This is an article that I wrote back in 2020, but I don’t believe that I published it anywhere, so I’m doing it now. I did discuss this on this episode of the HVAC School Podcast.
Heat Recovery Chillers
I have always had a love and passion for hydronic systems, perhaps it’s because they are not that common in most areas. The designs tend to be elegant and the flexibility is unparalleled by most equipment choices currently available in the U.S. I’m going to try to not get too far into the weeds in this article, but offer an overview of what I feel would be my dream system.
The Source
A heat recovery chiller will do simultaneous heating and cooling, as opposed to a traditional heat-pump or reverse cycle chiller that can only operate in one mode at a time. This system would have a second refrigerant to water heat exchanger and utilize the air-source when we don’t need to bank / store heat, or are running in heat only mode. There are several benefits to this style system, the main being that while operating simultaneous heating / cooling mode the COP of the system doubles. For example, the Multi-Aqua MHRC2 shows a COP of about 8, which is approximately equivalent to 30 EER or 34 SEER. This gives us geothermal level performance without the need of a field.
Indoor Portion
The indoor portion of the system, we would utilize buffer tanks as thermal storage for the chiller. The buffer tanks give us several benefits, including longer run times for the chiller, load matching, and the ability to size for larger load (heating or cooling) without some of the problems that occur when over-sizing a traditional system. The buffer tanks also allow sizing more aggressively by having storage during peak load conditions.
This shows a cooling buffer tank as well as a heating buffer tank. The horizontal pumps are what circulate water through the chiller, while the vertical pumps are what distribute the water to the load / emitters. This configuration allows for water to be used for the loads first and extra capacity to go into the buffer tanks. The buffer tanks also offer hydraulic separation for the pumps (meaning that they won’t interfere with each other if / when there are different flow rates). Another advantage of the buffer tanks is that you can connect multiple heating or cooling sources in parallel to the system. This is shown by the extra tees in the hot buffer tank where we could connect something such as solar water collectors, pellet or wood boiler, or a conventional boiler. You’ll also notice on the right side of the hot buffer tank, that I am showing a brazed plate heat exchanger that would be used to supply domestic hot water.
Distribution System
This could be a number of things, from radiant panels, in-floor, high-output baseboard radiators, however I’m going to model it as a 4-pipe hydronic air handler.
The hydronic air handler is able to provide cooling, heating, and re-heat dehumidification, all in one package. With a hydronic system, it would be very easy to load match by controlling the fan speed and utilizing a Delta-T pump or outdoor reset controls. These air handlers are available from several manufacturers and come in different styles from traditional (as shown), small duct high velocity systems, and even ductless styles.
Disadvantages
- Not highly available in the U.S.
- When the chiller is down nothing works (not much different than traditional)
- Lack of understanding / technicians afraid to work on this style system.
- Potentially higher upfront costs.
- Lack of design or planning could cause unhappy clients (same with traditional)
- Distribution systems need to be designed around low water temperatures (not a drop in replacement for traditional boiler systems)
Advantages
- Central plant for heating, cooling, and DHW (also a disadvantage)
- Flexible distribution options / methods.
- Self contained refrigerant circuit
- Ability to produce / store thermal energy during off peak electrical hours
- Ability to move BTU’s more efficiently through water than air
- Long life for distribution system (50-100+ years)
- Easier transition to natural / flammable refrigerants
Conclusion
In conclusion, there are many advantages to this style system, as well as disadvantages. One of the main points is to think of the system in 3 distinct components, the source(s), thermal storage, and distribution. As mentioned, this only one concept, but in reality, there are many ways to accomplish this, which is one of it’s advantages. For example, perhaps in certain scenarios it makes sense to dump excess heat into a pool, create ice storage during off peak hours, cascade into another water -> water heat pump for higher temperature distribution, incorporate solar collectors, and so on.
Thank you for reading all the way to the end!
