A multitude of pathways towards expanding the market for CO₂ in heat pumps in America were discussed at a session dedicated to heat pump technology case studies at ATMOsphere America conference on natural refrigerants in San Francisco, 18-19 July 2014: Direct exchange geothermal design, integrated heating and cooling with water source CO₂ heat pumps, the design of heat exchangers for heat recovery, as well as progress towards introducing a CO₂ heat pump water heater tailored to the needs of the North American market were key topics.

Expanding the opportunity for geothermal heat pumps with R744

Despite their tangible benefits with respect to energy efficiency, reliability and water use, ground source geothermal heat pump systems are limited to 1-2% of the HVAC market. This can be attributed to persisting market barriers to the use of conventional geothermal systems, such as the high cost of the ground heat exchanger (GHX) installation (~50-75% of total system), the disruption associated with the GHX installation, as well as the cost and difficulty of evaluating the suitability of individual installation sites.

The use of R744, however, expands the opportunity for geothermal heat pumps according to the presentation by Marc Portnoff, Thar Geothermal, LLC:

"R744 allows us to do a direct exchange design, because if you think about inserting synthetic refrigerants in and out of the ground there are a couple of problems, one is the cost, as the size of the charge gets larger - it is 10-fold more expensive than CO₂” noted Marc Portnoff. “Our oil management allows to separate the compressor oil from the refrigerant at the compressor skid, so we do not have to worry about compressor oil going into the ground heat exchanger. The other thing that R744 allows us to do is to design smaller diameter ground loops… The smaller diameter allows us to have much greater design flexibility in how we install the ground heat exchanger”.

Thar Geothermal has drilled over 100 boreholes, and has spent the last several years learning how to install a direct exchange geo-loop efficiently and effectively. They have worked towards successfully integrating their technology in the HVAC environment.

Demonstrations of a commercial scale (15-20 ton) geothermal system encompassing air side heat exchangers, as well as radiant floor & panels for both heating and cooling at Thar’s 210,000 ft2 (19,510m2) R744 Geothermal Facility, in Pittsburgh, Pennsylvania have shown enhanced energy efficiency and reduced environmental footprint.

“By and large, we are 30% more efficient than a standard ground source geothermal system, which tends to be 40-60% more efficient than an air side heat pump”, the presenter noted.

"We think we have a play here. We are not going to eliminate the market barriers for geothermal. We have a play by improving performance because CO₂ is an excellent refrigerant in this environment and by reducing the cost of installation so that the return on investment increases. Our goal is with our partners, that we will over time increase the market share from 1-2% in the HVAC market to 10%”, Marc Portnoff concluded.

Less than 3-year payback time for MAYEKAWA’s water source CO₂ heat pump system at 190-room hotel in Panama

A presentation by Troy Davis, Mayekawa USA MYCOM, discussed the introduction of Mayekawa's ECO Cute water source hot water heat pump at Torres De Alba Hotel in Panama City. With a commitment to reduce energy costs and lower carbon emissions at his hotel property, the owner of the hotel recently installed the first ever integrated CO₂ refrigerant water source heat pump heating and cooling system implemented in a hotel in Central America. The installation is at their luxury 190-room, three-tower hotel located in downtown Panama City in the El Cangrejo District. The hotel had already installed a solar thermal hot water heat pump for their laundry operations with much success, however they wanted to substantially reduce their reliance on butane hot water heaters that were used for hotel room hot water heating as well as the restaurant’s kitchen. The goal was to install a system that would cover the majority of the hot water demand, combine the 24-hour cooling load required in this climate while simplifying the retrofit installation. Working together GreeNRG - Latin America, Mayekawa USA designed an electric heat pump system for the hotel.

The hotel has three towers with guest rooms as well as a kitchen for the restaurant that were using standard butane hot water heaters located on the roof of each tower. On the third floor of the hotel is the chilled water plant consisting of three electric centrifugal water chillers for the air conditioning load. It was decided that the best location for the electric heat pump was the existing chiller mechanical room, which provided easy access to chilled water mains and electrical distribution panel. In addition, there was space for an additional hot water storage tank next to the chiller mechanical room.

The heat pump of choice was a Mayekawa Eco Cute electric-driven, water source, hot water heat pump that uses CO₂ as a refrigerant for water chilling and hot water heating all in the same unit. This Mayekawa CO₂ heat pump made its North American debut at the Somerston Wine Co. winery located in Napa Valley, US in 2010 and is backed with a proven record of performance and effectiveness in Japan and Europe, where there have been over 500 installations to date.

For this project the heat pump provides 74kW heating capacity at 75°F inlet and 194°F hot water outlet and 51kW cooling capacity at 54°F inlet and 44°F chilled water outlet. It is more efficient than a standard hot water heater and in this application, provides a 2.95 heating COP, 2.04 cooling COP and a combined 4.99 COP. In addition, by using a CO₂ heat pump instead of gas fired hot water heaters/boilers and HFC refrigerants, this system results in a 26% lower carbon footprint overall.

By combining the hot water heating system with the chilled water cooling system using the Mayekawa water source heat pump unit, a higher COP is realised versus individual systems. This integrated system has been in operation since summer 2013 and the calculated payback period for the new heat pump system is less than 3 years.

Sanden’s pathway towards introducing a US-tailored CO₂ heat pump water heater

Sanden has been participating in the CO₂ heat pump water heater project lead by the Energy Program of Washington State University and founded by Bourneville Power Administration. According to a joint presentation by Maho Ito of Sanden International USA and Ken Eklund, Washington State University Energy Program laboratory, test results of CO₂ heat pump water heaters have shown an Energy Factor (EF) of 3.35 and Coefficient of Performance (COP) of 4.2 at outside ambient temperature of 67°F(19.4°C), compared to average electric resistance water heater of less than 1.0 EF and an HFC heat pump water heater of 2.4 EF.

This means that the CO₂ heat pump water heater is more than 3 times more efficient than the conventional electric water heater.

"If electricity costs 10 cents per kilowatt hour and the average electricity use is 3,000 kilowatt hours, the average annual savings are $200”, noted Ken Eklund. “If the system is used as a combination space and water heater the savings increase to over $1,000 per year.”

Field test results of CO₂ heat pump water heaters installed in four single-family homes, in North Pacific States encompassing different ambient temperatures are currently ongoing. “What we see is performance that is very similar to what we predicted from the lab test results, with one difference, and that difference is that in a lab Energy Factor, the only impact one can measure is the tank loss”. On the other hand, in field tests, there are also pipe losses - which are determined by how much time the water is sitting in the pipe - that also have to be taken into account, which makes the analysis difficult. To address this, Sanden and Washington State University are carrying out one-minute data collection, so temperature readings are taken every single minute. At least 3 minutes of the same temperature are needed before the data can be used for calculating performance.

Sanden will also be participating in a Demand Response project to test how the CO₂ heat pump water heater can contribute to storing off-peak demand energy in order to control electricity peak demand.

Upon completion of field performance tests and demand response potential tests, lessons learned will be incorporated into next generation design & installation training, with a view to introducing a product tailored to the needs of the US in 2015.

Design of heat exchangers for heat recovery in transcritical CO₂ systems

Rolf Christensen, Alfa Laval, discussed the design of heat exchangers for heat recovery in transcritical CO₂ systems and water heating in particular, the optimum operating pressure, the minimum approach temperature for different operating pressures, as well as the optimum COP.

According to the presenter, the balance between first cost and performance can be achieved by selecting appropriate operating pressure. Moreover, in order to attain successful transcritical CO₂ heat pump performance:

• The operating pressure must be chosen with consideration to the thermal duty
• The heat exchanger design can not be based on the LMTD method
• A segmented model with local physical properties must be used
• Gas coolers should be long and slim, high-Θ