Hot water from thin… water?
Rheem technical marketing manager David Micallef provides an in-depth analysis of the features, benefits and applications of hot water heat pumps.
There is not a plumber around who doesn’t know what a heat pump is. Ask anybody to explain the technology and invariably you will get answers like, “It’s like an air conditioner for your water heater” or “It’s like a fridge in reverse”.
This type of analogy is a great way to describe what is, for most people in the plumbing industry, a bit of an enigma product in that ambient air, sometimes as low as 5oC or even lower can be used to produce hot water up to 65oC or higher.
What is missing from this description is that these products are air sourced, or air to water heat pumps. That is, they draw the energy required for heating from the atmosphere.
But there is a variant on this technology which can use the energy contained in water to produce hot water, known as water to water heat pumps.
The technology is not new, and many readers would be familiar with geo-thermal heat pumps, a similar technology, which extract heat from the ground to either heat a building or produce hot water. The big difference is that water to water heat pumps can be used in applications where sinking a whopping great hole in the ground is impractical.
How does this work?
The most practical use for a water to water heat pump is to produce hot water for showering purposes (domestic hot water) by extracting the waste heat from a building’s air conditioning system.
In most large buildings, chilled water at around 7oC is circulated around the building to cool (or more accurately extract heat from) the rooms or offices. As it cools the space, the water in the circuit rises to around 12oC. The chiller operates like a heat pump, but in reverse, and cools this water down to 7oC by ‘rejecting’ this heat to the atmosphere where the cycle begins again.
The water to water heat pump takes a portion of the returning 12oC water and uses this as the energy source for its refrigeration cycle to produce hot water. In the process, it cools the water down to 7oC before returning it to the chiller circuit.
Efficiency on efficiency
Commercial grade heat pumps operate with an efficiency of around 400%, more commonly known as Co-efficient of Performance or a COP of 4. That is for every kW used to run the compressor and pump or fan, it produces 4kW of heat. It is these efficiency boosts that make heat pumps an attractive energy reduction selection.
The truth is, heat pumps have two COP’s – one for heating and one for cooling. In air to water heat pumps, the cooled air is vented to atmosphere and generally lost. The water to water heat pump extracts energy from the chilled water circuit before the chiller and returns pre-cooled water. This is work that the chiller no longer needs to do.
The heat pump cools water with a COP of around 3, using the same energy (to run the compressor and pump) that it used to produce water with a COP of 4. This means that the combined COP is 4 + 3 = 7. This is not some mathematical trickery, but real energy saving that reduces a buildings carbon footprint and running costs.
The Building Code of Australia (BCA) has requirements for building energy efficiency. Naturally, as plumbers, we are interested in the efficiency that relates to that of heating hot water, but the heating and cooling of a building are even bigger users of energy.
Further, schemes such as National Australian Built Environment Rating System (NABERS) and Greenstar put an emphasis on holistic energy reduction. This not only relates to the actual energy consumed by a building, but the impact that building can have on reducing energy in other parts of life.
Enter end of trip facilities or EOTs. An EOT is a part of a building that facilitates the reduction of carbon in the way people travel to and from work by providing a place to store bikes and to shower at the ‘end of the trip’. For practical reasons, these are typically located in the basement of the building which is below ground level and it is considered counter-productive to circulate hot water to the basement for the express purpose of supplying hot water to a zone that will typically be used once per day.
Further to this, it is difficult to obtain fresh air and convey products of combustion for gas water heaters in the basement. The same applies to air sourced heat pumps which, without a ready supply of fresh air, will quickly cool the room down to a point where they can no longer heat the water.
Using an electric water heater with an effective COP of 1 goes against the entire philosophy of reducing carbon (unless of course it is supplied with solar or wind generated electricity, but that’s another story) and so a water to water heat pump provides an excellent solution by tapping into the available waste heat energy freely flowing around the buildings chilled water circuit and helping to reduce the buildings chilling load.
Wherever a relatively constant source of energy is available in water, this energy can be used to provide hot water.
Two examples include North Sydney swimming pool and the Sydney Opera House using Rheem Thermal (formerly Accent Air) commercial water to water heat pumps.
The former uses Sydney Harbour as the energy source to heat the pool to a constant and comfortable temperature all year round. The latter uses the harbour’s water to provide hot water for showers, toilets and catering for the Sydney Opera House.
Another application supplied by Solahart International (a division of Rheem Australia) uses river water to provide hot water to the Sun City Hotel in South Africa.
Features and Benefits
Rheem has been producing a WaterMark certified commercial air to water heat pump for over 10 years. The release of the water to water heat pump will be accompanied by an upgrade to the whole range. The new range includes circa 15kW and 35kW air to water and water to water heat pumps.
On top of this, the Rheem iQ control system is added across the range. Rheem iQ takes heat pump monitoring and control to a whole new level. Up to 10 sensors are used to provide feedback on the operating condition of the water heater, which can be readily viewed on the system monitor or via the central BMS system.
On the water side, monitoring includes hot water temperature and chilled water temperature in and out of the heat pump, tank temperature and building hot water flow temperature.
On the water to water heat pump the chilled water circuit is also monitored for water flow and outlet temperature and will shut down the heat pump should either of these not meet operational requirements. On the air to water heat pump, the ambient air and evaporator coil temperature are monitored and either initiate a de-ice or auxiliary boost function, depending on user selection.
On the refrigerant side, pressure transducers provide real time information on the suction and discharge pressures and the suction superheat temperature, without the need to fit pressure gauges. The benefits this has in terms of product life and maintenance costs can easily be realised as the system can be remotely monitored for operational parameters and alarm trips.
Another feature is the ability to cascade control up to four heat pumps without any external controls. The cascading allows the minimum number of heat pumps to operate to maintain building heat losses, but ensures all units are available when serious heating is required. The heat pumps then rotate so the wear is evenly distributed. These features work to extend the overall plant operational life.
Value for Money
A heat pump is a relatively slow means of heating hot water and therefore its benefit is realised by having enough stored hot water available to meet the peak demands and then letting it slowly recover over say 4 hours in readiness for the next peak.
The combination of the heat pump and bulk storage make it a higher cost option, kW for kW, than a high efficiency gas water heater system. However, when considering the COP of 7 for a water to water heat pump versus the ever rising cost of gas with an efficiency of typically 83%-84%, the payback period can be as low as 1 ½ years, making a water to water heat pump a very attractive offering for a variety of high water use applications where chilled water is also used.