S.A. Tassou

Design and construction of a LiBr–water absorption machine

Abstract The objective of this paper is to present a method to evaluate the characteristics and performance of a single stage lithium bromide (LiBr)–water absorption machine. The necessary heat and mass transfer equations and appropriate equations describing the properties of the working fluids are specified. These equations are employed in a computer program, and a sensitivity analysis is performed. The difference between the absorber LiBr inlet and outlet percentage ratio, the coefficient of performance of the unit in relation to the generator temperature, the efficiency of the unit in relation to the solution heat exchanger area and the solution strength effectiveness in relation to the absorber solution outlet temperature are examined. Information on designing the heat exchangers of the LiBr–water absorption unit are also presented. Single pass, vertical tube heat exchangers have been used for the absorber and for the evaporator. The solution heat exchanger was designed as a single pass annular heat exchanger. The condenser and the generator were designed using horizontal tube heat exchangers. The calculated theoretical values are compared to experimental results derived for a small unit with a nominal capacity of 1 kW. Finally, a cost analysis for a domestic size absorber cooler is presented.

Measures used to lower building energy consumption and their cost effectiveness

This study uses the TRNSYS computer program, for the modelling and simulation of the energy flows of modern houses, to examine measures to reduce the thermal load. For the calculations, a typical meteorological year (TMY) and a typical model house are used. The measures examined are natural and controlled ventilation, solar shading, various types of glazing, orientation, shape of buildings, and thermal mass. In summer, ventilation leads to a maximum reduction of annual cooling load of 7.7% for maintaining the house at 25 °C. The effect depends on the construction type, with the better-insulated house saving a higher percentage. Window gains are an important factor and significant savings can result when extra measures are taken. The saving in annual cooling load, for a well-insulated house, may be as much as 24% when low-emissivity double glazing windows are used, which are recommended since the payback period is short (3.8 years). Overhangs may have a length over windows of 1.5 m. In this way, about 7% of the annual cooling load can be saved for a house constructed from single walls with no roof insulation. These savings are about 19% for a house constructed from walls and roof with 50 mm insulation. The shape of the building affects the thermal load. The results show that the elongated shape shows an increase in the annual heating load, which is between 8.2 and 26.7% depending on the construction type, compared with a square-shaped house. Referring to orientation, the best position for a symmetrical house is to face the four cardinal points and for an elongated house to have its long side facing south. In respect to thermal mass, the analysis shows that increasing the wall and roof masses and utilizing night ventilation is not enough to lower the house temperature to acceptable limits during summer. Also, the analysis shows that the roof is the most important structural element of the buildings in a hot environment. The roof must offer a discharge time of 6 h or more and have a thermal conductivity of less than 0.48 W/mK. The life-cycle cost analysis has shown that measures that increase the roof insulation, pay back in a short period of time, between 3.5 and 5 years. However, measures taken to increase wall insulation pay back in a long period of time, of about 10 years.

Variable-speed capacity control in refrigeration systems

Abstract This paper presents a review of the application of variable-speed capacity control to refrigeration systems. The aim is to put together diversified information in a single source and to appraise recent advances in variable-speed technology. The review reveals that although variable-speed drives based on inverters have been applied successfully to control the capacity of rotodynamic machines, such as pumps and fans, their application to positive displacement machines, such as compressors, has so far been restricted to small-capacity air-conditioning units. There has been only a very small uptake of the technology in the medium-range capacity units, due to a number of problems, such as insufficient development and integration of components, poor reliability, high capital cost and the failure of demonstration installations to produce the expected energy savings. Although inverter-based variablespeed compressor technology offers the potential for energy savings, considerable research work is still required for the development of optimised and cost-effective systems.

Modelling and simulation of an absorption solar cooling system for Cyprus

Abstract In this paper a modelling and simulation of an absorption solar cooling system is presented. The system is modelled with the TRNSYS simulation program and the typical meteorological year file containing the weather parameters of Nicosia, Cyprus. Initially a system optimisation is carried out in order to select the appropriate type of collector, the optimum size of storage tank, the optimum collector slope and area, and the optimum thermostat setting of the auxiliary boiler. The final optimised system consists of a 15-m2 compound parabolic collector tilted 30° from the horizontal and a 600-l hot water storage tank. The collector area is determined by performing the life cycle analysis of the system. The optimum solar system selected gives life cycle savings of C£1376 when a nonsubsidized fuel cost is considered. The system operates with maximum performance when the auxiliary boiler thermostat is set at 87°C. The system long-term integrated performance shows that 84,240 MJ required for cooling and 41,263 MJ for hot water production are supplied with solar energy.

Modelling, simulation and warming impact assessment of a domestic-size absorption solar cooling system

In this paper the modelling, simulation and total equivalent warming impact (TEWI) of a domestic-size absorption solar cooling system is presented. The system consists of a solar collector, storage tank, a boiler and a LiBr–water absorption refrigerator. Experimentally determined heat and mass transfer coefficients were employed in the design and costing of an 11 kW cooling capacity solar driven absorption cooling machine which, from simulations, was found to have sufficient capacity to satisfy the cooling needs of a well insulated domestic dwelling. The system is modelled with the TRNSYS simulation program using appropriate equations predicting the performance of the unit. The final optimum system consists of 15 m2 compound parabolic collector tilted at 30° from horizontal and 600 l hot water storage tank. The total life cycle cost of a complete system, comprising the collector and the absorption unit, for a lifetime of 20 years will be of the order of C£ 13,380. The cost of the absorption system alone was determined to be C£ 4800. Economic analysis has shown that for such a system to be economically competitive compared to conventional cooling systems its capital cost should be below C£ 2000. The system however has a lower TEWI being 1.2 times smaller compared to conventional cooling systems.

Review of solar and low energy cooling technologies for buildings

The objective of this paper is to examine solar cooling and low energy cooling technologies. A brief review of various cooling systems is presented, including solar sorption cooling, solar-mechanical systems, solar related air conditioning, and other low energy cooling technologies. The relative efficiencies and applications of the various technologies are presented. These technologies can be utilized to reduce both the energy consumption and environmental impact of mechanical cooling systems.

Comparative performance evaluation of positive displacement compressors in variable-speed refrigeration applications

Abstract This paper presents the results of investigations into the performance of positive-displacement refrigeration compressors for variable-speed capacity control applications. Compressors tested include an open-type reciprocating, a semi-hermetic reciprocating and an open-type rotary vane. All three compressors were tested under constant and variable head-pressure operating conditions. The results indicate that all three compressors were designed for maximum efficiency at nominal speed. At constant head-pressure only the open-type compressor exhibited an improvement in the COP at reduced speeds. With variable head-pressure-control all three compressors showed an increase in the COP with a reduction in speed. The results of an energy analysis carried out based on the experimental results and weather conditions for two locations representing temperate and warm weather conditions showed that all three compressors when operated at variable speed offer energy savings compared to their fixed-speed counterparts. The analysis has shown the open-type reciprocating compressor to be the most efficient system offering 12% savings when operating in a temperate climate and 24% savings when operating in a warm climate.

Heat transfer and pressure drop of ice slurries in plate heat exchangers

Ice slurries can be used both for cold storage in place of chilled water or ice and as a secondary refrigerant since, up to certain concentrations, they can be pumped directly through distribution pipeworks and heat exchangers. For ice slurries to become more widely accepted, however, more engineering information is required on fluid flow and heat transfer characteristics. This paper reports on the results of experimental investigations into the melting heat transfer and pressure drop of 5% propylene/water ice slurry flowing in a commercial plate heat exchanger. Measurements were obtained for ice fractions between 0% and 25% by weight, and flow rates between 1.0 and 3.7 m3/h. In this flow range, increasing the ice fractions from 0% to 20% caused around a 15% increase in the pressure drop over the flow range tested. The overall heat transfer coefficient, based on the logarithmic mean temperature difference, was found to remain fairly constant as the ice fraction increased from 5% to 20%. The heat transfer capacity of the heat exchanger was found to increase by more than 30% with melting ice slurry flow compared to chilled water flow. In a practical application, for a given thermal load this would lead to greater than 60% reduction in flow rate and pressure drop compared to chilled water cooling systems.

Energy analysis of buildings employing thermal mass in Cyprus

In this paper the effects on the heating and cooling load resulting from the use of building thermal mass in Cyprus are presented. This is achieved by modelling and simulation with the TRNSYS program of a typical four-zone building with an insulated roof in which the south wall of one of the zones has been replaced by a thermal wall. Despite the fact that the diurnal temperature variations in Cyprus are ideal for the application of thermal mass, no such application is presently available. Therefore the main objective of this paper is to investigate the possible benefits resulting from such an application. The results of the simulation show that there is a reduction in the heating load requirement of the zone by about 47%, whereas at the same time a slight increase of the zone-cooling load is exhibited. Optimisations of the various construction parameters have also been carried out. The optimum overhang size is found to be equal to 1.2 m with minor variations in the range of 1 to 1.5 m. The effect of the air gap size between the glazing and the thermal wall is insignificant. The optimum value of wall thickness obtained is equal to 25 cm. The effect of roof insulation is investigated and it is found that insulation is a must for better comfort conditions. Also, the effect of applying ventilation whenever the ambient temperature is lower than the indoor temperature during summertime is investigated. A reduction of 7.5% is obtained when air at 3 air changes per hour is directed into the house. In conclusion it can be said that the thermal wall offers some advantages and should be used whenever buildings are erected with south-facing walls.

Modeling of the modern houses of Cyprus and energy consumption analysis

This study uses the TRNSYS computer program for the modeling and simulation of the energy flows of the modern houses of Cyprus followed by an energy consumption analysis. For the calculations, a Typical Meteorological Year for the Nicosia area and a typical model house are used. Initially, the Cyprus energy scene and an analysis of the number of houses employing heating and cooling equipment is presented from which it is observed that the number of systems installed has increased tremendously during the last decade. The results of the simulation show that the inside house temperature, when no air-conditioning is used, varies between 10–20°C for winter and between 30–50°C for summer. The effect on the temperature and the heating and cooling loads that various wall and roof constructions present is determined. This investigation indicates the importance of the roof insulation, which results in a reduction up to 45.5% of the cooling load and up to 75% of the heating load. The effect of mechanical ventilation, window shading, as well as that of the inclined concrete roof used for aesthetic reasons, is also examined. The life cycle analysis is used for the economic analysis of the various constructions. The results indicate that the wall insulation pays back in a twenty year period with marginal savings, whereas the roof insulation has considerable economic benefit, with life cycle savings up to EUR 22374 depending on the type of construction.