Disitrict heating: steam system optimization or hot water conversion?

For the past few years, the trend in district heating has been to use low-temperature hot water for thermal distribution instead of steam, and even to convert steam-based district heating systems to hot-water systems. But is this really the best approach? Should existing systems be converted, or optimized?

There are a number of advantages to using low-temperature hot water (i.e., with a temperature of less than 100°C) as the heat transfer fluid instead of steam.
- The main benefits of low-temperature hot water include:
- Easy to integrate renewable energy sources
- Easy to incorporate recovered energy
- Lower installation costs when preinsulated pipes are used
- Maintenance costs are usually lower
- Lower heat loss during distribution
However, steam also entails benefits:
- High-level energy density (BTU/lb)
- High temperature available for sterilization, processes, humidification, etc.
- Low mass density (no distribution pumps)
- Possibility of using combined cycles in summer to generate extra electricity from steam
- Improved performance for absorption chillers
Practically speaking, both systems coexist because each has its own pros and cons. From an economic perspective, it is rarely advantageous to convert a steam system to a hot-water system unless the existing system is dilapidated and in disrepair. Due to the cost of converting the heat source, adding pumps and making the necessary changes (e.g., replacing exchangers, adding electric humidifiers and replacing fresh air coils), conversion usually does not provide a worthwhile return on investment.

For existing systems, it is generally more cost-effective to upgrade the installations to improve their energy efficiency than to convert them to hot water.

In some cases, it may be beneficial to choose hot water to expand the systems using recovered heat, as was done for the district heating systems in Paris (CPCU) and Montréal (CCUM).

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