Showing papers in "International Journal of Energy Research in 1988"

The optimization of hybrid energy conversion systems using the dynamic programming model: RAPSODY

Abstract: The paper describes a stochastic, dynamic programming model, RAPSODY, which is designed to analyse and determine optimal operating strategies for a hybrid electricity generating system comprising up to three diesel sets with optional battery storage and augmented by variable wind or photo-voltaic power. The model takes capital, operating and maintenance, and fuel costs into account to assess the optimal operating strategy for the auxiliary and to calculate the average daily cost of satisfying an electrical load profile which may also contain a stochastic element. In doing so, the decreased fuel efficiency and lifetime of the diesel set as it is operated below full capacity is explicitly taken into consideration as is a further important cost component attributable to switching on. The model is provided with an efficient optimizing routine which allows the user to obtain optimal component sizes for a particular load profile and wind or solar resource. An example is given in which a hybrid wind power system incorporating battery storage and an auxiliary diesel generator is optimized. For the case studied, the auxiliary switching cost and the shape of its operating cost function were important in defining the optimal operating strategy which was a significant factor in minimizing generating costs.

Solar thermal storage systems using phase change materials

Abstract: Using Fourier series expansion of the involving temperatures and the forcing parameters i.e. the solar radiation and the ambient temperature, an iterative procedure has been developed to solve the heat transfer problem with moving boundaries. Calculations specific to a typical summer and winter day in Delhi have been presented for a numerical appreciation of the developed analysis. Experiments have been performed to validate the developed theoretical analysis. A good agreement is seen between theoretical and experimental results with in the domain of the applicability of theory.

Rural energy consumption patterns with multiple objectives

Abstract: The problem of designing a rural energy centre and thus obtain the emerging energy consumption patterns with multiple objectives, can be viewed as matching the various energy sources to cater fully to the energy needs of a wide variety of tasks such as cooking, lighting, ploughing, pumping water for irrigation, rural transport, etc. This, in fact, should satisfy several economic, technical as well as social objectives or goals. In order to arrive at a satisfactory solution, this has to be viewed as a multi-objective optimization problem. This communication develops a mathematical model for optimum energy planning in a rural environment. Goal programming approach is used in evolving a satisfactory solution to the above problem. The model developed is applied in the case of a typical South Indian village situated in a semi-arid region especially with respect to the major domestic energy needs of cooking and lighting. Results from the computer-simulated model are given.

Analysis of rankine cycle heat pump driers

Abstract: We present a simulation analysis of closed Rankine cycle convective heat pump driers in which the product air recirculation ratin within the chamber is high. The configurations include driers with both external and internal refrigerant evaporators. The effects of modifications to basic designs are examined and real plant details are added in successive stages. Throughout the study the efficiency of each system is related to the second law loss mechanisms discussed by Carrington and Baines (1988). A detailed audit of exergy losses is presented for two driers in particular situations using data based on operational timber driers. The simulation results illustrate the need for the drier control strategy to take advantage of the performance characteristics of the heat pump drier. In addition, it is important to carefully match fans and heat exchangers to the drier requirements because the fan power will often be comparable with the compressor input.

Identification of zeolites for heat transformer, chemical heat pump and cooling systems

Abstract: Among the gas-solid adsorption processes the zeolite water adsorption seems to be very interesting for use in thermodynamic systems such as chemical heat pump, heat transformer and cooling systems. This work evaluates theoretically the best type of zeolite for use in each system on the basis of the adsorption data of zeolites using prefixed operative conditions. Results obtained indicate that thermodynamically a given zeolite type achieves better results than the others for each of the abovementioned systems. Methodology and criterion of comparison used can be applied to other gas-solid processes aiming at realizing chemical heat pump, heat transformer and cooling systems.

Heat pump assisted distillation. X: Potential for industrial applications

Abstract: In heat pump assisted distillation, the working fluid can either be one of the components of the mixture to be distilled or it can be an external working fluid. The operating parameters of more than thirty potential heat pump assisted distillation systems have been listed. A detailed assessment can only be made if detailed thermodynamic data are available. A set of criteria has been prescribed for the choice of an appropriate mode for a particular application.

Hall effects on the hydromagnetic free convective flow and mass transfer through a porous medium bounded by an infinite vertical porous plate with constant heat flux

Abstract: Effects of Hall current on free convection and mass transfer flow through a porous medium bounded by a vertical surface when a uniform magnetic field acts in a plane which makes an angle x with the plane transverse to the plate have been analysed. An analytic solution of the problem is obtained and the effects of the Hall parameter and the permeability parameter, as well as the other parameters entering into the problem, are discussed and shown graphically.

Simultaneous heat and mass transfer on oscillatory free convection boundary layer flow

Abstract: The problem of simultaneous heat and mass transfer in two-dimensional free convection from a semi-infinite vertical flat plate is investigated. An integral method is used to find a solution for zero wall velocity and for a mass transfer velocity at the wall with small-amplitude oscillatory wall temperature. Low- and high-frequency solutions are developed separately and are discussed graphically with the effects of the parameters Gr (the Grashof number for heat transfer), Gc (the Grashof number for mass transfer) and Sc (the Schmidt number) for Pr = 0–71 representing aid at 20°C.

Second law limits in convective heat pump driers

Abstract: The loss mechanisms in convective drying systems are examined from a second law viewpoint. The avoidable and unavoidable loss processes in heat pump driers are identified and compared with those in conventional heat and vent driers. In the quasi-static limit the only unavoidable loss in a heat pump system is due to the irreversible diffusion of water vapour into the unsaturated atmosphere of the drier. Thermal losses, due to heat transfer within the drier and venting to the atmosphere are avoidable in principle by the use of a heat pump. In practice the efficiency of a heat pump drier depends on how effectively these processes are managed, whereas in conventionally-heated driers the major loss is due to the heating process. Manipulation of the residual thermodynamic loss can have only a restricted effect on the efficiency in such driers. The influence of drier design on the unavoidable losses in heat pump driers is shown to be important. In the most advantageous case, a multi-pass system with a high recirculation ratio, the unavoidable loss is related only to the relative humidity. Temperature is an important parameter in other systems, especially single-pass driers. The significance of the unavoidable thermodynamic loss is that it is closely related to the actual thermodynamic loss in an effective heat pump drier design because it sets the scale of the actual losses. Thus, the limiting loss is a relevant consideration when selecting the operating conditions for heat pump driers.

On the coupling of PCM stores to active solar systems

Abstract: This paper investigates the problems arising in the combination of active solar systems with PCM storage matrices. A new heat exchanger design is used, based on the rolling cylinder principle. The store is managed by a dedicated microcomputer which interacts with the system control computer. The store computer configures the store in real time to operate it in near optimal conditions, according to the current system operating point.

Exergy and energy analyses of absorption heat pumps

Abstract: This paper presents a detailed thermodynamic analysis of absorption heat pumps and heat transformers. First and second law thermodynamic methods are both applied to calculate the respective energy efficiency (COP) and the energy efficiency (n) for the systems as well as the individual components. Using LiBr/H2O as an example, two three-dimensional T-S-X diagrams for an absorption heat pump and a heat transformer are constructed by calculating the thermodynamic properties and the chemical potentials. From the calculated COP and n values, it is concluded that AHP for heating has better thermodynamic performance than both AHP for cooling and heat transformer.

Design and development of a solar cooker cum drier

Abstract: A solar cooker cum drier has been designed and developed. In designing this unit, optimum booster-collector geometry has been considered in order to eliminate the requirement of sun tracking. Tilting arrangements have also been provided to capture more solar energy. This dual purpose device has been found useful for cooking food and dehydrating fruit or vegetables.

Combined free and forced convection flow through a porous medium

Abstract: Local similarity solutions of the forced and free convection flow past a porous medium bounded by a semi-infinite vertical porous plate are obtained numerically.

Solar assisted biogas plants III: Energy balance of fixed dome biogas plants and enhancement of production in winters

Abstract: This paper presents a thermal model of a fixed dome biogas plant, with or without hot charging and having the ground surface above the plant, blackened and glazed. The rate of energy transfer, between the ground and the plant has been determined by electrical simulation experiments, using a small scale model of the plant (made of copper), suspended in copper sulphate solution. For hot charging a shallow solar pond, built on the ground has been considered. The monthly variation of the slurry temperature, corresponding to 1, 4, 8, 15 and 30 m3 plants have been evaluated for Madras, New Delhi and Srinagar, which correspond to typical hot, composite and cold climates; the following cases have been considered: 1 Bare plant, without hot charging and blackening/glazing of the ground, above the plant. 2 Plant, charged with hot slurry from a shallow solar pond. 3 Plant, with ground above blackened and glazed. 4 Plant, with ground above blackened and double glazed and also provision for heating the slurry by burning a part of the produced biogas. It is seen that blackening and single glazing of the ground above the plant is able to maintain the slurry temperature between 28 and 35°C throughout the year in composite climate such as Delhi; in cold climates even double glazing alone is not enough for that purpose. Hot charging from a shallow solar pond alone is not sufficient to maintain satisfactory slurry temperatures in winters of Delhi and of course Srinagar. In hot climates the slurry temperature is sufficiently high, except for a couple of months, when either hot charging from a shallow solar pond or blackening/glazing of the ground above is sufficient to have the slurry temperature in the desirable range. In cold climates such as Srinagar, the desired slurry temperatures can be maintained if a part of the gas produced is burnt for heating the slurry, in addition to blackening and double glazing of the ground above the dome. Theoretical predictions of slurry temperatures have been compared with the field data recorded for an 8 m3 digester at the Energy Complex, Masoodpur, Delhi; the agreement is satisfactory.

On the energy output estimation of wind turbines

Abstract: During the operation of the German test field for small Wind Energy Conversion Systems (WECS) on the island of Pellworm five wind turbines were tested following recommendations of the International Energy Agency (IEA) expert group. Possible errors in the estimation of a tested wind turbine's total energy output at a potential installation site are investigated. Different wind speed frequency distributions (the measured one, the Rayleigh and the two-parameter Weibull distribution) are used to calculate the total energy output. The differences between the various distributions are mostly below 10 per cent. An improvement of the energy output estimate by a Weibull-instead of a Rayleigh distribution was not found. It is also shown that the use of the recommended 10 min averages or any other average overestimates the WECS' efficiency, up to 14 per cent on average depending on turbulence intensity. Wind power instead of wind speed is the appropriate parameter for power performance testing. Spectra of wind power and electrical power output show three areas of different correlation. A resistance length for wind turbines is shown to be dependent on the WECS operation status.

Comparison results of two optimization techniques for a combined wind and solar power plant

Abstract: Two optimization techniques have been tested on an hour-by-hour simulation of a combined wind and solar power plant. The system also includes a battery storage system as well as a group of diesel generators. The two optimization techniques are: simplex from the package of MINUITS written at CERN and a modified steepest descent algorithm. Both techniques are suited to hour-by-hour simulation for the above system since the function being minimized is monotonically decreasing towards a minimum. The comparison results showed that the steepest descent algorithm converges slightly faster than the simplex one. Moreover, the application of the techniques for two different sites with different load profiles let us conclude that the results are stable.

Why is life-cycle costing important when retrofitting buildings

Abstract: Using life-cycle costing (LCC) gives us a means to find the best retrofit strategy for an apartment block. This method also shows us how important it is to consider the whole existing building as an energy system. If the best heating system is put into the house almost every shield retrofit is unprofitable. Having heating systems, with high variable costs combined with exhaust ventilation air pumps, sometimes makes it unprofitable to caulk the windows and doors. This article also shows the importance of using the accurate prices for the energy. Short-range marginal costs (SMRC) gives different retrofit strategies than normal tariffs used today. This also means that the retrofits do not correspond to the optimal use of the total national energy system and already scarce resources are used unnecessarily.

Energy and exergy analysis of the preheating of combustion reactants

Abstract: Influence of the enhancement of physical recuperation on fuel consumption in heating furnaces has been analysed. The multiplier of fuel energy and fuel exergy economy has been defined. The effects of chemical recuperation, increasing the chemical energy and exergy of fuel have been determined. The independent preheating of combustion reactants has been investigated. The possible increase of the lean fuel content in the gaseous fuel mixture has been determined. Numerical examples are included.

Transport of thermal energy by a simple two‐phase loop

Abstract: A loop filled with a phase changing fluid is proposed as a device capable of transferring heat power from a generation place to a user Through the solution of a set of non-linear equations, operating conditions of the system are determined It is seen that even large amounts of energy can be transported under relatively small mass flow rates and differences of pressure and temperature Then only a small degradation of the energy conveyed takes place The operating conditions of the system are sensitive to the filling degree and the inner diameter of the loop pipe

Experimental studies of low‐temperature drying by dehumidification. Part 2—Experimental

Abstract: Experiments are described in which several different types of cycles and dryers are compared, and it is shown that the best performance, in terms of both drying time and energy saving, is achieved by a recirculating cycle with a hybrid dryer with a heat pump dehumidifier and supplementary resistance heating.

Studies on collector‐cum‐storage type solar water heaters under arid zone conditions of india

Abstract: Conventional natural circulation type solar water heaters are very costly. Though manufactured in India, they are beyond the reach of the common man. To reduce the cost, a collector-cum-storage type solar water heater has been developed and its detailed study has been carried out and reported. The heater can provide 100 1 of hot water at a temperature of 55–60°C in the evening which can be retained at 40–45°C until the next morning. The efficiency of the heater varies from 57.4 to 66.0 per cent depending on design parameters.

Solar heating of open swimming pools

Abstract: This communication presents a thermal model for the performance of an open swimming pool. It is seen from numerical calculations that the combination of a pool cover and solar collectors (with an area equal to 75 per cent of that of the pool) enables the swimming season to be extended in Delhi, India to forty-seven weeks as compared to the period of twenty-nine weeks for an open swimming pool.

An experimental analysis of a solar assisted absorption heat pump with earth seasonal storage

Abstract: A plant composed of an energy roof, a seasonal earth storage and an absorption heat pump has been experimented. The purpose was to study the behaviour of the various components and their interaction. The surveys were carried out over a two year period. The following operations were considered: the charging of the earth storage by the energy roof and the working of an absorption heat pump connected either to the energy roof or to the earth storage.

Heat pump assisted distillation. VIII: design of a system for separating ethanol and water

Abstract: A heat pump assisted distillation system has been designed for the separation of ethanol from 7 per cent aqueous mixtures to produce 93 per cent ethanol by weight. The distillation column has been designed on the basis of conventional design procedures. Valve trays were chosen to provide operational flexibility. R114 was chosen as the working fluid for the mechanical vapour compression heat pump. The heat pump system has been designed to match the heat loads determined for the column. Auxiliary heat exchangers have been provided to aid flexibility and control of column operation.

Analysis of a pressure driven absorption refrigeration cycle

Abstract: This study presents an analysis of a pressure driven absorption refrigeration cycle which utilizes a membrane separation process to achieve refrigerant-absorbent (R/A) separation. Since the performance of such membranes cannot be predicted generally, the analysis is accomplished by computing cycle performance as a function of the effectiveness of the membrane separation process. The net refrigeration effect and work input are determined based on thermodynamic property data for several working fluid combinations, and desirable characteristics for refrigerant-absorbent pairs are identified. The solubility parameter is used to characterize the potential for separation by candidate membrane materials. The absorbent tetraethylene glycol dimethyl ether (E-181) is found to have good potential for separation from Refrigerants 21 and 22 by typical membranes such as cellulose acetate. The coefficient of performance of the proposed cycle is lower than that of a standard vapour compression cycle operating between the same temperature limits. Improved cycle performance may be achieved by development of a working fluid pair having a more nearly optimum combination of properties.

Modified refrigerant compressor as a reciprocating engine for solar thermal power generation

Abstract: The prime mover of a small solar thermal power generation system can be either a turbine or a reciprocating engine. In this paper a case is made for the use of a reciprocating engine instead of a turbine. A new design of valves is described which results in an extremely simple reciprocating engine. It is argued that the efficiency of such an engine would be at least as good as that of a comparable turbine. A one-ton refrigerant compressor was modified to incorporate the valves proposed in order to run it as an engine. The engine was run on compressed air and the pressure-time diagram of this engine has been analysed to evaluate the performance of the new valves. The present engine is compared with a similar engine developed by the Thermo Electron Corporation, U.S.A.

Solar assisted biogas plants II: Energy balance of floating drum type biogas plants

Abstract: In the present paper, a mathematical model is developed to predict i the heat losses and gains from various mechanisms in a KVIC-type biogas plant as a function of slurry temperature and other parameters, ii transient performance of the conventional KVIC-type biogas plants, and iii transient performance of KVIC-type biogas plants covered by a greenhouse. Numerical calculations have also been made to predict the monthly performance of biogas plants of different sizes having digester volumes ranging from about 4 to 220 m3 for the climate of Delhi, India. The heat losses from the digester to the ground through the sides have been evaluated following Sodha et al. (1980) by assuming the digester to be an infinite cylindrical cavity in the ground. Computations have also been made to study the effect of a greenhouse, its size and night curtains on the performance of biogas plants. Results indicate that the heat losses from the slurry to ambient through the annular space left between the gasholder and the digester are very significant for smaller plants, whereas charging represents the largest single component of heat losses for larger plants.

The value of natural environments in the extraction of finite energy resources: A method of valuation

Abstract: The value of the natural environment is invariably excluded from the costing of finite energy resources due to difficulties associated with noncommensurability. This paper illustrates a method of valuation for strip mining coal in a virgin natural environment. The method is based on the principles of irreversibility and opportunity costs. Although the values derived represent a bottom threshold, their inclusion would prompt an improvement in the allocation of scarce resources for the development of energy technologies.