Isolve's workshop in Norway. Photo credit: Isolve.
Isolve's workshop in Norway. Photo credit: Isolve.

Norwegian OEM Isolve Is Building a Home HVAC System Around a Dual-Source Reversible CO2 Heat Pump

The heat pump uses air as a heat source during the day and hot water from a thermal energy storage system as a heat source at night.

Norwegian OEM Isolve has developed a home HVAC system that comprises a dual-source reversible CO2 (R744) heat pump, ventilation unit and thermal energy storage system.

The company says its Isolve Air system features a patented compression cycle that enables it to use air as a heat source during the day and the hot water in the thermal energy storage system as a heat source at night when the temperature drops. Dual-source heat pumps are incredibly rare due to their complex designs. In 2023, a joint U.K.-Chinese research team said it had developed and tested a dual-source heat pump that used outdoor air and exhaust air as heat sources. Over in the U.S., scientists at Oak Ridge National Laboratory have created an air-ground source heat pump for buildings.

Outside of its unique design, the Isolve Air isn’t designed to stick out. The company says the entire system is “roughly the size of a refrigerator cabinet” and requires less than 1m2 (10.8ft2) of installation space. 

The sanitary hot water system has two cylinders, one for preheating and one for heating water up to 80°C (176°F). The heat pump has a capacity of 4kW (0.85TR) and a seasonal COP of 4.1 in combined operation (hot water and space heating).

The thermal energy storage includes a 150L (39.6gal) tank equipped with a DX evaporator, plus a heat exchanger connected to the heater/cooler in the air-handling unit. The energy storage capacity is close to 20kWh when operating as a heat source at night, with 14kWh of ice water cooling capacity, with the ice water reaching 0°C (32°F) before thawing.

The ventilation system maintains air quality by equally distributing fresh air into and removing stale air out of the home, with filtration capacities ranging from 100 to 350m3/hour (440.2 to 1326.5gal/hour). A water battery can preheat or cool the incoming air. spoke with Holm about the development of the Isolve Air system, its dual-source design and ice-making capabilities and the company’s future plans. This interview has been edited for length and clarity.

Can you tell us about the development process of the Isolve Air?

Per Erik Holm: The original idea was to leverage daily temperature variations by storing heat during the day in an energy efficient manner and using it at night. One weakness with air-source heat pumps is that heating capacity decreases when heating demand is increasing with falling temperatures. That usually happens at night and in the early morning. That is why we introduced an energy storage unit that functions similarly to a geothermal well. Utilizing the high energy density of water’s phase change (330kJ/kg) allows for compact storage.

The Isolve Air is a dual-source heat pump, which is not a common design. How is it able to use both air and water as heat sources?

The main heat source for the heat pump is outside air. The heat pump is integrated with a balanced ventilation unit. Fresh air and heat source air is supplied via a supply duct. The used air is united with the heat source air after it has exchanged heat with supply air in the AHU, before all the air enters the evaporator in the heat pump unit. An exhaust air fan then sucks all the air out of the house. This is the day mode operation of the unit.

At night when the outdoor air is colder, the air evaporator is bypassed and a DX coil in the thermal energy storage tank serves as the heat source for the heat pump. The water rapidly cools down to zero degrees [Celsius] and starts freezing. This phase change gives approximately 14kWh of evaporation heat at close to zero degrees. At the end of the night the energy storage is frozen to ice. The ice is thawed during the day, either by cooling the supply air in a water battery situated in the AHU or by subcooling the refrigerant R744, depending on house heating or cooling needs.

Actually, we then use outside air, exhaust air and excess room heat as heat sources. The energy storage serves as a facilitator for this.

R744 heat pump
Isolve CEO Per Erik Holm with a prototype of the Isolve Air. Photo credit: Isolve.

The ice storage system is another unique element of the Isolve Air. How does it work, and does it require a de-icing mechanism?

P.E.H.: The Isolve Air system can use ice generated overnight for cooling during the day. This is done by running water through the coils, which are cooled by the refrigerant. The cold coils freeze the water around them, creating pellets of ice. Offering free cooling that can be used during peak electricity times is a big asset.

The system originally included a thermal solar collector to thaw the ice during the day, recharging the energy storage. If there was no sun, the ice would be thawed by subcooling the CO2  in the energy storage DX coil. When we integrated the AHU in the system, we could use the excess heat in the building to thaw the ice instead of a thermal solar panel. Additionally, the system uses a two-temperature hot water system, significantly enhancing the efficiency because up to 60% more of the hot water is heated by the pump, thereby increasing the gas cooling from the hot water heating.

How does Isolve’s heat pump system integrate with home ventilation to optimize the use of R744 for heating, cooling and hot water production?

P.E.H.: Integrating the system allows for the utilization of all spare heat within it, offering a significant advantage in energy efficiency and heat recovery. Integration with the heating system makes it possible to reduce the gas cooler outlet temperature by 4–5K, which is essential for a high COP in a transcritical CO2  heat pump. Moreover, integration helps save installation space by consolidating components into a single unit, often referred to as “one box.”

A potential drawback of this design is that the unit can become quite heavy and challenging to maneuver through doors or lifts. However, Isolve Air is designed modularly, which means it can be assembled in parts, easing transportation and installation.

What partnerships or collaborations is Isolve pursuing to enhance the development and distribution of its technology?

P.E.H.: The goal is to establish this system as the preferred choice for builders of modern, low-energy houses. Our initial focus is on Norway and Sweden. Our prototypes have been built by Ensyro, a ventilation unit manufacturer, and us. We are looking for a company that sees the opportunities in a completely integrated system and that has the ambition to play an important role in the green shift. My dream is to see zero-energy houses for normal people with photovoltaic solar panels on the roof, electric cars with vehicle-to-grid battery chargers in the garage and Isolve Air systems for the indoor comfort system.

“My dream is to see zero-energy houses for normal people with photovoltaic solar panels on the roof, electric cars with vehicle-to-grid battery chargers in the garage and Isolve Air systems for the indoor comfort system.”

Per Erik Holm, CEO of Isolve

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