Development of Propane-Based Heat Pump Solutions to Replace Gas and Oil-Fired Systems

In the “LCR290 – Low Charge HP Solutions” project, Fraunhofer ISE collaborated with 20 companies to investigate practical heat pump solutions using the natural refrigerant propane (R290). These solutions are intended to replace gas and oil heating systems in existing multi-family residential buildings–ranging from individual-floor heating systems to central systems installed indoors or outdoors. The concepts account for different building types, heat sources, and space constraints, paving the way for a safe, efficient, and cost-effective rollout of climate-friendly heating technology in the housing sector’s existing building stock.

© Fraunhofer ISE
Visualization of an example building with a central air-to-water heat pump and decentral-ized domestic hot water reheating.

Initial Situation

In the energy system of the future, heat pumps will be the dominant heating technology. The housing sector therefore needs standardized solutions for typical applications when replacing gas and oil heating systems in multi-family buildings. Propane is recognized as a refrigerant solution in the heat pump industry and is widely used in outdoor heat pumps. For indoor installations, Fraunhofer ISE is researching practical solutions, as special safety requirements apply to propane charges exceeding 150 grams.

Objective

The objective of “LCR290” was to develop scalable system solutions for the rapid, safe, and cost-effective deployment of propane heat pumps in existing multi-family residential buildings. The focus was on replacing gas-fired individual-unit heating systems on a per-unit basis, as well as retrofitting fossil-fuel-based central heating systems with central heat pumps designed for indoor or outdoor installation. The concepts were designed to ensure safe operation–including through the reduction of refrigerant charges–and to guarantee good overall efficiency. In addition, specific constraints of the existing apartment building stock, such as minimally intrusive retrofitting while the buildings are occupied and limited available space, had to be met. Direct exchange between manufacturers and housing companies was a key component of the project.

Approach

Fraunhofer ISE first analyzed sample buildings from the portfolios of the participating housing companies and derived technical requirements from them. In a collaborative project with manufacturers and the housing industry, modular concepts were developed for heat sources, heat pumps, storage systems, hydraulics, control systems, and heat distribution. The study examined decentralized floor-level solutions with hydraulically connected heat sources, as well as central brine-to-water and air-source heat pumps for indoor and outdoor installation. Building on the LC150 preliminary project, the team reduced the propane fill volume and adapted this approach for higher capacities. Laboratory demonstrators, along with efficiency and defrosting tests, provided the basis for performance evaluation. In more than 100 leakage tests, the team evaluated five safety concepts. In addition to the standard-compliant ventilated enclosure, chimney and drainage concepts were examined. The results can be incorporated into manufacturers’ product developments and future planning tools.

© Fraunhofer ISE
Knowledge on refrigerant charge reduction gained from the LC150 project will be transferred to sys-tems with larger capacity in the new project.

Results

For outdoor central heat pumps, a prototype was developed whose propane charge is 20 percent lower than that of comparable units, while efficiency increased by 15 percent. A compact, indoor-installed ground-water heat pump with a heating capacity of 28 kW was also developed, which can be installed even in existing basements with limited space. The presented demonstrator of a residential heat pump can provide up to 4.9 kW (B0/W55) and is significantly smaller than a standard gas boiler.

More than 100 safety tests confirmed the concept of the ventilated enclosure as described in the standards EN 378 and IEC 60335-2-40, thereby providing manufacturers with the assurance needed to develop new products for indoor installation.

Overall, the project confirmed that heat pumps can replace gas and oil heating systems in typical existing multi-family residential buildings, provided that the heat pump, storage tank, hydraulics, control system, and heat distribution are tailored to the building.

The solutions developed are being incorporated into the follow-up project “MAMMUT,” which aims to simplify the planning, design, and quotation process for heat pump systems in multi-family buildings.