Cartesian, a Norwegian startup, and SINTEF, Norway’s largest independent research institute, have developed a heat storage system that leverages phase change materials (PCM) as part of a building heating system.

The heat storage system is a bio-based battery encased in a sleek metallic container filled with 3,000kg (6,614lbs) of biowax – a phase change material (PCM) – derived from vegetal oil. In a pilot project this system illustrated its ability to efficiently absorb, store, and release significant amounts of latent heat.

“Our newly developed PCM storage system is incredibly flexible, accommodating diverse heat sources,” said Alexis Sevault, Chief Technical Officer of Cartesian As. “This adaptability is a key to its potential in transforming energy solutions for both buildings and diverse industrial processes.”

Cartesian emerged in 2023 as a spin off from SINTEF Energy Research. The company, jointly owned by its four founders and SINTEF Venture and other investors, represents the culmination of extensive research and collaborative efforts in thermal energy storage technology. Sevault, with his research at SINTEF Energy Research, concentrated on developing compact heat storage technology for decarbonizing buildings and industrial processes.

The launch of Cartesian was marked by the successful operation of its first 200kWh pilot project with propane (R290) heat pumps at the ZEB (Zero Emission Building) laboratory in Trondheim, Norway, showcasing positive results for approximately two years. The lab is run by SINTEF and NTNU (Norwegian University of Science and Technology).

“The company is preparing to launch its second pilot, an integration of CO2 cooling with PCM storage in a supermarket, slated for operation in the beginning 2024. This is one of several upcoming projects aimed at demonstrating the versatility of the technology in various use cases,” said Håkon Selvnes, Chief Project Officer, Incharge of the pilot in Cartesian As.

The first pilot

The initial pilot project integrates an LHS (latent heat storage) unit with the central heating system at the ZEB laboratory. The research was carried out by Sevault and colleagues researchers at SINTEF Energy Research. It is described in the presentation, “200kWh latent heat storage unit using a pillow‐plate heat exchanger: demonstration in an office building,” given  at the 15th Gustav Lorenzen Conference on June 13-15 in Trondheim.

The ZEB laboratory is equipped with solar panels that have a maximum capacity of 184kW, two R290 heat pumps, each with 15kW (4.3TR) capacity, and a 200kWh LHS system with a connection to a district heating network. This lab is an active research center where data from 1,171 sources is collected and stored on a database server every two minutes. Over 14 months, this has amounted to an estimated 350 million data points.

The building heating system relies on a low-temperature hydronic network with two primary heat sources: the air-to-water R290 heat pumps and the return loop of the district heating network. The selected heat pump system has a nominal design to handle temperature lifts on the hot side from 35°C (95°F) to 40 °C (104°F). The return loop of the local district heating provides heat from 39 °C (102.2°F) to 47 °C (116.6°F), depending on the neighborhood heating needs.                          

The hydronic network has several uses, including pre-heating domestic hot water, room radiators, and heat exchangers providing heated air for ventilation. The overall heating system is designed to cover at least the maximum heat demand of the building, calculated to about 26kW (7.39TR), necessary to maintain all rooms in buildings at a comfortable temperature on the coldest days of the year.

Norwegian Startup Develops Novel Heat Storage System

The LHS unit can be charged from heat pumps or district heating and it can be discharged to pre-heat domestic hot water, radiators and ventilation or it can be discharged to support the heat pumps.

Over 14 months, the ZEB laboratory’s LHS unit, containing 3,000kg of biowax and equipped with pillow-plate heat exchangers, was closely monitored. It achieved a peak storage capacity of 234kWh and a charge rate of 13.7kW (3.89TR) over 11 hours. The unit experienced a minimal heat loss, only 0.68% of its daily capacity.

“Data from this period has provided insights into the unit’s efficiency and operational patterns,” said Sevault. “These findings are significant in optimizing energy storage and management for zero-emission buildings.”

The big silver container in the ZEB laboratory contains a PCM that melts at body temperature, or 37°C (98.6°F). “The device contains three [metric tons] of a liquid wax on a vegetable oil that cannot be used as food,” Sevault said. “In the same way water turns into ice, the wax becomes a solid, crystalline material when sufficiently cold. Cold for this particular wax means below 37°C(98.6°F).”

Sevault highlighted that other types of biowax have different melting points, all offering several opportunities for many similar applications, which Cartesian can install according to the need.

Cartesian Compact PCM storage,  Credit: Cartesian As,
Cartesian Compact PCM storage, Credit: Cartesian As,

“Our newly developed PCM storage system is incredibly flexible, accommodating diverse heat sources. This adaptability is a key to its potential in transforming energy solutions for decarbonising buildings and industrial processes.”

Alexis Sevault, Chief Technical Officer of Cartesian

Author Saroj Thapa