The open absorption system : energy savings in different applications

Sammanfattning: The open absorption system is a technique that makes it possible to recover energy in different processes using moist air. In most applications the latent heat in moist air is difficult to recover because of the low temperature. Processes using outdoor air to carry away moisture therefore have high energy demands. The open absorption system makes it possible to recover the latent heat and therefore decrease the outdoor air flow and the total energy demand. Two different pilot plants with an open absorption system have been realized in the northern part of Sweden. One is located at a sawmill where four timber dryers and one bio-fuel dryer have been operating. The other one is a pilot plant in a public swimming pool. In this thesis these pilot plants are described in detail and the energy savings when using the open absorption system are evaluated. The specific heat demand for a conventional drying system at the sawmill was about 5970 kJ/kg of evaporated water. When the open absorption system was installed the heat demand was reduced to approximately 1400 kJ/kg of evaporated water. At the same time, an additional 360 kJ/kg of electricity has to be supplied. 45 000 m3 per year of dried bio-fuel has been sold on the market as a result of the decreased heat demand in the wood dryers at the sawmill. Compared to other drying techniques the investment cost is high due to large airflows and therefore large apparatus. The main part of the investment cost, i.e. about 70% originate from the bio fuel dryer and the absorbers. In order to decrease the initial cost a parameter study has been made to investigate the possibilities to reduce the airflow of the drying process, i.e. bio fuel dryer and absorber. Parameters studied were, drying temperature, salt concentration and cooling of the airflow during the absorption process. The results show that it is possible to decrease the airflow by 31% when using a higher drying temperature. Higher salt concentration decreases the airflow by approximately 32% and cooling during absorption makes it possible to decrease the airflow by 50%. In order to minimize the airflow the three parameters were combined. In this case it is possible to decrease the airflow by approximately 60%. The energy demand for the conventional ventilation technique in indoor swimming pools has also been compared to two different heat recovery techniques, the mechanical heat pump and the open absorption system. The mechanical heat pump is the most widely used technique in Sweden today. The open absorption system is a new technique in this application. Calculations have been carried out on an hourly basis for the different techniques. Measurements from an absorption system pilot plant installed in an indoor swimming pool in the northern part of Sweden have been used in the calculations. The results show that with the mechanical heat pump, the electrical input increases with 63 MWh/year and with the open absorption system 57 MWh/year. However, both a mechanical heat pump and an open absorption system decrease the annual energy demand from 611 MWh to 528 and 484 MWh respectively, which corresponds to a decrease of approximately 14 and 20% respectively. The electricity input will increase when using the heat recovery techniques. Changing the climate in the facility has also been investigated. An increased temperature decreases the energy demand when using the conventional ventilation technique. However, when the mechanical heat pump and the open absorption system is used the energy demand is increased when the temperature is increased. In order to make the open absorption technique competitive the thesis shows that there is need for designing optimised systems that can be produced in large numbers, all in order to decrease the production costs and lower necessary investment. Another potential advantage with this system is that it does not require electricity as the driving energy. It seems reasonable to conclude however that the price difference between electricity and fuels must be very large before a new system like open absorption technology will be able to penetrate a market dominated by an established technology, like the mechanical heat pump.It appears that the most interesting market for the open absorption technology would be larger installations at industrial plants. If the reductions of the investment estimated in paper B can be realised and if fuel prices increase, the open absorption process would be more competitive with conventional drying technologies. For large installations, mass production is not essential for achieving competitive manufacturing costs and the risks taken by the companies that decide to introduce the system on the market would therefore not be so large.