Techno-economic analysis of a combined cooling, heating and power system based on hot sedimentary aquifer for hotel building in tropical countries

. Low enthalpy geothermal based Combined Cooling, Heating and Power （ CCHP) system for building is expected can increase energy efficiency and reduce carbon dioxide emission. The aim of this study is to assess techno-economic performances of a CCHP for hotel building in tropical countries with total energy demand of 7.64 MW. The fresh geothermal fluid was divided by 15% to power generation system of Organic Rankine Cycle (ORC) and the waste heat merged with the rest fresh geothermal to absorption refrigeration system and heating systems. Cycle Tempo and COMSOL are used to simulate CCHP system and a borehole of hot sedimentary aquifer (HSA), respectively. The simulation results indicate that total CCHP system efficiency is 36.61% consist of ORC efficiency and absorption cycle efficiency are 12.77% and 57.88%, respectively with emission reduction of 1.4 ton CO 2 eq. per year compare to conventional grid electricity. The best business scheme was BOO with financial incentives, which were tax allowance, soft loan, and grant. Therefore, the production prices were 0.08 USD/kW t h, 0.10 USD/kW e h, and 12.03 USD/MMBtu for cooling, electricity, and heating respectively.


Introduction
Regarding to increase energy efficiency, Indonesia has target to reduce 1.7% it in every year commercial sector [1]. Despite energy demand in commercial sector for space cooling, lighting, space and water heating are growing significantly, but current energy efficiency implementations are not having a substantial effect in achieving the target. Therefore, the study must be taken care. The importance of energy efficiency in Indonesia was shown on the escalation of energy demand by 3.99% in 2000 -2014 [2]. Final energy consumption of Indonesian's commercial building was 5.64% or equal to 38 million BOE in 2016 [3]. Electricity and cooling are the main energy needs for commercial building in tropical countries like Indonesia. The number of hotel building in 2016 up to 2,387 hotels with 233,007 rooms [4].
Indonesia has geothermal potency in all area for both high-temperature and low-temperature. The potency of high-enthalpy geothermal is 16,134 MWe and lowenthalpy geothermal near high-temperature resource is 7,886 MWe [5]. In fact, most locations in Indonesia can be considered as medium enthalpy because the temperature range on aquifer is about 100 o C -200 o C [6]. Geothermal resources in urban area can be retrieved from Hot Sedimentary Aquifer (HSA).
CCHP is combinations that produce electricity and heat from the same energy resource. Previous studies on energy technologies related to energy efficiency are cooling and heating from geothermal and photovoltaic for a building in Romania [7], CCHP from geothermal and natural gas for an office building in China [8], combine heat and power from lignite and geothermal for sustainable buildings in Turkey [9], and combine heat and power from natural gas for small commercial buildings in North America [10]. Many studies use natural gas as main energy resource [8,10-12] and combined energy resource or hybrid [7,9]. The study in tropical country was done in Thailand, which Raksaskulwong [13] reviewed direct usage of lowenthalpy geothermal as direct use and Chaiyat [14] worked on the prototype of CCHP from low-enthalpy geothermal.
The aim of current study is to assess technoeconomic performances of a CCHP using HSA as energy resource in tropical country. The geothermal energy is taken from HSA with one production well and one injection well. Moreover, the simulation is demanddriven so that the downstream is going to be simulated first and then checked if the medium-enthalpy geothermal resource is sufficient. Finally, the result is compared with conventional for economic aspects to find the most feasible business scheme.

Energy demand
The assumptions of projected hotel are total hotel area is 45,000 m 2 or equal to 45 floors, total hotel room of 600 in which 1 room consists of 2 guests, and occupancy rate of 61.5% [15]. Total energy consumption in tropical countries to achieve satisfactory is 320 kWh/m 2 per year [16]. In tropical countries, cooling is very important so that cooling demand is 56% of total energy consumption [17] and the rest of it is electricity demand. Total water consumption per guest night is 812 L [18]. Heating demand for hot water is calculated with specific heat formula for temperature range of 25 o C to 60 o C. Additionally, facilities' heating demand volume size is assumed. The temperature for hot tub, sauna, and hot spring pool is 60 -70 o C, 70 -100 o C, and 35 -70 o C, respectively. Table 1 summarizes energy demand for the proposed hotel. Water is heated with geothermal heat in a heat exchanger from 25 -60 o C. Fig. 1 shows the CCHP system used in this study.

Fig. 1. CCHP system
Both hot tub and sauna facilities are in the same area so that fresh water circulates from facility that needs higher temperature to the lower one. As sauna needs higher temperature than hot tub, the circulation goes to sauna and hot tub with temperature range of 70 -100 o C and 60 -70 o C.

Borehole simulation
The assumption for borehole simulation is steady-state. The borehole simulation uses COMSOL and only simulates production geothermal well as the best CCHP scenario. The heat source comes from Talang Akar aquifer (West Java) which has averagely 300 m thick and 145 o C [21]. There are three layers, which top soil tends to have constant surface temperature in depth of 10 -20 m [22]. The total depth to the aquifer is 2500 m. The three layers have different soil thermal conductivity of 1, 2 and 5 W/m K [23] for top soil, soil and aquifer respectively. Since the mass flow rate is 36.7 kg s -1 so the diameter borehole diameter is 8-in nominal diameter and 12-in casing diameter [24].
The simulation uses two steps on COMSOL, which are heat transfer in porous media (ht) and turbulence k-ε. Since the minimum required temperature is 140 o C using turbulence flow for heat transfer in fluid. Input the boundary conditions as listed on Table 2. . The economics parameter must attain payback period of 10 years and IRR>WACC. There are three business schemes option that is going to be studied, which are engineering, procurement and construction (EPC), build, operate and transfer (BOT), and build, own, and operate (BOO) with financial incentives (BOO + FI). Table 3 shows the assumptions for three business schemes.

Energy demand
Energy demand of five-star hotel building is estimated using engineering model based on quantity of services and energy intensity of services, comprises of thermal energy and electricity. The total energy demand estimation is 7,640.88 kW with its distribution is shown in Fig. 2.

CCHP system
For the simulation, CCHP system splits 15% of fresh geothermal for ORC, then the waste heat merges with the rest fresh geothermal to enter the ARS and heating systems. The optimum conditions of ORC with R134a as working fluid before entering turbine are 120 o C and 38 bar, while outlet turbine conditions are 50.5 o C and 8.5 bar. The optimum conditions of ARS with ammoniawater as working fluid are affected by incoming geothermal temperature so the outlet temperature of generator in ARS 123.8 o C. Because of that, the mass flow is only 36.7 kg s -1 . Mass flow leads to pump power as it needs 185.28 kW to circulate the whole system. The net energy produced by CCHP is revealed in Fig. 3. The energy efficiency of ORC, ARS, and total CCHP system is 12.77%, 57.88% and 36.61%, respectively. It implies the ability of the systems to deliver the energy services. The efficiency of ORC with R134a as working fluid is 10% [25] and only depends on operating conditions. The most optimum generator outlet temperature is 130 o C [20], therefore the closest temperature to the optimum makes the ARS has higher efficiency. Since the system cannot reach the optimum temperature, the efficiency is only 57.88%. This is due to the fresh geothermal is merged with waste heat from power generation system that results lower outlet temperature of generator. . Both PLN electricity and gas-based power higher emission rate than HSA-based CCHP system, thus the difference up to 1.4042 ton CO2eq per year.

Borehole supply
The simulation indicates that the output condition of geothermal fluid is 143.2 o C as shown on Fig. 4. This can fulfil the needs on CCHP system. The model assumes that the pipe is located in low conductivity rock so that heat loss to the environment is negligible. Horizontal profile for velocity and temperature indicate turbulent flow along the borehole. Although the total temperature decline is small, temperature profile along borehole depth is segmented because of different soil layer characteristics.

Economics
The capital expenditure (CAPEX) of the whole system is 18.64 million USD, the cost breakdown is shown on Fig.  5.

Fig. 5. Cost Breakdown
The geothermal drilling is share equipment for polygeneration. Hence, the cost of geothermal drilling is based on joint cost of heating, electricity, and cooling. The share of heating, electricity, and cooling is 72.37%, 22.46%, and 5.17%, respectively.
The target of production costs is to reach willingness to pay. It means that production costs must be lower than conventional, which is levelized cost of electricity (LCOE) of air handling unit (AHU) (0.08 USD/kWth), PLN electricity tariff (0.11 USD/kWeh), and pipeline gas tariff (12.81 USD/MMBtu).
From the production costs comparation, BOO with FI scheme is the best option for the project as represented on Fig. 6.

Figure 6. Production Costs Comparation
The BOO + FI can reach willingness to pay with final production costs of 0.08 USD/kWth, 0.10 USD/kWeh, and 12.03 USD/MMBtu for cooling, electricity, and heating respectively. A fiscal incentive through tax allowance from government is regulated by Ministry of Finance regulation number 3 year 2012 (No.3/PMK/2012). Tax allowance reduces income tax to 5% of 70% of net income for the first 6 years and 10% of net income for the rest lifetime. Although the government gives loan facility with lower bank loan rate for geothermal project, soft loan offers lower bank loan rate. The rate depends on London Interbank Offered Rate (LIBOR), which is 2% per year. Soft loan can be obtained from international bank such as Asian Development Bank or World Bank. Moreover, international organization grant on energy efficiency is feasible to get like Global Environment Facilities (GEF) that also focus on energy efficiency and help private sector to reach the same goals. In this experiment case, the grant is 3 billion USD, while it can be bigger depending on the organization's review.
CCHP with low-enthalpy geothermal resource is a good investment for both SPV and hotel management. SPV obviously gets profit with IRR of 13%, payback period of 10 years, and NPV of 142,013 USD. Also, the credibility of SPV to have another project on energy efficiency is possible. Nonetheless, hotel management gets premium quality facilities and possible green building certification.

Conclusion
Total energy demand of hotel building is 7,649.88 kW in which mostly used for facilities. Besides, the energy supply arrangement divides 15% of fresh geothermal for ORC and then the waste heat merges with the rest fresh geothermal following systems. The total energy supply is 7,921.69 kW and total efficiency of 36.61%. Additionally, the borehole system of 2,500 m depth and 8-in diameter production well lifts surface geothermal fluid of 143.2 o C. To have production costs reach willingness to pay, build, own, operate (BOO) with fiscal incentives, soft loan, and international grant is applied. At last, the final production costs are 0.08 USD/kWth, 0.10 USD/kWeh, and 12.03 USD/MMBtu for cooling, electricity, and heating, respectively.