Polygeneration District Heating and Cooling Systems Based on Renewable Resources

Abstract: Traditional district heating (DH) and district heating and cooling (DHC) systems have to address two principal challenges: phase-out of fossil fuels in favor of renewables; and profit instability related to declines in heating and cooling demands along with electricity price fluctuations. These obstacles can be overcome at once through upgrading these systems to a polygeneration concept by means of a retrofitted air-steam gasifier and gas upgrading equipment, enabling the use of renewable feedstocks such as refuse derived fuel (RDF) and municipal solid waste (MSW). In particular, the polygeneration DHC system will be able to produce simultaneously heating, cooling, electricity and value-added products – char, syngas, synthetic natural gas (SNG) and hydrogen. This work investigates the retrofit of these DHC systems through a case study based on the existing Climaespaco facility, located in Lisboa, Portugal. Thermodynamic, exergy, economic, exergo-economic and environmental models were built in Engineering Equation Software (EES) and Matlab. Overall, both RDF and MSW were found to be technically feasible and economically viable for using as feedstocks in the polygeneration DHC system. SNG production integrated in the polygeneration DHC system through the air-steam gasification and gas upgrading equipment is judged to be practical and also boosts revenues. The highest energy efficiency is achieved for cases where char is the sole by-product. System efficiencies drop as other value-added products are included as system outputs, although the extent of the efficiency decline can be adjusted by regulating syngas and SNG production. The highest discounted net cash flows are found for the scenario where SNG, syngas and char are produced simultaneously. A payback period of 3 years was determined for this and the other cases. From the exergy and exergo-economic perspectives, the scenario of simultaneous char and syngas production is the most promising as the overall exergy efficiency has the highest value and product exergo-economic costs are the lowest. However, from the products diversity viewpoint, the simultaneous production of char, SNG, syngas and H2 is advantageous.