Aiming for the European and North American tri-generation market, highly-efficient systems are being developed. At the moment, single-stage absorption chillers are typically coupled to co-generation engines through a single hot water loop at temperatures below 100°C (210°F). In this configuration, the heat from the exhaust gas at temperatures of about 400-500°C (750-930°F) is transferred to the water loop, which is accompanied by a significant loss of exergy. A substantial increase in system performance can be achieved by a stepwise utilisation of the exhaust gas enthalpy in a Double-Effect and a Single-Effect cycle. In this combination of one-and two-stage chillers the coefficient of performance (COP) increases from about 0.7 to almost 1.0 whereby the cooling capacity rises by about 25%. In order to facilitate optimum adaptation of the aggregates - i.e. motor engine and absorption chiller - and to give maximum flexibility an innovative system concept has been developed. The new coupling scheme is based on a standard direct-fired Double-Effect chiller and introduces only minor design changes, like adaptation of the thermal layout of the exhaust gas driven regenerator heat exchanger of the chiller. No additional low temperature regenerators are required. In case of simultaneous heating and cooling the system supports a continuous switchover between maximum cooling and maximum heating capacity. In this mode up to 80% of the driving heat for cold production can be recovered as useful heat at temperatures up to 100°C (210°F). A description of the coupling scheme together with a discussion of energetic and operational characteristics of the concept is presented. Full-scale demonstration projects are under preparation.
It has already been shown that efficiency of direct-fired absorption chillers or tri-generation systems (CCHP) can be increased if the hot flue gas - from internal gas burner or from motor engines or gas turbines - is successively utilized in the absorption cycle at several temperature levels. For successful realization of such concepts, however, efficient heat exchanger designs are required. An increase in complexity on the flue gas side of the chiller must not introduce a proportional increase in size, cost, and pressure drop. Thus, the development of a compact and efficient flue gas fired regenerator design with low flue gas pressure drop is a major step towards COP increase in direct-fired and exhaust-fired absorption chillers. The potential of different regenerator concepts including common smoke tube design and alternative boiling tube designs with natural circulation of the lithium bromide solution and flue gas flow across bundles of plain or finned tubes is discussed. Promising designs are identified on basis of numerical calculations. A semi-industrial sized prototype of a direct-fired regenerator with consecutive sections of plain and finned vertical boiling tubes has been tested in laboratory over a range of thermal inputs up to 256 kW (net. CV). Experimental results of heat transfer and combustion side pressure drop investigations at the novel regenerator are presented. Accordance of experimental results with theoretical predictions is shown.
For energy efficient tri-generation, solar cooling or residential heating compact sorption appliances using the working pair water/lithium bromide (LiBr) can be applied. Standard plant technology established in the field of large capacity chillers and heat pumps with conventional tube-and-shell configuration cannot be easily adapted to compact units for economic and operational reasons. For a successful marketing of compact sorption units, major effort is required to achieve tolerable plant dimensions and weight together with affordable first cost and reliable operation. In a research project, the design of all key components, i.e. main heat exchangers - evaporator, absorber, condenser and generator - and the liquid pumps for refrigerant and absorbent solution has been revised. In order to achieve uncomplicated manufacturing and improved vacuum tightness, concepts for standardized heat exchanger modules have been developed. A single effect absorption plant has been erected for the experimental investigation of the newly designed finned heat exchangers and plate heat exchanger configurations as main heat exchangers in an absorption cycle. Concepts for sizing these compact heat exchanger units in compliance with the given thermo-hydraulic conditions of a water/lithium bromide absorption cycle will be presented. Estimations of the wetting of the heat exchangers, the pressure drop in the vapor flow, as well as the achieved heat transfer per heat exchanger volume will be discussed. Experimental results for falling film absorption with compact heat exchangers will be presented.
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