Performance study of an OTEC system

This paper describes a series of studies carried out to analyse the performance of an Ocean Thermal Energy Conversion (OTEC) system. The objective function, $\text{A}\text{W}{}_{\mathrm{net}}$, where A is the total heat exchanger area and W_net is the net work output of the system, was used for the parametric and optimisation studies. By using $\text{A}\text{W}{}_{\mathrm{net}}$ the heat exchangers were directly related to the remaining OTEC components. Since changes in one component of the system invariably affect the rest of the system, it was thus possible to evaluate the combined effects on the OTEC power plant.The effects of the following parameters on system performance were investigated: ocean fluid velocity through the exchanger, log-mean temperature differences of the heat exchangers, heat transfer enhancement and cold seawater pipe diameter. It was concluded that for a 1 MW_e OTEC power plant, the net output of the plant becomes zero when ΔT (the temperature difference between the hot and cold ocean streams) approaches 12·80°C. The power cycle used in this study was a simple closed Rankine cycle with ammonia as the working fluid.     

Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources

A novel cogeneration system was proposed and techno-economically investigated, consisting of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger subsystem and a heat pump subsystem. The main purpose is to identify suitable working fluids (among 27 fluids with boiling point temperature ranging from −47.69 to 47.59 °C) and optimized cycle parameters for the ORC-based power generation subsystem. The screening criteria include net power output per unit mass flow rate of hot source (P_net), the ratio of total heat transfer area to net power output (A/W_net), and electricity production cost (epc). Results show that there exists optimum evaporating temperatures maximizing the P_net value and minimizing the A/P_net and epc values. The optimum temperatures vary with different screening criteria and fluids. Optimized fluids based on each screening criteria are not the same. E170, R600 and R141b show the lowest A/W_net and epc values with averagely 3.78% lower P_net value than R236ea which presents the largest P_net value.     

Characteristic of local boiling heat transfer of ammonia and ammonia / water binary mixture on the plate type evaporator

Power generation using small temperature difference such as ocean thermal energy conversion (OTEC) and discharged thermal energy conversion (DTEC) is expected to be the countermeasures against global warming problem. As ammonia and ammonia/water are used in evaporators for OTEC and DTEC as working fluids, the research of their local boiling heat transfer is important for improvement of the power generation efficiency. Measurements of local boiling heat transfer coefficients were performed for ammonia /water mixture (z = 0.9−1) on a vertical flat plate heat exchanger in a range of mass flux (7.5–15 kg/m² s), heat flux (15–23 kW/m²), and pressure (0.7–0.9 MPa). The result shows that in the case of ammonia /water mixture, the local heat transfer coefficients increase with an increase of mass flux and composition of ammonia, and decrease with an increase of heat flux.     

Maximum-power-point tracking control of solar heating system

The present study developed a maximum-power point tracking control (MPPT) technology for solar heating system to minimize the pumping power consumption at an optimal heat collection. The net solar energy gain Q_net (=Q_s ? W_p/η_e) was experimentally found to be the cost function for MPPT with maximum point. The feedback tracking control system was developed to track the optimal Q_net (denoted Q_max). A tracking filter which was derived from the thermal analytical model of the solar heating system was used to determine the instantaneous tracking target Q_max(t). The system transfer-function model of solar heating system was also derived experimentally using a step response test and used in the design of tracking feedback control system. The PI controller was designed for a tracking target Q_max(t) with a quadratic time function. The MPPT control system was implemented using a microprocessor-based controller and the test results show good tracking performance with small tracking errors. It is seen that the average mass flow rate for the specific test periods in five different days is between 18.1 and 22.9 kg/min with average pumping power between 77 and 140 W, which is greatly reduced as compared to the standard flow rate at 31 kg/min and pumping power 450 W which is based on the flow rate 0.02 kg/s m² defined in the ANSI/ASHRAE 93-1986 Standard and the total collector area 25.9 m². The average net solar heat collected Q_net is between 8.62 and 14.1 kW depending on weather condition. The MPPT control of solar heating system has been verified to be able to minimize the pumping energy consumption with optimal solar heat collection.     

An economic and environmental assessment of transporting bulk energy from a grazing ocean thermal energy conversion facility

An ocean thermal energy conversion (OTEC) facility produces electrical power without generating carbon dioxide (CO₂) by using the temperature differential between the reservoir of cold water at greater depths and the shallow mixed layer on the ocean surface. As some of the best sites are located far from shore, one option is to ship a high-energy carrier by tanker from these open-ocean or “grazing” OTEC platforms. We evaluate the economics and environmental attributes of producing and transporting energy using ammonia (NH₃), liquid hydrogen (LH₂) and methanol (CH₃OH). For each carrier, we develop transportation pathways that include onboard production, transport via tanker, onshore conversion and delivery to market. We then calculate the difference between the market price and the variable cost for generating the product using the OTEC platform without and with a price on CO₂ emissions. Finally, we compare the difference in prices to the capital cost of the OTEC platform and onboard synthesis equipment. For all pathways, the variable cost is lower than the market price, although this difference is insufficient to recover the entire capital costs for a first of a kind OTEC platform. With an onboard synthesis efficiency of 75%, we recover 5%, 25% and 45% of the capital and fixed costs for LH₂, CH₃OH and NH₃, respectively. Improving the capital costs of the OTEC platform by up to 25% and adding present estimates for the damages from CO₂ do not alter these conclusions. The near-term potential for the grazing OTEC platform is limited in existing markets. In the longer term, lower capital costs combined with improvements in onboard synthesis costs and efficiency as well as increases in CO₂ damages may allow the products from OTEC platforms to enter into markets.     

基于海底黑烟囱的海洋温差发电热力循环系统研究

与传统的海洋温差发电系统不同,海底黑烟囱海洋温差发电系统是以海洋地热为热源,以深海冷水为冷源的发电系统。文章分别分析和计算了以水蒸气为工质的开式系统和以纯氨为工质的闭式系统的循环热效率、换热器负荷、泵耗以及循环净功等相关参数。结果表明,与以纯氨为工质的闭式系统相比,开式系统的热水泵功耗过大,降低高温海水的温度和提高闪蒸压力对开式系统是不利的;以水蒸气为动力循环工质有利于降低换热器的负荷,这对换热器的设计是十分有利的。     

Design of an ocean thermal energy plant ship to produce ammonia via hydrogen

The 43°F (24°C) temperature difference that exists between surface water and deep water at selected sites in tropical oceans can be used to drive a heat engine to produce electric power, electrolyze water, and produce ammonia from the resulting hydrogen plus nitrogen from the air. A baseline design has been developed for a 100-MWe Ocean Thermal Energy Conversion (OTEC) plant-ship that would produce 313 tons per day of ammonia. The cost estimates for this design have been extrapolated to 500-MWe plant-ships to produce ammonia (for fertilizers and chemicals) or liquid hydrogen for shipment to the U.S. It is judged that ammonia will be producible at competitive cost ($96/short ton in 1975 dollars) by the sixth and subsequent plant-ships in the mid-1980s. This production by OTEC/ammonia plants would conserve supplies of natural gas or other fossil fuels now used to produce ammonia on shore. For the longer term (1990s), liquid hydrogen from OTEC plants should become competitive as demands for this clean fuel and efficient ways for employing it in larger markets (fuel cells, transportation, etc.) come to maturity.     

Heat transfer characteristics of a slot jet impinging on a semi-circular convex surface

Surface heat transfer characteristics of a heated slot jet impinging on a semi-circular convex surface have been investigated by using the transient heating liquid crystal technique. Free jet velocity, turbulence and temperature characteristics have been determined by using a combination of an X-wire and a cold wire anemometry. The parametric effects of jet Reynolds number (Re_W) ranging from 5600 to 13,200 and the dimensionless slot nozzle-to-impingement surface distance (Y/W) ranges from 2 to 10 on the local circumferential heat transfer have been studied. Local circumferential Nusselt number (Nu_S) decreases with increasing the dimensionless circumferential distance (S/W) from its maximum value at the stagnation point up to S/W=3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3?S/W?4.2 which coincided with a secondary peak in heat transfer. Correlations of local and average Nusselt numbers with Re_W, Y/W and S/W have been established for the stagnation point and the circumferential distribution. The rate of decay of average circumferential Nusselt numbers around the semi-circular convex surface is much faster than that which occurs laterally along the flat surface. As Y/W increases, the effect of surface curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced.     

An ocean thermal energy conversion based system for district cooling,ammonia and power production

In this study, a novel Ocean Thermal Energy Conversion (OTEC) based tri-generation system that produces ammonia, cooling and power is developed and analysed. This OTEC plant operates on the naturally existing temperature difference that exists in various depths of the ocean. The OTEC plant used in this study is operated using a single-stage ammonia Rankine cycle. The discharge seawater from the condenser in the organic Rankine cycle is used to provide district cooling. Two different operation cases of the analysed system are considered, where for the first case 50% of the power produced is stored in the form of ammonia during the off-peak hours. The second case is for complete power production proposed for peak hours. For the case where 50% of the power produced (case 1) is used to produce ammonia the highest energy and exergy efficiency is found to be 1.37% and 56.17% respectively. As for the case where, only power is produced (case 2) the maximum energy and exergy efficiency of the OTEC plant is found to be 1.83% and 78.02% respectively. The corresponding maximum power production was 6612 kW and 13,224 kW for cases 1 and 2. The maximum hydrogen and ammonia production rate is found to be 94.35 kg/h and 534.7 kg/h at peak efficiency values. The cooling duty at the peak energy and exergy efficiency is found to be 64.4 MW where the condenser temperature is 11.38 °C.     

Micro-/nanoscaled irreversible Otto engine cycle with friction loss and boundary effects and its performance characteristics

An irreversible cycle model of the micro-/nanoscaled Otto engine cycle with internal friction loss is established. The general expressions of the work output and efficiency of the cycle are calculated based on the finite system thermodynamic theory, in which the quantum boundary effect of gas particles as working substance and the mechanical Casimir effect of gas system are considered. It is found that, for a micro-/nanoscaled Otto cycle devices, the work output W and efficiency η of the cycle can be expressed as the functions of the temperature ratio τ of the two heat reservoirs, the volume ratio r_V and the surface area ratio r_A of the two isochoric processes, the dimensionless thermal wavelength λ and other parameters of cycle, while for a macroscaled Otto cycle devices, the work output W₀ and efficiency η₀ of the cycle are independent of the surface area ratio r_A and the dimensionless thermal wavelength λ. Further, the influence of boundary of cycle on the performance characteristics of the micro-/nanoscaled Otto cycle are analyzed in detail by introducing the output ratio W/W₀ and efficiency ratio η/η₀. The results present the general performance characteristics of a micro-/nanoscaled Otto cycle and may serve as the basis for the design of a realistic Otto cycle device in micro-/nanoscale.     

Analysis of optimization in an OTEC plant using organic Rankine cycle

This study quantified the effects of evaporation temperature, condensation temperature, and the inlet- and outlet-temperature differences of deep cold seawater and warm seawater on the performance of an ocean thermal energy conversion (OTEC) plant using an organic Rankine cycle (ORC), and also investigated the optimal operations required for the performance. A finite-temperature-difference heat transfer method is developed to evaluate the objective parameter, which is the ratio of net power output to the total heat transfer area of heat exchanger in the system, and R717, R600a, R245fa, R152a, and R134a were used as the working fluids. The optimal evaporation and condensation temperatures were obtained under various conditions for maximal objective parameters in an OTEC system.The results show that R717 performed optimally in objective parameter evaluation among the five working fluids, and that R600a performed better than other fluids in thermal efficiency analysis. The optimal seawater temperature differences between the inlet and outlet of the evaporator and condenser are proposed. Furthermore, the influences of inlet temperatures of warm and cold seawater in the ORC are presented for an OTEC plant. The simulation results should enable the performance of an ORC system to be compared when using various organic working fluids.     

A chart for predicting the possible advantage of adopting a suction/liquid heat exchanger in refrigerating system

This paper presents the effects produced by a suction/liquid heat exchanger installed in a refrigerating cycle, evidencing that, its use can improve or decrease the system performance depending on the operating conditions. Attention is focused on developing an easy operating method in order to predict the behaviour of the system introducing the heat exchanger, changing the operating conditions and/or the refrigerant fluids. To this aim, 19 different ozone friendly fluids (R-22, R-32, R-152a, R-125, R-134a, R-236a, R-227a, RC-318, R-410A, R-413A, R-407C, R-417, R-502, R-507A, R-717, R-290, R-600, R-600a and R-1270) have been considered, varying evaporating and condensation temperatures, respectively in the range −40 °C/10 °C and 25 °C/50 °C. The advisability of the installation of the heat exchanger can be evaluated as a function of thermodynamic properties. Furthermore, a simple chart allowing to verify the effectiveness of installation of heat exchanger has been developed for each refrigerating fluids and for the specified operating conditions.     

On the selection of fin profiles for OTEC plate-fin evaporators

This paper considers the plate-fin heat exchanger for an OTEC power plant on account of its compactness. Ammonia is selected as the working fluid for a plant of 1 MW gross output. Choice of seawater side velocity is made on the basis of pressure drop and biofouling. Fourteen different fin profiles are considered in order to choose a suitable configuration. The selection of the plate-fin heat exchanger depends on minimum exchanger volume, minimum pumping power requirement and low ammonia side pressure drop.     

Ocean thermal energy conversion (OTEC): Social and environmental issues

OTEC converts the solar energy, collected and stored in tropical waters, into electricity. The electricity may be either cabled to shore or used in situ for the manufacture of energyintensive products. Two countries, U.S.A. and Japan, are seriously pursuing OTEC. The development programs in both countries are similar. Presently, the emphasis is on the closed Rankine cycle with ammonia as the working fluid. The power plants are to be housed on floating platforms. If the electricity is to be cabled to shore, the platforms will be moored to the ocean floor. If the plants are to produce chemical products, they will graze from one location to another on the open sea to capture the largest available thermal resource.Technical feasibility of OTEC appears certain. In the near term, OTEC can be economical for U.S. islands, which depend on imported oil for power generation. OTEC can enter the U.S. mainland market in the Southeast if projected capital cost for large plants is realized and high voltage, underwater d.c. transmission is developed beyond current state of the art. The islands market amounts to 8 GW and the U.S. market is estimated to be much larger. Penetration of the island market can begin in the early 1990s and of the mainland market after the year 2000.A potential impediment to OTEC's accelerated deployment is capital. Although there are numerous important environmental and institutional questions, they are secondary to the economic and cost issues.This paper addresses the economic, social and environmental issues pertinent to the commercialization of OTEC. The a priori assumptions are that technical problems can be solved and that in certain locations. OTEC can be competitive with conventional base-load power systems.     

Equilibrium of NH4SCNNH3 system for thermal energy storage

A solid-gas reaction of ammonium thiocyanate (NH₄SCN) and ammonia produces liquid ammoniate (NH₄SCN·nNH₃). The region of the liquid phase and the equilibrium properties of the ammoniate have been determined. Crystalization of the ammoniate was not observed, though the liquid was cooled to − 10°C. The enthalpy changes in the liquid phase were also estimated. Accordingly, this system has a wide range of liquid phase and offers a medium for thermal energy storage or a chemical heat pump system.     

Non-linear analyses of temperature oscillations in a closed-loop pulsating heat pipe

In this paper, the chaotic behavior of wall temperature oscillations in a closed-loop pulsating heat pipe was investigated using non-linear analyses on temperature data. The tested heat pipe, consisting of 5 turns, was made of copper capillary tube and had an internal diameter of 2 mm. Ethanol was selected as the working fluid with filling ratios (FR) of 30%, 50% and 70%. Wall temperature fluctuations were recorded under three different heating power inputs of 37, 60, and 87 W. Various methods, including pseudo-phase-plane trajectories, correlation dimensions (D_E), Lyapunov exponents, and recurrence plots, were used to analyze the non-linear dynamics characteristics of temperature oscillation data. Three types of attractors were identified under different power inputs. All of the calculated positive largest Lyapunov exponents were found to be less than 0.1, demonstrating the weak chaos characteristics of the pulsating heat pipe. The increase of the power input augments the correlation dimensions and contributes to the improvement of the thermal performance of the pulsating heat pipe. For each power input, the correlation dimensions have the trend of D_E,FR=50% > D_E,FR=70% > D_E,FR=30%, and the best thermal performance was obtained at 50% filling ratio. At least four independent variables are required in order to describe the heat transfer characteristics of a PHP. The average time of the temperature oscillation stability loss, i.e., the inverse of the largest Lyapunov exponent, decreases as the power input increases. In the recurrence plots, chaotic states were observed. The Recurrence Quantification Analysis indicates larger values of the order-2 Renyi entropies K₂ at the evaporation section than at the condensation section. Moreover, the trend that K_2,Q=87W > K_2,Q=60W > K_2,Q=37W at each filling ratio both for T_e4 and T_c4 collaborating with the positive, finite largest Lyapunov exponent gives a hint of the maximum entropy self-organization process of the temperature oscillations with the increase of power input.     

Finite time thermodynamics study and exergetic analysis of ammonia–water absorption systems

This paper presents an optimization study of a single stage absorption machine operating with an ammonia–water mixture under steady state conditions. The power in the evaporator, the temperatures of the external fluids entering the four external heat exchangers as well as the effectiveness of these heat exchangers and the efficiency of the pump are assumed fixed. The results include the minimum value of the total thermal conductance UA_tot as well as the corresponding mean internal temperatures, overall irreversibility and exergetic efficiency for a range of values of the coefficient of performance (COP). They show the existence of three optimum values of the COP: the first minimises UA_tot, the second minimises the overall irreversibility and the third maximises the exergetic efficiency. They also show that these three COP values are lower than the maximum COP which corresponds to the convergence of the internal and external temperatures towards a common value. The influence of various parameters on the minimum thermal conductance of the heat exchangers and on the corresponding exergy efficiency has also been evaluated. From an exergetic viewpoint it is interesting to reduce the temperature at the desorber and at the evaporator and to raise the values of that parameter at the condenser and the absorber. However these changes must be accompanied by an important increase in the total UA if it is desired to conserve a constant COP. The internal heat exchangers between the working fluid and the solution improve both the overall exergy efficiency and the coefficient of performance of the absorption apparatus.     

Open-circuit analysis of thin film heterojunction solar cells

A new technique is proposed for the calculation of the width W of space charge region and consequently the concentration of uncompensated acceptors N_A–N_D, which is based on the open-circuit analysis of thin film heterojunction solar cells illuminated by monochromatic light with the wavelength within photosensitivity region. The proposed method was simulated under different values of the theoretically considered parameters (ideality coefficient n, saturation current I₀ and shunt resistance R_sh) of a CdS/CdTe heterojunction solar cell. The calculated values of W and N_A–N_D were established to be dependent on the mentioned above electrical parameters.     

Influences of nozzle-plate spacing on boiling heat transfer of confined planar dielectric liquid impinging jet

We have investigated the single-phase and boiling heat transfer of dielectric liquid under the Reynolds numbers (2000, 3000 and 5000) and under nozzle-plate spacing (H/W; 0.5, 1.0 and 4.0) in a submerged impinging jet system. The boiling incipience increases in proportion to the Reynolds number and in inverse proportion to the nozzle-to-surface spacing. The critical heat flux at H/W = 1.0 is lower than those of outer spacings, such as H/W = 0.5 and 4.0, due to the characteristics of the jet impingement heat transfer distribution. We suggest a correlation equation of nozzle-plate spacing (H/W) having the lowest CHF for various jet velocities.