تجزیه و تحلیل سیستم در یک چشم انداز اروپا از عرضه جدید خنک کننده صنعتی در یک سیستم CHP

In the municipality of Södertälje two large industries use much of the electricity, district heating (DH) and chilled water in the area. The Södertälje energy system is not isolated, however, but is connected to the DH systems of southern and central Stockholm, and a change in the Södertälje energy system will also influence the connected energy systems in Stockholm. The cooling demand in Södertälje is currently covered by lake water cooling and compression chillers, but in order to reduce the use of electricity, conversion to absorption cooling or increased lake water cooling can be considered. The large combined heat and power (CHP) plant in Södertälje is not used to its full potential today, but investment in absorption cooling and/or a cold condenser unit integrated with the CHP plant could increase the plant’s operation hours. In this paper the system effects of introducing new industrial cooling supply in Södertälje has been investigated through optimizations of a model including both the industries and the district heating supply in Södertälje and Stockholm. The results show that, independently of whether condensing power production is feasible in the CHP plant or not, investments in both increased lake water cooling and absorption cooling are profitable. A sensitivity analysis of how energy market prices affect the results shows that even though the system cost will change depending on energy market prices, the optimum cooling technology mix will remain the same. However, a sensitivity analysis of the transfer DH capacity between the Södertälje and Stockholm energy systems shows that if the transfer DH capacity is increased, absorption cooling will be less profitable since more heat can be sold from Södertälje to Stockholm while at the same time reducing the use of fuel resources.

Increased focus on carbon dioxide emissions related to global warming has created a need to adopt energy conservation and efficiency as well as energy resource abatement measures. According to the EU directive concerning climate and energy, the EU will have to reduce the emissions of greenhouse gases by 20% and increase the use of renewable energy by 20% by 2020 [1]. The industrial sector stands for 39% [2] of the total amount of energy usage in Sweden, which makes efficiency in industries concerning both the manufacturing processes and supporting processes, such as heating and cooling, essential factors in reducing the total use of energy and resources in this country. Previous studies have shown that a joint system perspective is beneficial when studying industries located in close proximity to a district heating (DH) system. Through modeling the industries and the DH system as a coherent system, the wider system perspective can provide information about possibilities of cooperation concerning heating and cooling and the system effects of investments in both industries and energy companies [3], [4], [5] and [6]. In a research project called SEAST (System design for Energy efficiency – Astra Zeneca, Scania and Telge Nät), two large industries and an energy company in the municipality of Södertälje very near Stockholm are working together to reduce the region’s global climate influence. The two industries (Astra Zeneca and Scania) are the municipality’s major energy users, together using about 150 GW h/year of heat which can be compared to the municipality’s total DH supply of about 700 GW h/year. Combined, the industries also use about 450 GW h of electricity annually of which a substantial part is used for cooling. This corresponds to about 0.6% of the total amount of electricity used in Swedish industry and 40% of the total amount electricity used in Södertälje, which has a population of about 85,000. In Södertälje the need for energy efficiency, combined with an increased cooling demand due to increased industrial production, will create a possibility for investments in new efficient technologies in the industries as well as system solutions for cooling supply. The present cooling system is based on free cooling using lake water which is complemented by compression chillers. The increased cooling demand could be supplied by investments in increased capacity of the lake water cooling system, as well as in new cooling technology like an absorption cooling system. An absorption cooling system can provide longer operation hours for the combined heat and power plant in Södertälje, but another option to increase the operation hours would be to invest in an additional condenser operating with lower condensing temperature for the plant. Due to the connection between the Södertälje and the southern and central Stockholm DH systems, any system change in Södertälje will also influence the system performance of the Stockholm energy system. The energy system of Södertälje and Stockholm is unique in its size, but the results can be implemented also in other industrial areas with large CHP plants and district heating and cooling systems. Extensive energy conservation and efficiency-related measures in large industries may have a great impact on the regional energy system.

Independently of the energy market scenarios or DH transfer capacity between the Södertälje and Stockholm DH grids, the results of the optimization study show that investments in new cooling supply system will reduce the system cost of the system as a whole. The system cost of today’s system based on compression chillers and lake water cooling can be reduced through replacing the compression chillers with either absorption chillers or increased lake water cooling. Absorption chillers increase the heat demand in the system and thus create a possibility for increased production of electricity. As a consequence of this, the reduced system cost is mainly related to the electricity sold from the CHP plant; hence the economic benefit of the investment is confined to the energy companies Telge Nät and Söderenergi. A prerequisite for implementing investments in absorption chillers in the industries is therefore that the investment cost and the system revenue would somehow be distributed between the different parties so that everyone will benefit from the proposed measure. The factor that has the greatest influence on the trade-off between different cooling technologies in Södertälje is the DH transfer capacity between the Södertälje and the Stockholm energy systems. An increased DH transfer capacity would result in a more efficient use of the most economically profitable plants, and reduce the use of plants using more expensive fuel resources and fuels that are more heavily burdened with taxes and less favored by policy instruments such as coal. An expansion of the DH transfer capacity would also lead to lower CO2 emissions, since less fuel resources are used in the system as a whole. The possibility for condensing power shows a potential for a further increase of the revenue from the CHP plant as well as reduced global CO2 emissions if the produced electricity is assumed to replace marginal electricity. An investment in condensing power would to a large extent reduce the CHP plant’s dependence on the heat demand for electricity production. However, an expansion of the DH transfer capacity between the central Stockholm and southern (Södertälje) energy systems would achieve the same effect and, in addition, reduce the use of fuel resources in the entire system. A problem with this solution is once again the distribution of the system profit. Increased DH transfer capacity between the studied energy systems does not necessarily benefit both systems equally. Thus, the profit of increasing the exchange between the systems would have to be shared in order to be advantageous for all parties.