Decarbonized district energy systems: Past review and future projections

Abstract

A significant portion of building energy usage globally goes toward space heating and cooling, and whether using individual building systems or district systems, those loads are often met with carbon-based sources. As we shift to decarbonize the electrical grid, we must also consider how to best decarbonize our heating and cooling loads in a way that aligns well with a renewable electrical grid. District energy systems (DES) distribute thermal energy to buildings in a community using shared resources and infrastructure. Unlike other decarbonized solutions, DES has the potential to reduce strain on the electrical grid and integrate renewable thermal sources and waste heat. This review will focus on current technology for decarbonizing DES and will discuss important design considerations as well as a qualitative comparison to individual systems.

A DES consists primarily of energy sources and storage, a distribution network, heat conversion, and user loads (such as buildings). We classify heating and cooling sources as constant, variable, or dispatchable, and review carbon-free options. The design of a DES depends on multiple factors including the nature of the energy sources, the loads to meet, central or distributed plant design, and the potential need for redundancy and resilience. We review design decisions including what sources and loads to connect, what distribution network design to implement, and the modeling and control of DES, and consider how to best integrate with a fully renewable electrical grid. Currently, DES designs are unique for each installation and require tailoring for each site. Due to the large number of distributed components, controls are important for DES, both at a component and system level. Future trends to consider include rising cooling demand loads, winter electrical peak load, conversion of traditional DES to state-of-the-art decarbonized systems, and the changing costs and economics of DES.