient decreased in the drying time.
The variation of moisture content of Curcuma xanthorrhiza Roxb with drying time for SAHPD and HPD are shown in Figure 9. The moisture content of Curcuma xanthorrhiza Roxb was reduced from 3.167 dry basis to final moisture content of about 0.065 dry basis. The time to reach the final moisture content was found of about 10.5 hours and 8hours for HPD and SAHPD, respectively. The SAHPD had a shorter drying time compared to the HPD. In other words, the SAHPD reduced the drying time 24% compared to the HPD. This due to, its moisture content transfer rate is higher than the HPD, this caused by the difference in the partial vapour pressure between Curcuma xanthorrhiza Roxb and the drying air obtained in the SAHPD is higher than in the HPD. This difference value is very dependent on the drying air temperature and relative humidity, when drying air temperature is high and relative humidity is low, the difference in the partial vapour pressure between Curcuma xanthorrhiza Roxb and the drying air is also high, and vice versa.
The variation of drying rate with drying time for SAHPD and HPD are shown in Figure 10. The drying rate of Curcuma xanthorrhiza Roxb was calculated, for the HPD, the drying rate is varied from 0.60 kg/h to 1.65 kg/h and in average is 1.05kg/h. whereas, for the SAHPD, the drying rate is varied from 0.85 kg/h to 1.85 kg/hand in average is 1.36kg/h. Referring to Figure 10, the drying rate decreased with increase by time. Then, the evaporation rate of moisture decreased by time.
The variation of SMER and SEC with drying time for SAHPD and HPD are shown in Figure 11. The SMER is varied from 0.533 kg/kWh to 1.467 kg/kWh and 0.327 kg/kWh to 0.676 kg/kWh and in an average of about 0.931 kg/kWh and 0.521 kg/kWh for HPD and SAHPD, respectively were calculated. Whereas, the SEC are varied from 0.68 kWh/kg to 1.88 kWh/kg and 1.27 kWh/kg to 3.53 kWh/kg, and in an average of about 1.17 kWh/kg and 2.07 kWh/kg for HPD and SAHPD, respectively were calculated. As observed from the figure that the SMER decreased and SEC increased with increase by time. This due to, the drying rate decreased in time, and also the SMER of the HPD is higher that the SAHPD, this due to the total energy input to the HPD is lower than to the SAHPD. The variation of dryer thermal efficiency and pickup efficiency with drying time for SAHPD and HPD are shown in Figure 12. The dryer thermal efficiency is varied from 33.50% to 97.07% and 21.45% to 44.39%, and in an average of about 61.03% and 34.28% for HPD and SAHPD, respectively were calculated. The pickup efficiency is varied from 34.04% to 93.37%and 30.30% to 92.78%, and in an average of about 57.51% and 59.21% for HPD and SAHPD, respectively were calculated. As observed from the figure that the pickup efficiency of the SAHPD is higher than the HPD. This due to, the evaporation rate of moisture in the SAHPD is higher than the HPD.   Figure 11. Variation in SMER and SEC with drying time Figure 12. Variation in thermal efficiency and pickUp efficiency with drying time

CONCLUSIONS
The performances of a SAHPD and a HPD have been evaluated for drying of Curcuma xanthorrhiza Roxb. The HPD and SAHPD reduced mass of Curcuma xanthorrhiza Roxb from 30.70 kg to 7.85 kg needed 10.5 hours and 8 hours with averages temperature and relative humidity of about 49.2 o C and 26.5%, and 57.7 o C and 19.8%, for SD and SAHPD respectively. The moisture of Curcuma dried from 3.167 db to 0.065 db with an air mass flow rate of 0.121 kg/s. The SAHPD reduced the drying time of about 24% compared to the HPD. The drying rate and the specific energy consumption rate were calculated in an average of about 1.05 kg/h and 1.36kg/h, and 1.17kWh/kg and 2.07kWh/kg for HPD and SAHPD, respectively. The specific moisture extraction rate and dryer thermal efficiency were calculated in an average of about 0.931 kg/kWh and 0.521 kg/kWh, and 61.03% and 34.28% for HPD and SAHPD, respectively. Whereas, the pickup efficiency and coefficient of performance of the heat pump were calculated in an average of about 57.5% and 59.2%, and 4.03and 4.35 for HPD and SAHPD, respectively. Whereas, the average of the solar collector efficiency was calculated to be about 39.2%. The result shows that the SAHPD is capable of drying Curcuma quickly because of the high drying rate and high pickup efficiency.