Thore Berntsson

Heat sources - technology, economy and environment

Heat sources for heat pumps in buildings as well as in industry are discussed. Furthermore, some environmental aspects concerning choice of heat source are highlighted. Only systems for heat pumping are included, i.e. air-conditioning types which can also partly work as heat pumps are excluded. Recent heat pump installations in Sweden are mainly in small systems. Ambient air, exhaust air, soil and rock are the most common heat source types. Data on COP, investment costs, working fluids, present Swedish market etc. for these types of heat sources are given. Data on industrial heat pump installations in some countries and distribution of these according to heat pump type and industry sector as well as heat source temperatures are reported. Process integration aspects when choosing heat source size and temperature are also discussed as well as the relation between these parameters and the choice of heat pump type. Finally, the influence on the economy of the heat source temperature is presented. The cost-effectiveness of heat pumps for reducing CO2 emissions compared with other heating technologies is discussed. The main results are: (1) heat pumps can in many cases in the future contribute to a reduction of CO2; (2) there is a rather big difference, for larger systems a major difference, between the water-based and the ambient air-based heat pumps in terms of efficient reduction of greenhouse gas emissions.

CHP in the pulp industry using black liquor gasification: thermodynamic analysis

Black liquor supplies the major share of the energy used in the production of chemical kraft pulp, the dominant pulping process worldwide. Traditionally, black liquor, which contains dissolved organic and inorganic substances from pulping, is recycled to a recovery boiler that performs the dual function of recovering chemicals and energy. Gasification of the black liquor as an alternative to the conventional recovery system is under intense development, because it offers potential advantages in both functions. The realization of the energy recovery potential of black liquor gasification is strongly connected to changes in the pulp mills system for cogeneration of heat and electric power. In this work, pinch analysis of the integrated gasification cogeneration system, is used to identify systems which maximize power and heat yield under given process constraints and before considering integration with the mill.

Optimization study of the compression/absorption cycle

Abstract For each external situation optimum working conditions for the compression/absorption cycle can be found. The improvement in cycle performance which is gained by optimizing the temperature gradient in the absorber is considerable, particularly for situations with small external temperature gradients. Theoretically, the external and internal temperature gradients should be equal to maximize the cycle performance. The introduction of a solution loop, however, changes this and the optimum internal temperature gradient is always larger than the external gradients. The optimum point of operation is found by studying the changes in the compressor and pump and the heat loss obtained in the solution heat exchanger with the working conditions. A comparison of a compression/absorption cycle, using NH3-H2O, and a compression cycle working with pure R12, always results in a higher coefficient of performance for the former. The capacity of the NH3-H2O system is also considerably higher.

Co-ordination of pinch technology and the MIND method-applied to a Swedish board mill

By combining the pinch technology and the MIND method, it is possible to identify beneficial and energy-efficient measures in a complex industrial energy system. By tackling a problem on the two different aggregation levels, the result is thoroughly evaluated and durable measures are achieved. The strength of the combination of methods is elucidated in a case study where a Scandinavian pulp and paper mill is analysed. The studied problem concerns pre-evaporation of effluents in a board mill using excess heat. Different alternatives are evaluated, taking into account economic, technical and practicable constraints. The results show that it is cost-effective to pre-evaporate the effluent using excess heat in the studied mill.

The compression/absorption heat pump cycle—conceptual design improvements and comparisons with the compression cycle

Performance improvement of an industrial single-stage compression/absorption heat pump (CAHP) using an ammonia/water mixture as the working fluid has been studied theoretically. By allowing a higher absorber pressure (40 bar) than the highest design pressure of todays screw compressors (25 bar), higher COPs could be obtained. Longer falling-film tubes in the vertical shell-and-tube absorber and desorber also increased the COP. These two modifications together increased the COP of the CAHP by 10%. The improved design has a lower optimal absorber glide (temperature difference due to composition change in absorber) and reduced solution heat exchanger sizes. The study was performed with a constant total area. Furthermore, the CAHP performance was studied for five heating cases. Its performance was compared to that of a two-stage compression heat pump (CHP) using isobutane as working fluid, on the basis of approximately equal investment cost. It could be concluded that only heating cases where both the sink and the source temperature changes are high (>20 K) give superior performance for the CAHP.

Utilization of excess heat in the pulp and paper industry - a case study of technical and economic opportunities

Newly developed methods and tools based on pinch technology are used in a case study to investigate the potential and economy of using excess heat for pre-evaporation of chemo thermo mechanical pulp effluent and heat pumping in an integrated pulp and paper mill. The new tools give information about the system that traditional pinch tools such as the grand composite curve or the composite curves would not reveal. For example, the highest temperature levels possible where excess heat can be released are identified together with the amount of excess heat at each temperature level. The new curves are also able to provide information about where heaters and coolers are placed in an existing system. The matrix method has been used successfully in order to find an economically feasible heat exchanger network retrofit for the release of the excess heat found with the curves. The results of the case study show that a pre-evaporation plant can be integrated with the overall process with just a few modifications in the existing process. There are also opportunities for heat pumping in the system. Both projects have a pay-back period shorter than required for implementation.

Heat pumps in industrial processes—An optimization methodology

The optimal integration of heat pumps in industrial processes has not yet been fully understood. In this paper an optimization methodology and a method which uses the composite curves as a guideline to the correct choice of heat pump type are outlined. The selection is done by matching the shape of the composite curves against the specific characteristics of several heat pump types. Furthermore, a methodology for the optimization of the most important parameters in a heat pump system is presented. In the optimization methodology the annual cost is minimized by varying heat source and heat sink temperatures, the heat pump size and the stream or streams to be utilized as heat source and heat sink. To reveal the potential for electrically driven compression heat pumps two different examples were studied with the methodology. The first example had close composite curves and was thought to be a poor heat pump candidate. The second one had open composite curves and was thought to be a promising example. The results showed that for both examples, heat pump installations were advantageous under good economic conditions for the heat pump, i.e. low electricity price, high fuel price and low investment costs. Also reasonable payback periods were achieved. With more unfavourable conditions the payback period increased, and in extreme cases a heat pump was no longer a better alternative than pure heat exchange. This decline in potential for heat pumping was much less in the example with open composite curves than in the example with closed ones. However, the conclusion to be drawn is that there exists today a potential for heat pumps in industrial processes.

Calculation methods for comparing the performance of pure and mixed working fluids in heat pump applications

Three methods for comparing cycle performance of working fluids, pure as well as non-azeotropic mixtures, are investigated for two applications and for two mixture pairs, HCFC22-CFC114 and HCFC22-HCFC142b, and their pure components. The methods differ in the way of calculating the heat exchange processes. They assume, respectively, equal minimum approach temperatures, equal mean temperature differences and equal heat transfer areas. Changes of coefficient of performance (COP) with composition are explained for all methods. It is shown that transport properties must be taken into account when making rigorous comparisons between working fluids. To predict the relations between fluids with high accuracy, one must use the method with equal heat transfer areas. By the method with equal mean temperature differences, the COP can be estimated with the same accuracy for mixtures as for pure fluids, and can be used for rough estimations of the COP level with different fluids. The method of equal minimum approach temperatures should be avoided for non-azeotropic mixtures.

Integration of heat pumps in industrial processes

In spite of several technical and economic advantages, the number of heat pumps in industry is still very low compared to those for house heating. There are several reasons for this; one of the important ones being a lack of knowledge of how to find good, economic applications with the aid of process-integration principles. With the aid of these principles, the appropriate design in terms of heat pump type, size and heat source and sink temperatures can be identified. In doing that, the characteristics of both the industrial process and the heat pump must be taken into account. For the process the pinch temperature, the shape of the composite curves and the number of heat exchangers in the system are the most important factors. For the heat pump, the possible COPs that can be achieved and the ratio of heat to heat sink/heat from heat source are the most important factors, in addition to investment costs, energy prices etc. Methods for optimization of the main parameters in a grassroot design and for finding the most appropriate designs in a retrofit situation have been developed. With the aid of such methods, the potential for heat pumping in industry can be shown to be higher than earlier anticipated. Studies in real plants have verified this.

Potential for greenhouse gas reduction in industry through increased heat recovery and/or integration of combined heat and power

The potential for greenhouse gas (GHG) reduction in industry through process integration measures depends to a great extent on prevailing technical and economic conditions. A step-wise methodology developed at the authors department based on pinch technology was used to analyse how various parameters influence the cost-optimal configuration for the plants energy system, and the opportunities for costeffective GHG emissions reduction compared to this solution. The potential for reduction of GHG emissions from a given plant depends primarily on the design of the industrial process and its energy system (internal factors) and on the electricity-to-fuel price ratio and the specific GHG emissions from the national power generation system (external factors).