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District heating and cooling networks based on decentralized heat pumps: energy efficiency and reversibility at affordable costs

The future systems for heating and cooling need to meet both economic and environmental expectations. Not least since the cooling demand is increasing. By comparing and analysing five different scenarios this article shows a realistic way to reach both goals.

The five scenarios have been compared regarding costs and emissions of CO2. The common setting is a Southern European climate (Rome) with a relatively high need for cooling, and a city zone with 500 small multifamily houses of 500 m2 each, with an overall land area of 1 km2. It is assumed that no low-cost or low-emission source for high-temperature district heating is available.

The five scenarios
The first scenario includes individual systems, where all the buildings are equipped with gas boilers and electric cooling units. The main costs of this “Individual” scenario are the investment costs for individual boilers and chillers, plus corresponding gas and electricity costs.

In the second scenario, the individual boilers are substituted with traditional district heating while keeping individual cooling units. Costs are reduced since the total investment costs for the network, substations and a centralized boiler are lower than for the individual boilers. Also, gas prices are lower for industrial customers than for private consumers. The traditional network is assumed to operate with a supply temperature of 80 °C and a supply-return temperature difference of 30 K.

The remaining three scenarios are all based on a so-called Flexynets network. In this system, heating and cooling are both provided through a low-temperature district heating network connected to decentralised, reversible heat pumps, making it possible to balance heating and cooling demands. It is also possible to introduce low-temperature waste heat sources, such as excess heat from shopping malls. The three different scenarios include 0 %, 35 %, and 60% of waste heat, respectively. The network supply temperature is set at 30 °C, with a supply-return temperature difference of 10 K. Costs are reduced by applying industrial prices for both gas and electricity. However, this is offset by increased investment costs.

Comparison of costs and emissions
The comparison shows that the Individual scenario comes with the highest costs, when both investments and operation are taken into account. This is mainly due to high operation costs. The other four scenarios are relatively equal regarding costs. The environmental comparison shows larger differences. While the traditional district heating network gives rise to the highest CO2 emissions, the emissions from the Flexynet system decreases as the share of waste heat used increases.

In conclusion, this comparison shows that under the assumed conditions, in areas with a cooling need, a Flexynet system can compete financially, while at the same time resulting in lowered CO2 emissions. On the other hand, the introduction of heat pumps leads to increased use of electricity. If renewable electricity is used, the environmental profile becomes even better.

Marco Cozzini, Italy
Matteo D’Antoni, Italy
Simone Buffa, Italy
Roberto Fedrizzi, Italy
Federico Bava, Denmark

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