Ocean Thermal Energy Conversion System Research Articles

A study of the temperature distribution in the OTEC cold water pipe using a heat and mass transfer approach

Abstract The difference between sea water temperature at a depth of around 1000 m and sea water temperature at sea level is generally used as a parameter in the design of Ocean Thermal Energy Conversion (OTEC). In practice, electricity generation is determined by the difference between the temperature of the cold seawater coming out of the Cold Water Pipe (CWP) and the temperature of the seawater at the surface. The temperature of cold sea water increases due to heat transfer experienced by cold sea water flowing through the CWP, which comes into contact with surrounding sea water which has a higher temperature. This in turn provides a lower actual temperature difference, and therefore reduces the design power capacity. However, many previous studies did not consider these lower temperature differences. This may be acceptable for cases with practically small heat transfer such as CWP with low thermal conductivity combined with good insulation used in 1000 m CWP vertical floating systems. Unfortunately, this may not be the case for many of OTEC’s proposed alternative sites, which are located on land systems that require CWP lengths of five km or more. This raises the need for careful investigation to determine the temperature of the sea water coming out of the CWP, where it is necessary to calculate the temperature distribution of the cold sea water flowing through the CWP. This paper aims to estimate the temperature distribution of cold sea water flowing through the CWP and the increase in temperature of cold sea water leaving the CWP. Analysis based on the principles of mass and heat transfer was carried out in this research, where modelling was carried out numerically using a finite volume approach. For the case considered, the change in sea water temperature at CWP from depth to the surface occurs 1-3°C, which is the accumulation of each change in sea water depth. The results of this research illustrate that designing an OTEC system with a long CWP must consider the temperature distribution of cold sea water flowing through the CWP to produce a more realistic design.

A numerical and experimental study of breakdown of falling film on horizontal circular tubes

Falling film horizontal tube heat exchangers are used in several industrial applications such as sea water desalination, multi effect distillation, chemical industries, ocean thermal energy conversion systems and refrigeration. Different flow patterns that occur during falling film evaporation on an array of horizontal tubes are droplet, droplet-columnar, columnar, columnar-sheet and sheet modes. From the heat transfer point of view, a sheet mode (continuous falling film spanning the intertube space) is the most desirable falling-film mode. Falling film breakdown leads to the occurrence of dry patches on tubes which cause heat transfer deterioration. In the present study, falling film breakdown on horizontal tubes under adiabatic condition has been investigated both numerically and experimentally. The study focuses on the influence of tube pitch to diameter ratio on the threshold film Reynolds number required to maintain continuous falling film. Results indicate that the threshold film Reynolds number increases with the increase in the pitch to diameter ratio for a given diameter of the tube. The study also shows that as the number of tubes in the column increases, the threshold film Reynolds number required to maintain continuous film over all the tubes increases. The study highlights the need for correlations for the column-sheet mode transition that take into account the effect of number of horizontal tubes in a vertical array.

System Parameters Sensitivity Analysis of Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) is a technology to harvest the solar energy stored in the ocean by utilizing the temperature difference between warm surface and cold deep seawater. Considering that the OTEC system works in a low-temperature range, the present paper assessed the technical resources comprehensively by acquiring in-situ thermocline data and conducting a sensitivity analysis of the system parameters. The in-situ temperature profile data were measured in the waters of North Bali, Indonesia. The temperature gradient data based on field measurements were then compared with the HYCOM consortium model. The data were then used as input in the OTEC power and efficiency estimation through a single-stage ranking cycle. The analysis was conducted by varying the type of working fluid, the performance of the heat exchanger, and the location to investigate how the system parameters influenced the power produced. Using an unusual combination of parameters made it difficult to analyze the resulting data multiple times. However, with reference-based analysis and the formulation of calculations, the sensitivity of each parameter could be assessed at both locations. As a result, the ammonia working fluid provided the highest net power output of the system but had the lowest efficiency of all working fluids. The heat exchanger performance in terms of net power and efficiency cannot be separated from the seawater mass flow requirement. This referred to the results where the heat exchanger with a temperature difference of 3°C before and after the seawater passed through the heat exchanger and produced the highest net power and efficiency. Additionally, the net power output reached its convergence level at a water depth of 400m for the Bungkulan site and 450m for Celukan Bawang, which was proportional to the thermocline tendency. Doi: 10.28991/ESJ-2024-08-02-04 Full Text: PDF

Thermodynamic, economic, exergoeconomic analysis of an integrated ocean thermal energy conversion system

This paper proposes an integrated ocean thermal energy conversion (OTEC) system that couples power generation, refrigeration, and desalination. Based on the mathematical model, the thermodynamic, economic, and exergoeconomic performances are explored by evaluating several crucial parameters such as the ammonia-water concentration of the power subsystem (Xgen), the turbine inlet pressure (Pturb), etc. The results indicate that integrated OTEC system significantly improve the exergoeconomic performance since the efficient utilization of the exhausted cold seawater. The integrated system can achieve maximum net output power of 16.79 kW, refrigeration capacity of 268.87 kW, freshwater production of 53.22 t/d, as well as exergetic efficiency of 31.69% when the power subsystem's flow rate is 1.0 kg/s. Moreover, there is a maximum net output power when Pturb is under 800 kPa, and increasing Pturb leads to higher optimal Xgen. The maximum net output power, for instance, is 2.18 kW, 4.62 kW, and 9.79 kW when Pturb value is 600 kPa, 700 kPa, and 800 kPa, respectively, with corresponding optimal ammonia concentration values of 0.73, 0.83, 0.93. Furthermore, the maximum exergetic efficiency increases as Pturb enlarges. The maximum exergetic efficiency rises from 20.40% to 31.69% when Pturb increases from 600 kPa to 950 kPa. Besides, compared to the single OTEC plant's levelized cost of energy at 3.56 $/(kWh), the integrated system's can be as low as 0.098 $/(kWh).

Experimental investigation on the performance of a Solar-Ocean Thermal Energy Conversion system based on the Organic Rankine Cycle

To improve the performance of the Ocean Thermal Energy Conversion (OTEC) system based on the Organic Rankine Cycle (ORC), a solar-assisted cycle is added, and the power generation experimental setup for the Solar-Ocean Thermal Energy Conversion (S-OTEC) system has been constructed. Through energy and exergy analysis, the effects of solar hot water temperature and working fluid flow rate on the components and system performance are investigated, and the performance differences between S-OTEC and OTEC at the same seawater temperature and working fluid flow rate are compared. The results show that: in the S-OTEC system, as the solar hot water temperature is increased from 54 °C to 72 °C, the isentropic efficiency of the working fluid pump and expander is improved, but the heat transfer coefficient of the heat exchanger is decreased. Futhermore, the thermal efficiency, power generation efficiency, and exergy efficiency increase in the ranges of 2.02–3.26 %, 2.93–3.15 %, and 37.93–41.01 %, respectively. When the working fluid flow rate is increased from 124.33 kg/h to 216.73 kg/h, the isentropic efficiency of the working fluid pump and expander as well as the heat transfer coefficient of the heat exchanger are improved. Additional, the thermal efficiency, power generation efficiency and exergy efficiency increase in the ranges of 2.05–2.75 %, 2.46–2.97 % and 38.01–48.13 %, respectively. What's more, the thermal efficiency, power generation efficiency and exergy efficiency of the S-OTEC system can be increased by 154.32 %, 39.33 % and 27.87 % respectively compared to the OTEC system.

Basic design optimization of power and desalinated water for hybrid cycle ocean thermal energy conversion system integrated with desalination plant

AbstractOcean thermal energy conversion (OTEC) is a heat engine application that utilizes the Rankine cycle to extract energy from the thermal gradient between surface seawater and deep seawater. Hybrid cycle OTEC (H-OTEC) is a combination of an open cycle desalination system and a closed-cycle power generation system that leverages the features of both cycles. Unlike other desalination technologies that require extensive energy to operate, H-OTEC relies entirely on renewable energy. In addition, a desalination plant can be coupled with the H-OTEC system (H-OTEC + D) to improve its performance. Conventionally, the total heat transfer area of heat exchangers per net power is used as an objective function to achieve optimal performance with the lowest capital expenditure cost. The proposed objective function, unlike the conventional one, considers both power and water. In this study, the optimization of H-OTEC + D and H-OTEC is carried out by minimizing the proposed objective function, considering several independent variables. The performance of both systems is evaluated in terms of the objective function, power consumption, seawater flow rates, and desalination ratio. The findings also indicate the effectiveness of the proposed objective function over the conventional one as an effective tool for maximizing power and desalinated water generation.

Efficiency Enhancement in Ocean Thermal Energy Conversion: A Comparative Study of Heat Exchanger Designs for Bi2Te3-Based Thermoelectric Generators.

This research focuses on enhancing the efficiency of Bi2Te3-based thermoelectric generators (TEGs) in ocean thermal energy conversion (OTEC) systems through innovative heat exchanger designs. Our comparative study uses computer simulations to evaluate three types of heat exchangers: cavity, plate-fins, and longitudinal vortex generators (LVGs). We analyze their impact on thermoelectric conversion performance, considering the thermal energy transfer from warm surface seawater to TEGs. The results demonstrate that heat exchangers with plate-fins and LVGs significantly outperform the cavity heat exchanger regarding thermal energy transfer efficiency. Specifically, plate-fins increase TEG output power by approximately 22.92% and enhance thermoelectric conversion efficiency by 38.20%. Similarly, LVGs lead to a 13.02% increase in output power and a 16.83% improvement in conversion efficiency. These advancements are contingent upon specific conditions such as seawater flow rates, fin heights, LVG tilt angles, and locations. The study underscores the importance of optimizing heat exchanger designs in OTEC systems, balancing enhanced heat transfer against the required pump power. Our findings contribute to a broader understanding of materials science in sustainable energy technologies.

Effect of working fluid on the ORC cycle performance of the ocean thermal energy conversion system

Effect of working fluid on the ORC cycle performance of the ocean thermal energy conversion system

An innovative ocean thermal energy conversion system with zeotropic Rankine cycle and direct contact membrane distillation for enhanced efficiency and sustainability

This research introduces an integrated ocean thermal energy conversion water and power cogeneration system (OTEC-WPCS), combining a Zeotropic Rankine Cycle (ZRC) with Direct Contact Membrane Distillation (DCMD) for efficient power and water production. The hydrofluoroolefins (HFOs), chosen for their zero ozone depletion potential (ODP) and extremely low global warming potential (GWP), are used as working fluid components in the ZRC. Additionally, DCMD is implemented for freshwater generation. A mathematical model is established, incorporating both thermodynamic and CFD simulations. The goal is to identify optimal operating conditions that maximize the output of the system. By coupling DCMD and ZRC in series, the system's efficiency reaches 8.92 %, an improvement of 5.71 % compared to a standalone ZRC system. This configuration allows for more efficient use of surface seawater heat compared to an ammonia-based Organic Rankine Cycle (ORC)-DCMD system. The enhancements include an increase in power and water production per unit mass flow of surface seawater by 0.55 kW/(kg·s) and 30 %, respectively. Over 60 % of the system's exergy destruction occurs in the DCMD modules and evaporator, while nearly 80 % of the capital cost is associated with the evaporator, condenser, and DCMD heat exchanger. Enhancing the DCMD module structure and heat exchanger can improve both energy and exergy efficiency, and economic feasibility.

Performance analysis of ocean thermal energy conversion system integrated with waste heat recovery from offshore oil and gas platform

To improve ocean thermal energy conversion (OTEC) system efficiency, four systems utilizing the waste heat recovery from offshore oil and gas platform are proposed, including flue gas boosting OTEC (system I), production water boosting surface seawater OTEC (system II), production water boosting working medium OTEC (system III), and production water boosting vapor OTEC (system IV). The system thermodynamic performance are investigated. The results show that system IV has larger power generation (Wnet), thermal efficiency (ηth) and exergy efficiency (ηex) than others. Compared with single OTEC system, for system IV, Wnet, ηth and ηex are increased by 1569.13 %, 70.35 % and 138.26 %, respectively. For system IV, with the increase of flue gas waste heat quantity from 2000 to 4000 kW, Wnet, ηth and ηex are increased by 562 %, 390 % and 181 % respectively; with the increase of production water waste heat quantity from 0 to 100 kW, Wnet, ηth and ηex are 12.59 %, 5.73 % and 2.86 % respectively. Wnet rises first and then decreases with the increase of evaporation pressure (Peva) or base fluid concentration (xb), presenting an optimal Peva of 1.5 MPa and xb of 0.82 corresponding to the maximum Wnet; Wnet decreases with the increase of condensation temperature.

Performance analysis of a novel small-scale radial turbine with adjustable nozzle for ocean thermal energy conversion

Ocean thermal energy is acknowledged as one of the most promising ocean renewable energy sources in low latitude sea areas. In the ocean thermal energy conversion system, the turbine plays a significant role, and it is responsible for converting the working medium enthalpy into the shaft output power. The present study is focused on the performance analysis of a novel radial inflow turbine with an adjustable nozzle in the OTEC system in order to adapt to the changing operating conditions of the turbine, which vary with the change in seawater temperature. At the design point, the predicted overall isentropic efficiency is 86.5%, and the shaft output power is 15.3 kW, slightly higher than the expected 15 kW. Furthermore, a parametric study is performed, respectively, for the nozzle vane stagger angle and the nozzle-impeller radial clearance to explore the favorable geometric parameters for different conditions. The turbine’s overall efficiency increases slightly with deceasing nozzle-impeller radial clearance, and the variation of the nozzle vane stagger angle is much more influential on the turbine shaft power and overall efficiency. The optimum stagger angle point moves from 32° to 36° gradually with the increase in nozzle-impeller clearance. Finally, the feasibility of an adjustable nozzle for the turbine under off-design conditions was verified by combining the radial clearance and nozzle stagger angle.

Role of R717 blends in ocean thermal energy conversion organic Rankine cycle

Ammonia (R717) is widely deemed to be the potential working fluid of the ocean thermal energy conversion (OTEC) system. To further improve the performance of OTEC Rankine cycle, R717-based binary non-azeotropic blends are employed as the working fluids, where R134a and R125 are selected as the additives. The thermodynamic model as well as the cost estimation model of OTEC system is developed to demonstrate the potential of power generation. The energy, net efficiency, and Levelized cost of electricity are evaluated and examined for OTEC system employing different blends, encompassing R717/R134a and R717/R125. The results indicate that the thermodynamic and economic indexes of R717-based blends are superior to pure fluids (i.e., R134a, R125, and R717). To maximize the net power, the mass fractions of R717 are 0.1 and 0.5 for the R717/R134a and R717/R125 blends, respectively, in a 30 kW-scale OTEC system, where the net power are higher than that of the pure R717 by 134% and 87%. In addition, R717-based blends perform better in energy cost than pure fluids, with 0–67% lower Levelized cost of electricity. Especially, a large-scale OTEC power plant is more economically profitable than a small one.

Reliability assessment of the ocean thermal energy conversion systems through Monte Carlo simulation considering outside temperature variation

AbstractThe ocean thermal energy conversion (OTEC) systems, as renewable energy-based power plants, have the potential to play a significant role in meeting future electricity demands due to the vast expanse of the world's oceans. These systems employ the temperature difference between surface ocean waters and deep ocean waters to drive a thermodynamic cycle and produce electricity. The temperature of deep ocean waters, approximately 1000 m below the surface, is approximately 4 °C, while surface ocean temperatures typically range between 20 and 30 °C. The generated power of OTEC systems is dependent on these temperature differences and may vary with changes in surface ocean temperatures. In this study, the main focus is to find the impact of temperature variation on the failure rates of OTEC system components and the generated power output of these plants. The findings indicate that as the demand for the power system increases, its reliability decreases. In order to improve the reliability of the power system, the integration of a new generation unit, such as the close cycle OTEC power plant under investigation, could be necessary. The findings also indicate the importance of considering temperature variation in the evaluation of the reliability of such types of power plants based on renewable energy.

Ocean thermal energy conversion (OTEC) system driven with solar-wind energy and thermoelectric based on thermo-economic analysis using multi-objective optimization technique

In the present study, we investigated a multiple energy generation system based on the use of solar, wind and ocean thermal energy for the study area with high renewable energy potential. In this study, all existing capacities in coastal areas (wind, sun, ocean temperature difference) are used to design a renewable system. The present system consists of subsystems including organic Rankin cycle (ORC), wind turbine, thermoelectric, heat exchanger and flat panel solar collector. EES thermodynamic software has been used to model the system and obtain thermodynamic results. The system designed for the city of Bushehr, which has good ocean thermal energy, wind energy and solar radiation potential, has been studied. The most important effective and practical parameters in the proposed system are collector area, solar radiation, and wind turbine speed. The results of the investigation of the system in Bushehr City showed that the production power of the system is 4,840,913.8 and 1,138,957.02 kWh compared to the climate changes. Also, the amount of freshwater produced by the system is 103,495.99 and 429,080.51 m3/h compared to the climate changes. Finally, in order to optimize the designed system, the response level multi-objective optimization method has been used to find the best set of objective functions and decision variables. The two objective functions of this optimization included the exergy efficiency of the whole system and the system cost rate. The most optimal value of exergy efficiency was 13.25% and the cost rate was 72.17 $/hour.

Integration of ocean thermal energy conversion and pumped thermal energy storage: system design, off-design and LCOS evaluation

Increasing the penetration of Variable Renewable Energy (VRE) in electric generation systems is a fundamental goal in reducing greenhouse gases emission. To reduce power fluctuations in electricity networks and avoid curtailment, large-scale energy storages represent one of the most promising solutions. Thermally-Integrated Pumped Thermal Energy Storage (TI-PTES) systems are an interesting technology that can be used for this scope if the heat source adopted for thermal integration can provide significant thermal power. The ocean temperature gradient in tropical areas is an attractive heat source to be coupled with the PTES system to realise efficient electric storage when integrated with an Ocean Thermal Energy Conversion (OTEC) system. In this study, a heat pump refrigerated by the warm tropical surface water uses electricity surplus from VRE to heat an amount of water contained in an end-life cargo ship used as water storage. The system discharges the stored energy through an ORC cycle refrigerated by the cold deep seawater when VRE production is low. A preliminary sensitivity analysis of the storage size and temperature is proposed through detailed system modelling to define the optimal design and layout. Therefore, the part-load analysis of the system is assessed to characterise the off-design performances and evaluate the potentialities of this system when applied to a plausible case study that includes VRE generation and an electric demand profile. Finally, the Levelised Cost Of Storage (LCOS) is evaluated and compared to other storage technologies. Results show that the round-trip efficiency may achieve values higher than 60 %, and an equivalent electric battery capacity of 20 MWh is feasible using end-life ships acting as energy storage. In contrast, the obtained LCOS of 388 €/MWh is still not competitive in the energy market. However, since tropical areas have high energy prices, considering this application for remote island electrification could be an interesting solution.

Investigation of Dynamic Behavior of Ultra-Large Cold-Water Pipes for Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) is a process that can produce electricity by utilizing the temperature difference between deep cold water and surface warm water. The cold-water pipe (CWP) is a key component of OTEC systems, which transports deep cold water to the floating platform. The CWP is subjected to various environmental and operational loads, such as waves, currents, internal flow, and platform motion, which can affect its dynamic response and stability. In this paper, we establish a computational model of the mechanical performance of the CWP based on the Euler–Bernoulli beam theory and the Morrison equation, considering the effects of internal flow, sea current, and wave excitation. We use the differential quadrature method (DQM) to obtain a semi-analytical solution of the lateral displacement and bending moment of the CWP. We verify the correctness and validity of our model by comparing it with the finite element simulation results using OrcaFlex software. We also analyze the effects of operating conditions—such as wave intensity, clump weight at the bottom, and internal flow velocity—on the dynamic response of the CWP using numerical simulation and the orthogonal experimental method. The results show that changing the wave strength and internal flow velocity has little effect on the lateral displacement of the CWP but increasing the current velocity can significantly increase the lateral displacement of the CWP, which can lead to instability. The effects of waves, clump weight, internal flow, and sea current on the maximum bending moment of the CWP are similar; all of them increase sharply at first and then decrease gradually until they level off. The differences in the effects are mainly reflected in the different locations of the pipe sections. This paper suggests some design guidance for CWP in terms of dynamic responses depending on the operating conditions. This paper contributes to the journal’s scope by providing a novel and efficient method for analyzing the mechanical performance of CWP for OTEC systems, which is an important ocean energy resource.

Study on the Vibration Characteristics of Marine Riser Based on Flume Experiment and Numerical Simulation

This study investigated vortex-induced vibrations of marine risers in Ocean Thermal Energy Conversion systems. Flume experiments were conducted under two conditions: Condition 1 was with a fixed riser bottom, and Condition 2 was with a fixed bottom on a mooring platform. The cross-flow acceleration of the riser was measured at different current velocities, and corresponding vibration responses were analyzed. Numerical simulations based on the flume experiments were employed to validate the reliability of the simulation method. Results from the flume experiments revealed vortex-induced resonance in Condition 1 when the flow velocity approached the riser’s natural frequency. In Condition 2, similar vibration responses were observed, with maximum acceleration occurring during flow velocity-induced vortex-induced vibrations. However, at higher flow velocities, the acceleration response showed a decrease followed by an increase, indicating the excitation of higher-order modes. The numerical simulations matched the flume experiments in Condition 1. In Condition 2, while the acceleration response and frequency agreed during vortex-induced resonance, discrepancies arose in the flow velocity that caused vortex-induced vibrations compared to the flume experiments. This study demonstrated the accuracy of numerical simulations in reflecting vortex-induced vibrations of risers, providing a foundation for further research on complex riser systems.

OTEC PERFORMANCE EVALUATION USING DIFFERENT WORKING FLUIDS AND VARIATIONS IN OPERATING ORC CONDITIONS

Ocean Thermal Energy Conversion (OTEC) is a system that produce energy of electricity by exploiting the temperature difference between surface part of seawater and deep region of seawater. The thermal energy at the surface seawater is used to convert working fluid into vapor which expands in the turbine to generate electricity and revert into liquid state by rejecting heat to cold seawater pumped from deep region of seawater. Basically, there are various types of working fluids that can be applied in an OTEC system depending on its contribution towards the performance of the system. In this study, the thermodynamics efficiency and net power output of basic OTEC system will be evaluated by varying three parameters which are the turbine inlet temperature, turbine inlet pressure and condenser outlet temperature using various working fluids through simulation using MATLAB. Simulation results indicate that R717 excels in producing the highest net power output but R600a has the best average value in thermodynamic efficiency. The findings of this work can contribute in giving an insight on which working fluid has the optimal characteristics in obtaining the best performance of the OTEC system.

THERMODYNAMICS ANALYSIS OF SOLAR-EJECTOR PUMP OTEC (SEP-OTEC) RANKINE CYCLE USING AMMONIA AS REFRIGERANT

Organic Rankine Cycle (ORC) applications include ocean thermal energy conversion (OTEC), in which mechanical work is generated from heat energy to rotate generators and generate electricity. The OTEC system heated and cooled its refrigerant by taking advantage of the relatively small temperature difference between the warmer surface seawater and the colder deep seawater. The low-temperature difference between seawater and the rest of the system meant that the thermal efficiency of the system was relatively low; to address this problem, the OTEC cycles needed to be revised. To increase the basic OTEC cycle's thermal efficiency by 3.3–4.0%, various modifications have been developed. Two such cycles are the Solar Boosted OTEC (SOTEC) cycle and the Ejector Pump cycle (EP-OTEC). While the two improvements alter the rotating turbine parameters in different ways, they can be combined to create an improved OTEC cycle through the use of thermodynamics. In this study, an algorithm for revised OTEC was developed using MATLAB, and the performance of the system after the modifications was further quantified. This SEP-OTEC cycle thermal efficiency gives a 1.2-fold improvement when compared to the previous OTEC cycle thermal efficiency, which was 3.1%.

Thermodynamic process control of ocean thermal energy conversion

The small temperature difference in ocean thermal energy conversion (OTEC) causes the high sensitivity of thermodynamic process to perturbations such as temperature and power load, which requires an efficient control strategy of thermodynamic process for OTEC. To this aim, an integral dynamic model is established to study the dynamic response of thermodynamic process in an OTEC system. To ensure the system efficiency and operating safety of power cycle, a multi-objective active-disturbance rejection control (ADRC) strategy is proposed and compared with corresponding single-objective ADRC and PI controller. Multi-objective ADRC includes a load-following control loop and two auxiliary control loops (evaporator control loop and condenser control loop). The results indicate that the multi-objective ADRC is most efficient to track the change of power load, in which the overshoot and integration absolute error (IAE) of load-following are reduced by 50% and 16% when compared with the single-objective ADRC. In addition, the multi-objective control strategy can maintain the stability of superheat degree and condensation pressure, and hence improves the system efficiency by 8.64% for the output power of 35 kW. Especially, the feedback-feedforward controller and feedback controller are attractive candidates to maintain the stable superheat degree and condensation pressure.