Environmental isotope investigations of New Zealand geothermal systems—A review

Environmental isotope investigations of geothermal systems at the New Zealand Institute of Nuclear Sciences have concentrated in recent years on combining several isotopes with chemical analyses. Geothermal hydrology has been studied by the use of hydrogen and oxygen isotopes with chloride and other water analyses. Rocks and minerals in well cores have added information to the geology and chemistry through the use of oxygen isotope techniques. Oxygen, hydrogen, carbon and sulphur isotope geothermometry have been combined in an attempt to derive temperature profiles with depth. ²²²Rn measurements in soil indicate leakage through faults, while ²²²Rn measurement of well discharges show evidence of underground processes. Future work will include noble gas isotope measurements. Measurements of ¹H and ¹⁴C are reported in an associated paper on tracing of underground water movements.     

Chemical equilibria in icelandic geothermal systems—Implications for chemical geothermometry investigations

Chemical geothermometry represents the most important tool for estimating reservoir temperatures in the exploration of geothermal resources. Chemical equilibria between alteration minerals and solution are generally attained in geothermal systems for all major components except chloride. For the interpretation of analyses of natural waters involving geothermometry major emphasis should be placed on assessing the overall water composition with respect to mineral equilibria, rather than attempting to distinguish geothermal waters from shallow waters by a classification involving the relative abundance of major anions and major cations. Generally, cold waters may be distinguished from geothermal waters by low chloride (< 10 ppm), in conjunction with relatively low pH (6–7) and low Na/K ratios (same as the associated rock), calcite undersaturation and low √Ca²⁺ H⁺ activity ratios.     

Simulation of solar heating systems—an overview

Process simulation has become an accepted tool for the performance, design, and optimization of thermal processes. Solving the mathematical models representing solar heating process units and systems is one of the most tedious and repetitive problems. Nested iterative procedures are usually needed to solve these models. To tackle these problems, several researchers have developed different methods, techniques, and computer programs for the simulation of very wide verity of solar heating process units and systems.It is of interest in this work to characterize and classify these methods, techniques, and programs in order to better understand their relations, types, structures, and procedures.The simulation problems are outlined; the simulation programs are grouped into two main types; special purpose, and general-purpose programs. Sequential and simultaneous computational sequences are illustrated. Simulator structure, program evaluation, and numerical techniques are summarized.By considering the unit and/or system entropy generation as well as the energy and material balances equations, more realistic models can be obtained. Also, rapid development of computer hardware and software will suggest new techniques and programs to be considered. These progress directions are noted.     

微型燃气轮机燃烧室热力性能的实验研究

对一微型燃气轮机燃烧室在不同负荷下的热力性能进行了实验研究,得到了该燃烧室在不同负荷下燃烧效率,出口温度分布,总压保持系数及NOx排放浓度等燃烧室性能指标参数的变化情况.结果表明,该燃烧室在各个实验负荷工况下均具有优异的性能,同时随着负荷的增加,压力损失增大,NOx排放浓度降低.     

Theoretical and experimental investigations on thermal management of a PEMFC stack

In recent years micro-cogeneration systems (μ-CHPs), based on fuel cells technology, have received increasing attention because, by providing both useful electricity and heat with high efficiency, even at partial loads, they can have a strategic role in reduction of greenhouse gas emissions. For residential applications, the proton exchange membrane fuel cell (PEMFC), is considered the most promising, since it offers many advantages such as high power density, low operating temperature, and fast start-up and shutdown.In this paper the electrical and thermal behaviors of a PEMFC stack, suitable for μ-CHP applications, have been investigated through experimental and numerical activity.The experimental activity has been carried out in a test station in which several measurement instruments and controlling devices are installed to define the behavior of a water-cooled PEMFC stack. The test station is equipped by a National Instruments Compact DAQ real-time data acquisition and control system running Labview™ software.The numerical activity has been conducted by using a model, properly developed by the authors, based on both electrochemical and thermal analysis.The experimental data have been used to validate the numerical model, which can support and address the experimental activity and can allow to forecast the behavior and the performance of the stack when it is a component in a more complex energy conversion system.     

A parametric study of closed loop solar heating systems—II

A nondimensional equation describing the closed loop solar heating system with either parallel or series auxiliary heaters is derived. Using European weather data the monthly solar fraction is calculated for variations in the major non-dimensional groups. A correlation is given relating the solar fraction to three non-dimensional parameters M, Kc and Rc. M is the ratio of energy available on the collector aperture to energy demand. To calculate the energy available it is necessary to know the monthly utilizability. Kc is the ratio of the store temperature required for a 100 per cent solar contribution to the average monthly collector peak stagnation temperature. Both these temperatures are referenced to the demand temperature. Rc is the effective “turn over time” of the store, i.e. the number of days to empty the energy contents of the store. The magnitudes of Kc and Rc depend on the load heat exchanger size. Comparisons between the solar fraction predicted with a dimensional hour by hour computer model and that by the correlation are made for two system types. The agreement is good and it is concluded that the correlation can be used as a reliable method to optimise closed loop solar heating systems.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

Impact of wind power in autonomous power systems—power fluctuations—modelling and control issues

This paper describes a detailed modelling approach to study the impact of wind power fluctuations on the frequency control in a non‐interconnected system with large‐scale wind power. The approach includes models for wind speed fluctuations, wind farm technologies, conventional generation technologies, power system protection and load. Analytical models for wind farms with three different wind turbine technologies, namely Doubly Fed Induction Generator, Permanent Magnet Synchronous Generator and Active Stall Induction Generator‐based wind turbines, are included. Likewise, analytical models for diesel and steam generation plants are applied. The power grid, including speed governors, automatic voltage regulators, protection system and loads is modelled in the same platform. Results for different load and wind profile cases are being presented for the case study of the island Rhodes, in Greece. The scenarios studied correspond to reference year of study 2012. The effect of wind fluctuations in the system frequency is studied for the different load cases, and comments on the penetration limits are being made based on the results. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental investigations on start-up performance of static nuclear reactor thermal prototype

Nuclear power is most suited to satisfy the energy demands of future deep space exploration. In this paper, we propose a static nuclear reactor (the nuclear static thermoelectric reactor [NUSTER]), which offers the advantages of superior modularization, simplification, a fully static state, and passive operation. Based on the conceptual design of a static nuclear reactor, an electrical heating principle prototype was designed and fabricated to validate the feasibility of the fully static passive energy conversion concept. Skutterudite thermoelectric generators (TEGs) were used for static energy conversion, and potassium heat pipes were employed for passive heat transfer. The system start-up performance, restart performance, and thermoelectric performance were investigated using the thermal principle prototype. We proposed a new approach to analyze the heat pipe start-up process based on the heat transfer performance. The experimental results indicated that the restart process can be used to reduce the start-up time, because the low heat flux stage is avoided. During the start-up process, the TEGs hot side heat flux and temperature difference were gradually established, and the TEGs open circuit voltage and power density gradually increased. A maximum open circuit voltage and power density of 38.2 V and 0.92 W/cm², respectively, were achieved when the TEGs temperature difference reached 575°C. The high performance of the thermal principle prototype demonstrated the feasibility of the NUSTER conceptual design, and the experimental data can serve as a valuable reference for optimization of static reactor designs.     

Liquid phase thermochemical energy conversion systems—An application of Diels-Alder chemistry

Thermochemical energy conversion at moderate or low temperature (> about 400°C) employing liquid phase components throughout a cycle is suggested as a promising concept for high-efficiency conversion of various energy sources (e.g. solar or industrial waste heat) to a convenient chemical form. In particular, we propose liquid phase Diels-Alder cycloaddition chemistry as an important class of reversible reactions for such low or moderate temperature thermochemical energy conversion systems. One of the important attributes of thermally driven Diels-Alder reactions is their concerted mechanism, with consequent high yields and efficiencies relative to liquid photochemical systems. Since the systems we propose involve organic species, with thermal stability concerns above about 400°C, it is important to demonstrate equilibrium shift capability for the highly energetic reactions sought. We have therefore carried out experimental studies with model liquid Diels-Alder systems that clearly demonstrate the degree of control over equilibrium available through substituent entropy effects. For example, K_eq is unity at about 420°C (T*) for the anthracene/maleic anhydride system (in solvent) while a phenyl substituent on the anthracene or isopropyl substituent on the anhydride reduce T* to about 200°C at constant ΔH⁰. These results are of importance as regards subsequent systematic identification of Diels-Alder reactions having ideal thermochemical and physical properties. We also have developed a rapid NMR technique for qualitative screening of candidate reactions, and have applied this technique to the study of various bicyclic diene/fumaric acid ester systems. Our paper further points to the need for better understanding of the catalysis likely required for these liquid phase Diels-Alder reactions.     

The influence of the composition of Si–Ge mixed crystals on thermal diffusivity—photoacoustic approach

The paper presents the results of the photoacoustic experiments performed on a series of Si_1−xGe_x samples for x in the range 0–8%. The influence of the alloy composition on the thermal diffusivity and the correlation between the thermal diffusivity and thermal conductivity are presented and discussed.     

Applications of photovoltaic systems—An economic appraisal with reference to india

A simple methodology for analyzing the economic viability of photovoltaic systems is proposed. The methodology is applied to the different near-term applications of photovoltaic systems both in the commercial and the rural sector with reference to India conditions. It is concluded that most of the photovoltaic systems are economically viable provided some long-term financial policy is envisaged and a few technical innovations in the balance of systems are made.     

Theoretical considerations in the use of small passive-solar test-boxes to model the thermal performance of passively solar-heated building designs

Theoretical considerations are presented for the thermal modelling of passively solar-heated building designs with passive-solar text boxes. Multi-room passive solar buildings, passive solar buildings having realistically massive walls, thermocirculating passive-solar designs, air-infiltration effects, edge effect corrections, and microclimate shading effects are discussed.     

Diffuser design influence on the performance of solar thermal storage tanks

The effect of the inlet and outlet diffuser design on the performance of thermal stratification in a vertical water tank is investigated experimentally. Two sets of diffusers are used in the experiments, which are conducted with a moving thermocline (both up and down) for different flow rates. The results indicate that the preservation of the initial thermocline is excellent when using a settling mesh. It is also shown that the extraction efficiency of the tank is higher at low flow rates during charging, whereas it is lower at low flow rates during discharging.     

Optimisation of merged district-heating systems—benefits of co-operation in the light of externality costs

Studies have shown that separate actors can benefit from co-operation around heat supply. Such co-operation, for example, might be between an industry selling waste heat to a district-heating system or two district-heating systems interconnecting their respective systems. Co-operation could also be expected to reduce the environmental impacts of the energy systems by choosing the plants with the lowest emissions. It is widely accepted that the production of heat and electricity causes damage to the environment. This damage often imposes a cost on society, but not on company responsible. In general, using a broader system perspective when analysing local energy systems results in a lower total cost, more efficient use of plants and a greater potential for producing electricity in combined heat-and-power (CHP) plants. Internalising the externality costs in the energy system model facilitates the study of what co-operation can mean for reducing emissions. This study shows that co-operation between the two systems is on the whole cost-effective, but the benefits are greater when external costs are not included in the calculation. Considering externality costs in combination with current electricity prices would lead to a higher system cost, but the quantity of emission gases will be lower. If, on the other hand, the calculation is made taking externality costs and corresponding adjusted electricity prices (the adjustment being necessary to compensate for the additional cost due to externality costs) into consideration, the quantities of emission gases will rise because more heat-and-power will be generated by one of the CHP plants.     

Theoretical thermal performance analysis of two solar‐assisted heat‐pump systems

The thermal performance of two different schemes of solar‐assisted heat‐pump systems has been theoretically studied. In first scheme, the evaporator of the heat pump is taken directly as the solar collecting plate and always maintained at the ambient temperature. As there is no heat loss from the collecting plate, the thermal efficiency of the collector is high and equals the solar absorptivity of the collecting plate. As suggested, the heat‐pump evaporator of the second scheme is placed in a novel fresh water solar pond/tank with high efficiency. Since the evaporator operates at a relatively high temperature, the COP of the heat pump can be increased. The calculated results show that the COP of a solar‐assisted heat pump using the second scheme is considerably higher than that of the first scheme. Copyright © 1999 John Wiley & Sons, Ltd.     

Ocean alternative energy: The view from China—‘small is beautiful’

The potential harnessing of tidal power has moved, spurred on by the recurring oil crises, from the back- to the front-burner, again. Concerns over global warming seem to point to an absolute and urgent necessity to limit the burning of fossil fuels. China has huge reserves of coal and they are used on a very large scale to provide heating and energy. The replacement of that source of power is not to be expected in the near future, but China has been looking for several decades at alternative sources. The ocean is one of them. The paper does not aim at comprehensiveness but attempts to provide a review of China's efforts in that domain based on several sources and trips to China and Japan.     

Thermoeconomic analysis method for optimization of combined heat and power systems—part II

In this paper, a thermoeconomic functional analysis method based on the Second Law of Thermodynamics and applied to analyze four cogeneration systems is presented. The objective of the developed technique is to minimize the operating costs of the cogeneration plant, namely exergetic production cost (EPC), assuming fixed rates of electricity production and process steam in exergy base. In this study a comparison is made between the same four configurations of part I. The cogeneration system consisting of a gas turbine with a heat recovery steam generator, without supplementary firing, has the lowest EPC.     

The role of hydrogen for the long term development of sustainable energy systems—a case study for Germany

Based on different current long-term energy scenarios the paper discusses the future perspectives of hydrogen in the German energy system as a representative example for the development of sustainable energy systems. The scenario analysis offers varying outlines of the future energy system that determine the possible role of hydrogen. The paper discusses the possibilities of expanding the share of renewable energy and the resulting prospects for establishing clean hydrogen production from renewable energy sources. Emphasis is given to the questions of an ecologically efficient allocation of limited renewable energy resources that can only be assessed from a systems analysis perspective. Findings from recent studies for Germany reveal a strong competition between the direct input into the electricity system and an indirect use as fuel in the transport sector. Moreover, the analysis underlines the paramount importance of reducing energy demand as the inevitable prerequisite for any renewable energy system.     

Theoretical and experimental investigations on the performance of dual fuel diesel engine with hydrogen and LPG as secondary fuels

The mathematical models to predict pressure, net heat release rate, mean gas temperature, and brake thermal efficiency for dual fuel diesel engine operated on hydrogen, LPG and mixture of LPG and hydrogen as secondary fuels are developed. In these models emphasis have been given on spray mixing characteristics, flame propagation, equilibrium combustion products and in-cylinder processes, which were computed using empirical equations and compared with experimental results. This combustion model predicts results which are in close agreement with the results of experiments conducted on a multi cylinder turbocharged, intercooled gen-set diesel engine. The predictions are also in close agreement with the results on single cylinder diesel engine obtained by other researchers. A reasonable agreement between the predicted and experimental results reveals that the presented model gives quantitatively and qualitatively realistic prediction of in-cylinder processes and engine performances during combustion.