Residential Water Heating from Air Conditioning Unit Heat Recovery
Water Heating: Retrofit Condensing Air Conditioning Units for Heat Recovery vs. Waste Heat
Installation of waste heat recovery units on residential air conditioning condensing (outside) units to supplement domestic water heating.
Item ID: 8
Multiple Energy Systems--Combined Space Conditioning and Water Heating
Technical Advisory Group: 2009 HVAC TAG (#2)
Technical Advisory Group: 2014 Residential Building TAG (#10)
Average TAG Rating: 2 out of 5
TAG Ranking Date: 04/10/2014
TAG Rating Commentary:
- No cost indicated here but this is going to be a high cost measure. Savings may or may not be cost effective. I would be more accepting of this than HPWH alone. Could also lead to a whole house HP system where the water heater, air conditioning, refrigerator, and dryer all utilize a central HP.
- To get this to work we need to deal with the asynchronicity of cooling and water heating demand. Tricky - but it can be dome - just not cheaply.
- Concerned about cost-effectiveness in the NW and about equipment reliability/maintenance requirements
- This is a commercial technology. It would pose significant challenges to incorporate it into typical residential situations. Its viable retrofit potential likely would be very limited.
- There is not enough cooling load in the PNW to make this a good investment.
- Integrating systems within homes and businesses will be critical as we look toward making super-efficient homes the new normal. This is only one example of where integrated heating and cooling would be possible.
- This is a great concept, but I don't see it as a best bet to pursue in the PNW with our limited A/C demands.
By incorporating heat recovery into the air conditioning (AC) unit, significant savings can be obtained in certain applications and climates. The system essentially functions as a heat pump water heater, but instead of extracting energy from the ambient air, a heat recovery unit (also known as a desuperheater) is installed between the compressor and the condensing coil (assuming a typical AC unit design). Waste heat from the refrigerant (which is much hotter than ambient air) is captured and transferred to the water heater storage tank. By pre-cooling the AC refrigerant at its hottest point by taking the heat and heating water, the cooling coefficient of performance (COP) of the air conditioning unit is improved substantially.
Certain residential and some commercial applications can benefit from this technology, including those with a fairly steady demand for both hot water and cooling, especially homes in hot climates. For most systems, the desuperheater can only operate when the building calls for air conditioning. At other times, electric or natural gas is needed to heat the tank.
Note that some HVAC equipment manufacturers may void their warranty unless they provide the desuperheater.
Baseline Description: Electric Resistance Water Heater
Baseline Energy Use: 3030 kWh per year per unit
The cooling load for Northwest homes in cooling zones 2 and 3 is about 0.52 and 0.85 kWh/sf-year, respectively (Northwest Power and Conservation Council) Assume an average of 0.75 kWh/sf-year. This value will be taken as representative of single-family and manufactured homes in those cooling zones. Heat recovery from single family air conditioners in Western Washington and Oregon makes little sense due to low operating times and annual energy use (about 0.13 kWh/sf-year). With a typical house size of 2,006 sf, the average air conditioning annual energy use for cooling zones 2 and 3 is 1,504 kWh/year (Baylon, 2012). Assuming a COP of 2.5, with a cooling load of 1,504 kWh, approximately 12.8 MMBtu of thermal heat energy is available for recovery and hot water heating. Unfortunately, load coincidence is not good as air conditioning loads peak in the late afternoon and hot water heating needs peak in the morning (after showers) and perhaps late at night when dishwashers and clothes washers are in operation. Assuming that 25% of the potentially recoverable energy can be used for domestic supply water preheating, the available energy is equivalent to about 937 kWh/year (0.25 x 12.8 MMBtu/3413 = 937 kWh/year). This is about 30.9% of the typical electrical hot water heating load of 3,030 kWh/year (RBSA--Metering). Note that the energy recovery could likely be increased with the provision of storage capacity.
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 30%
Low and High Energy Savings: 20% to 31%
Energy Savings Reliability: 2 - Concept validated
This analysis is for single family homes.
Energy Use of Emerging Technology:
2,121 kWh per unit per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
Approximately 1,265,000 houses are in zones 2 and 3 (Ecotope, 2014 Table 8).
Regional Technical Potential:
About 51.6% of the homes in zone 2 have cooling equipment while 85.4% of the homes in cooling zone 3 have cooling equipment (Baylon, 2012). The percentage of electric water heaters by state varies. It is 52% for Idaho, 38.1% in Montana, 54.6% in Oregon, and 58.8% in Washington. It will be assumed that 50% of the homes in cooling zones 2 and 3 have electric water heaters and that 75% of these homes also have cooling. Thus 474,375 units might be installed (0.75 x 0.50 x 1,265,000 = 474,375 units).
0.43 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Currently no data available.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.