The primary objectives of this project were to compare the baseline 80-gallon electric tank-type water heater that was currently in use on-site to the performance of a novel heat pump water heater. Energy usage, reliability, capacity, and water heater functionality including temperature control were all analyzed. The analysis was set to be performed over a 12-month period during which the baseline system and the iE1 heat pump system would be operated separately on a rotating schedule to allow for analysis of both. 

Continuous improvement of the iE1 system was a stated goal, with several software updates performed during the 12-month period in addition to site visits aimed at making physical adjustments to the unit (such as adjusting the CO₂ pressure on the compressor circuit). These modifications were recorded to develop a clear picture of the progress made in tweaking the system performance.

The iE1is an integrated heat pump water heater that utilizes R744 (CO₂) refrigerant and a unique thermal battery design to store heat. It is marketed as a “tankless” heat pump water heater due to its lack of potable water storage. Energy is stored in a propylene glycol solution in the 57-gallon thermal battery reservoir. This thermal battery fluid is kept at ~190˚F near the top of the vessel and has its temperature monitored by two temperature sensors located near the top and the bottom. A 316L stainless steel coil is immersed in the battery fluid through which potable water flows during fixture hot water use. This allows water usage in the building to pull heat from the thermal battery on-demand.

iE1 sequence of operation is based on the temperature of the thermal battery. When there is hot water usage in the building system, the heat stored in the thermal battery is discharged, and the temperature drops. This is monitored by a term in the software called State of Charge (SOC). The SOC is what triggers the heat pump and resistive element controls to engage and recharge the thermal battery.

Analysis of system performance is broken into two parts:

1. Performance metrics from the field were compared to the published specifications of the iE1. The notable metrics compared were rated coefficient of performance (COP), first hour rating, and temperature stability. 
2. The iE1 performance was compared to the performance of the baseline electric water heater.  

Performance of the iE1 improved over the demonstration period, as updates were made to the equipment and controls. It’s important to note that the most recent performance values were collected during the spring/summer months, allowing the heat pump to operate in the most favorable ambient conditions. 

Nevertheless, comparing iE1 performance in the summer of 2024 when the equipment was first installed with the COP values being recorded in recent months shows a substantial 275% improvement in performance. Comparing Period 3 performance of the iE1 to the baseline water heater confirms a 2 to 1 difference in COP between the two water heaters. This indicates the potential for considerable future savings.

The deployment of the iE1 provided several insights into the performance and reliability of the instantaneous heat pump water heater. This demonstration allowed Intellihot to make several iterative updates to the software and hardware of the technology in a controlled and heavily monitored setting. These updates to the technology had a positive impact on the performance, denoted by sharp increases in calculated daily COP values, which rose by 275% from Period 1 to Period 3. At the end of the analysis period, the iE1 was outperforming the baseline system by a factor of 2. This encouraging trend indicates the potential for substantial energy savings over the life of the equipment. 
 

Heat pump water technology is still in its infancy. Technology like the iE1 is going to be critical for the future of sustainable solutions on military bases. This study highlights the need for careful implementation and execution of heat pump technology to potable water systems. The potential for large-scale energy savings is obvious, but it is critical to consider all of the variables. (Project Completion - 2025)