MWM heat pump for single-family and two-family homes

A special MWM Moment from Heinrich Baas

Energy-saving heat pumps for single-family and two-family homes are not a new topic that emerged because of the current energy crisis and climate change. In fact, the oil crisis in the 1970s had already highlighted the need for more cautious use of energy and raw materials. Many in Germany still remember the driving bans on Sundays and the empty highways where pedestrians were free to roam. Back then, engineers already started thinking about technical solutions to the energy problem. One of the options identified for the significant reduction of the energy consumption and of the dependence on raw material was the use of heat pumps (HP).

Of course, MWM also took up the topic and supplied gas and diesel compressor sets to drive heat pumps used in the industry, sports facilities, indoor swimming pools, and slaughterhouses (see images 1 and 2).

However, the commercial use of heat pumps was not the only area focused on: In 1979, MWM and two partner companies started developing their own small heat pump for single-family and two-family homes. In early 1980, I joined the team as a test engineer and took over the HP test benches in what was then MWM Factory II on Friesenheim Island. Additionally, I supervised the company’s own field-testing facilities and those of the partner companies in selected residential buildings. This was my first and also challenging task after I graduated from university. Thanks to the training I had received in the core subjects of combustion engines, process engineering, and heating and air-conditioning technology, I was well prepared for this task.

A total of seven heat pumps were built with the D 893-1 diesel engine, three for the test benches and four for field testing. One additional heat pump was equipped with the G 893-1 gas engine for TWS Stuttgart. The D 893-1 has an output of 8.0 kW at 2,200 rpm (images 3 and 4).

The small heat pump was a monovalent air/water HP that extracted the environmental energy from the outside air and fed it into the heating circuit via a refrigeration circuit with the engine exhaust heat. The heat pump was able to provide the buildings with sufficient heat all year round. The heat output was 17 kW at an outside temperature of -15°C, enough for a single-family or two-family home (image 5).

The results of the field-testing facilities showed that compared to a boiler, some 50 percent of the fuel used could be saved. Nevertheless, a comparison of the different technologies showed that at the bottom line, the cost savings would only amount to about DM 100 a year. This was mainly due to the very low fuel price of DM 0.078/kWh at the time, which made the deployment of a more maintenance-intensive and failure-prone heat pump rather unattractive. Moreover, there was no tight-knit service network (image 6). Therefore, the innovative project was abandoned at the end of the 1983 heating season, and the focus shifted to the development of cogeneration power plants.

Within the scope of this HP project, the first electronic MWM engine control was developed in collaboration with the Mannheim-based company Sine. This control was also used in cogeneration power plants. Demand for HP drives with a higher output also declined due to the low cost of raw material in Germany. Nevertheless, other countries, e.g. in Scandinavia, continued to benefit from this economical technology. Today, in 2023, the majority of households in the region are with heated with heat pumps.