Technical and Business Evaluation of Geothermal Dryhouse for Coffee in Kamojang, West Java, Indonesia

The coffee farmers in Kamojang still currently rely on conventional drying method using direct sun radiation. However, the cold ambient temperature and fluctuative weather in Kamojang brings challenges to this method. Therefore, PT PGE Area Kamojang built a trial geothermal dryhouse with a heating pipe connected to a blowdown on the existing geothermal steam pipeline as the heat source. In this study, the existing geothermal dryhouse is evaluated from the technical and business aspects. The technical evaluation studies the thermal performance of the heating pipe to calculate the dryhouse heating load, the required heating pipe length, the effect of heating pipe operation on the dryhouse temperature, and the suggested technical improvements. Meanwhile, the business evaluation studies qualitatively about the relationship between the potential farmer profit enhancement and the possible coffee-drying duration reduction from previously 2-3 weeks to 1-2 weeks by substituting conventional drying method with the use of geothermal dryhouse. Aside from that, it was expected to retain the coffee’s pleasant flavor and not have any sulfur odor problems. The technical evaluation is performed using analytical and experimental methods. The analytical results show that the required heating load is 10.94 kW and the heat transfer rate per unit length of the heating pipe is 0.2618 kW/m. Hence, the required heating pipe length is 41.79 m. The Subsequently, the heating pipe was tested operational three times on 16 – 18 January, 27 April – 4 May, and 31 May – 5 June 2023. On Trial 1, the heating pipe performance was not optimal due to condensate built-up inside the heating pipe. Therefore, steam traps were installed to reduce the condensate built-up. On Trial 2, the heating pipe performance was better, which was indicated by the higher and stable dryhouse temperature during the night. Hence on Trial 3, the technical improvement involving rearrangement of steam trap and bypass valve considerably improved the heating pipe performance. It reaches the design temperature and being maintained above 40°C for a maximum of 50°C longer than it did previously. The business evaluation expects a significant profit enhancement for the farmers if the drying duration could be cut into 1-2 weeks using the geothermal dryhouse.


Introduction
Indonesia has a geothermal energy potential of 23.9 GW of which only 10% of the geothermal potential is utilized [1].The utilization of geothermal potential can be divided into two methods, namely direct and indirect utilizations [2].The main example of indirect utilization is for electricity generation.

Existing Geothermal Dryhouse
The existing geothermal dryhouse has a width of 6 m, length of 8 m, and height of 4.5 m, as shown in Figure 1.The heating pipe utilizes the blowdown from the main pipeline as the heat source so that the geothermal steam flows from the blowdown towards the dryhouse to transfer its heat to the dryhouse air.The total length of the existing heating pipe is 43.5 m.The actual documentations of the existing geothermal dryhouse and heating pipe connection to the main pipeline's blowdown are shown in Figure 2. The dryhouse has three coffee table with a single Uloop heating pipe under each table.The heating pipe is made of carbon steel with the specification code of SMLS Pipe CS API 5L Gr B 1" SCH 40.

Methodology 3.1. Technical Evaluation
The technical evaluation studies the thermal performance of the heating pipe to calculate the dryhouse heating load, the required heating pipe length, the effect of heating pipe operation on the dryhouse temperature, and the suggested technical improvements.The technical evaluation is performed through analytical and experimental methods explained in the following.

Analytical Method.
The analytical method is used to evaluate the dryhouse technically based on the necessary governing equations.In order to maintain the dryhouse temperature at a desired value, the heating load must be equal to the rate of heat loss and the rate of trapped radiation.The energy balance equation of dryhouse is expressed in Equations ( 1) and ( 2) as follows: ̇ =  ̇ −  ̇ (1) where  ̇ ,  ̇ ,  ̇ ,  ̇ ,  ̇ ,   ,   , , and are heating load, structural heat loss rate (kW), heat loss rate through air infiltration (kW), heat gain through trapped radiation (kW), construction factor, and climate factor, respectively.The structural heat loss rate is calculated using Equation (3) as follows: where , ,  ∞,ℎ , and  ∞, are structural overall heat transfer coefficient (kW/m 2 ·K), wall and roof area (m 2 ), dryhouse temperature ( o C), and ambient temperature ( o C) respectively.The heat loss rate through air infiltration is calculated using Equation (4) as follows: where ,  ℎ ,  ,ℎ ,  ,,ℎ ,  ∞,ℎ , and  ∞, are air exchange rate (s -1 ), dryhouse inner volume (m 3 ), density of air inside dryhouse (kg/m 3 ), isobaric specific heat of air inside dryhouse (kJ/kg·K), dryhouse temperature ( o C), and ambient temperature ( o C) respectively.
The geothermal steam flows inside the heating pipe and transfer its heat to the dryhouse air.This involves convection heat transfer on the inner and outer pipe walls, conduction heat transfer at the pipe wall, and radiation heat transfer on the outer pipe walls.The thermal circuit of the heat transfer process is shown in Figure 3.The geothermal steam flowing inside the heating pipe is at saturated vapor phase.Therefore, film condensation occurs as the steam transfers heat to the dryhouse air.The forced convection heat transfer coefficient and thermal resistance with film condensation for internal flow inside a horizontal cylindrical pipe are expressed in Equations ( 6) and (7), respectively, according to [6] , = 1 ℎ ,    (7) where ℎ , ,  , , ,   ,   ,   ,   ,   , , ℎ  ,  , ,   , and  , are convection heat transfer coefficient on inner pipe wall (kW/m 2· K), convection thermal resistance on inner pipe wall (m·K/kW), gravitational acceleration (m/s 2 ), condensate density (kg/m 3 ), steam density (kg/m 3 ), condensate thermal conductivity (kW/m·K), condensate viscosity (Pa·s), inner pipe diameter (m), pipe length (m), condensate isobaric specific heat (kJ/kg·K), saturation temperature ( o C), and inner pipe wall temperature ( o C), respectively.
The outer pipe wall is exposed to the dryhouse air flowing across the pipe.The natural convection heat transfer coefficient and thermal resistance of external flow across a cylindrical pipe are expressed in Equations ( 8) and ( 9), respectively, according to [6] as follows: where ℎ , ,  , ,   ,   , ,   , and  are convection heat transfer coefficient on outer pipe wall (kW/m 2· K), convection thermal resistance on outer pipe wall (m·K/kW), fluid thermal conductivity (kW/m·K), Rayleigh number, Prandtl number, outer pipe diameter (m), and pipe length (m), respectively.
The heating pipe also emits radiation to the surrounding.The radiation heat transfer coefficient and thermal resistance are expressed in Equations ( 10) and (11), respectively, as follows: the rest was caused by air infiltration of 1.79 kW.Since the dryhouse obtains heat through trapped radiation of 0.2 kW, the heating power required to keep the dryhouse air temperature constant is 10.94 kW, as shown in Table 2.The convection heat transfer coefficient with film condensation on the inner wall of the pipe is 27 kW/m2•K.Convection heat transfer involving a phase change has a higher heat transfer coefficient than convection without a phase change.The total thermal resistance from convection, conduction and radiation is 561.26 m•K/kW.Thus, the heat transfer rate per unit pipe length is 0.2618 kW/m and the required length of heating pipe to meet the heating power requirement (10.94 kW) is 41.79 m, as shown in Table 3.Therefore, the existing pipe length (43.5 m) has met the required pipe length.Since the geothermal steam is in the saturated vapor phase when it enters the inlet of the dryhouse, the heat transfer from the steam to the air of the dryhouse causes the formation of condensate from the steam.Based on the results of the calculation of the heat transfer parameters in Table 2, the rate of condensate formation per unit pipe length is 0.000235 kg/s•m or 0.8462 L/h•m.With an existing pipe length of 43.5 m, the total condensate formation rate is 0.010224 kg/s or 36.81L/h, as shown in Table 4.The trial operation of the heating pipe was performed twice.The first trial was on 16-18 January 2023, while the second trial was on 27 April -4 May 2023.During the first trial, the blowdown valve from the main pipeline was opened on 16 January 2023 at 02:00 p.m.The valve at the end of the heating pipe at the outlet of the dryhouse remains closed.The dryhouse temperature profile after heating pipe operation on Trial 1 is shown in Figure 4.The initial dryhouse temperature when the blowdown valve was opened was 33 o C.After the blowdown valve was opened, the dryhouse temperature gradually increased until reaching the maximum temperature of 36.5 o C was achieved 28 minutes after operation started (at 02.28 p.m.).Afterwards, the dryhouse temperature showed a declining trend.Therefore, the heating pipe could not reach the dryhouse design temperature of 40 o C.During the night, the dryhouse temperature declined to as low as 23 o C before increasing again in the morning.The failure to reach the design temperature was caused by the condensate buildup in the heating pipe which cannot be removed because the end valve is closed.With an inner heating pipe diameter of 0.02664 m and a length of existing heating pipe of 43.5 m, the volume of the cavity in the heating pipe is 0.024 m 3 or 24 L. If the rate of condensate formation based on Table 3 is 36.81L/h, then the condensate will fill all the cavities in the pipe within 40 minutes after the heating pipe is operated.
Based on the evaluated results of the first trial, three technical improvements are suggested: 1.Two steam traps are needed, each installed at the inlet and outlet of the dryhouse.The steam trap at the inlet reduces condensate in geothermal steam that flows into the dryhouse.Meanwhile, the steam trap at the outlet removes the condensate that forms while the steam is in the dryhouse.Based on the results of calculating the rate of formation of condensate in Table 4 of 0.03681 tph and by providing an allowance factor of 50%, the required capacity of each steam trap is 0.055 tph of condensate.2. The geothermal steam will experience a pressure drop when flowing in the heating pipe.While the steam pressure at the main pipeline blowdown can be known from the operational data of Kamojang GPP, a pressure gage should be installed at the dryhouse outlet to calculate the pressure drop experienced by the steam inside the heating pipe.3. Before the heating pipe enters the drying house, there is a 15 meter-long heating pipe segment between the main pipeline blowdown and the drying house inlet.To reduce heat loss in this pipe segment and maximize the heat transferred to the dryhouse, this heating pipe segment should be insulated.

Experimental Method: Second Trial (27 April -4 May 2023)
In response to the suggested technical improvements based on the first trial, two steam traps, a pressure gauge, thermal insulation were installed before the second trial.During the second trial, the blowdown valve from the main pipeline was opened on 27 April 2023 at 09:00 a.m.The dryhouse temperature profile after heating pipe operation on Trial 2 is shown in Figure 4.The initial dryhouse temperature when the blowdown valve was opened was 39 o C.After the blowdown valve was opened, the dryhouse temperature gradually increased until surpassing the dryhouse design temperature of 40 o C at 09:03 a.m.The dryhouse temperature kept increasing until reaching the maximum temperature of 43 o C at 10:42 a.m.(1 hour and 42 minutes after heating pipe operation).The dryhouse temperature remained higher than 40 o C until 11:19 a.m. and then was followed by a declining trend with a slight increase between 00:44 p.m. and 01:27 p.m. On Trial 2, the dryhouse temperature during the night was maintained higher than on Trial 1.This indicates that the technical improvements managed to improve the heating pipe's performance on maintaining the dryhouse temperature.However, the presence of condensate buildup inside the heating pipe suggests that the newly installed steam traps did not work properly.
Based on the evaluated results of the second trial, two technical improvements are suggested: 1.To discover why the condensate buildup still occurred, recheck the newly installed steam traps by exchanging the inlet and outlet steam traps.2. To improve the air circulation and minimize the sulfur's smell, two access doors and distant air exhaust should be added into the dryhouse.

Business Evaluation
Geothermal dryhouse has advantages compared to conventional methods such as drying using the sun.
With temperatures that can be maintained in the range of 30-43oC, geothermal dryhouse can accelerate drying time up to 75% faster (from 4 weeks to 1 weeks) under normal conditions.This method is also not influenced by the season so that drying time can run 24 hours and is not affected by weather in mountainous areas that often can be unpredictable and changes quickly.With this methods, coffee farmers can extend their drying season and dry their beans even during inclement weather.this condition also makes the drying temperature and humidity more stable so as not to damage the coffee flavor.In The achieved reduction of coffee drying duration after using the geothermal dryhouse compared to the conventional drying method is shown in Table 5.The desired moisture content of the dried coffee is 14%.This is intended to compensate for the further moisture content reduction caused by roasting process, which hopefully will set the moisture content of the coffee to 10-12%.This value refers to the international coffee moisture standard to maintain the coffee quality.Based on the testimony of local coffee farmers, the dried coffee using the geothermal dryhouse has a good taste and does not have any sulfur's smell issues.

Conclusion
Geothermal dryhouse has the advantages of being weather-independent, available heat around the clock, and naturally-controlled temperature and humidity.This method is similar to conventional drying method but relying on geothermal as a constant heat supply instead of the fluctuative sun radiation.Moreover, the existing dryhouse has been improved to mitigate sulfur smell issues by using double door access and a distant steam exhaust.In this study, the existing geothermal dryhouse for coffee drying in Kamojang is evaluated from the technical and business aspects.Based on technical evaluation, the required heating power is 10.94 kW with a heat transfer rate per unit length of heating pipe is 0.2618 kW/m.Thus, the required length of heating pipe is 41.79 m which has been fulfilled by the length of the existing heating pipe with a length of 43.5 m.The results indicated that the heating pipe was able to increase the dryhouse temperature.However, the condensate buildup inside the heating pipe reduces the dryhouse temperature minutes after the heating pipe operation.Therefore, it is responded by installing two stream traps, each installed at the inlet and outlet of the drying house to reduce condensate in the steam entering the drying house and remove the condensate formed while the steam is in the drying house.Subsequently, the heating pipe was tested operational twice on 16-17 January and 27-28 April 2023.On Trial 1, the heating pipe performance was not optimal due to condensate buildup inside the heating pipe.Therefore, steam traps were installed to reduce the condensate buildup.As a result, on Trial 2, the heating pipe performance was better, which was indicated by the higher and stable dryhouse temperature during the night.

Figure 2 .
Figure 2. Existing geothermal dryhouse and heating pipe connection to the blowdown.

Figure 3 .
Figure 3. Thermal resistance of heat transfer from geothermal steam to dryhouse air.

4. 4 .
Experimental Method: Third Trial (31 May -5 June 2023)In response to the suggested technical improvements based on the second trial, the inlet and outlet steam traps' were exchanged to discover whether the condensate buildup issue was caused by the steam traps themselves or by a systemic issue.During the second trial, the blowdown valve from the main pipeline was opened on 31 May 2023 at 11:00 a.m.The dryhouse temperature profile after heating pipe operation on Trial 3 is shown in Figure4.The initial dryhouse temperature when the blowdown valve was was 39 o C.After the blowdown valve was opened, the dryhouse temperature gradualy increased until surpassing the dryhouse design temperature of 40 o C at 11:18 a.m.The dryhouse temperature kept increasing until reaching the maximum temperature of 43 o C at 00:01 p.m (1 hour and 1 minute after heating pipe operation).The dryhouse temperature remained higher than 40 o C until 02:39 p.m. and then was followed by a declining trend.On Trial 3, the dryhouse temperature during the night was maintained higher than on Trial 3.This indicates that the technical improvements managed to improve the heating pipe's performance on maintaining the dryhouse temperature.

Table 2 .
Geothermal dryhouse heat loss and heat load.

Table 3 .
Results of analytical heating pipe heat transfer calculation.

Table 4 .
Results of condensation rate calculation.
8 4.2.Experimental Method: First Trial (16-18 January 2023) dryhouse can also reduce the risk of contamination that often occurs in conventional drying such as dust, dirt, and insects.

Table 5 .
Reduction of coffee drying duration (to reach 14% moisture content).