Heat Pump System with Water / Ice Storage Source LEO_2: How Does It Work?

(Last changed: 2018-10-28.)

Each heat pump needs a source in the vicinity of the building. Energy is extracted from this rather cool source and transported to the heating loops and 'pumped' to a higher temperature. The required electrical input energy and thus the efficiency of the heat pump is sensitive to the temperature of the heat source and the temperature of the heating water: The higher the source temperature and the lower the heating water supply temperature the higher is the efficiency of the heat pump.

The heat pump system LEO_2 - using an 'ice storage' plus 'energy fence' heat source - was designed for optimization of these two critical temperatures: punktwissen has developed custom control logic for managing all components of the system: Heat source, heat pump, and heating circuits.

punktwissen Heat Pump System

(1) Brine-water heat pump. (2) Energy-efficient brine pump. (3) Underground water/ice tank. (4) Ribbed pipe unglazed solar collector. (5) 3-way valve: Diverting brine to flow through the collector, depending on ambient temperature. (6) Hot water is heated indirectly using a large heat exchanger in the tank. (7) Buffer tank with a heat exchanger for cooling. (8) Heating circuit pump and mixer, for controlling the supply temperature. (9) 3-way valve for switching to cooling mode. (10) 3-way valve for toggling between room heating and hot water heating.


The operating statuses throughout one year are described in this post that features a slide-show of the hydraulic schematic, with the important temperatures at different points indicated:

[ ► Image gallery: Monthly operating statuses, typical temperatures  ]

LEO_2 uses a simple - cheap - off-the-shelf brine-water heat pump without 'smart' features. (The same type of heat pump is used with ground loops.)

The heat source is a combination of a water / ice tank ('Ice Storage') and an unglazed solar collector ('Energy Fence'). The heat exchanger in the water tank is built from ribbed pipes as well.

[ ► Water tank heat exchanger and solar collector, built for a new building ]
[ ► Re-purposing an existing cellar. Fence-like design of the collector ]

Harvesting ambient energy. The solar collector extracts energy primarily from ambient air; solar radiation is only a secondary, minor source of heat. In the water tank energy is harvested using these processes:

Even in the winter months the collector provides for the major part of the ambient energy the heat pump needs. The tank is used to buffer energy for days or weeks.

[ ► Energy balances for the different sources ]

Heat source and control. Depending on ambient temperature and tank temperature an automated 3-way valve diverts the brine flow either to the heat exchanger in the tank only or brine flows through both the solar collector and the tank. Energy not needed for heating is used for melting ice in the tank or for heating up the water again (e.g. during warm spells of weather in winter).

Heating mode. A pump for heating water is built into the heat pump. Depending on the state of an external 3-way-valve the heating water flows either into the hygienic hot water tank or the buffer tank for space heating.

The temperature in the hygienic storage tank does not need to be higher than about 50°C (No issues with legionella!).

The temperature in the buffer tank is kept low by optimized control of the heating circuits. One or more heating loops are integrated via standard mixers and circuit pumps.

Cooling mode. In summer the heating loop becomes the cooling loop. Brine is diverted to the heat exchanger in the buffer tank by a 3-way valve, so that the water in the underground tank is used for passive / 'natural' cooling. Mixer control makes sure that the supply temperature of the heating circuit does not drop below the dew point temperature. During cooling the heat pump can heat hot water: This helps passive cooling as energy is extracted from the underground tank. Prerequsite: A large-area heating system such as floor or wall heating.

[ ► Using the underground water tank for passive cooling ]

Added value. The collector can also be used as a 'fence' or visual protection; the water tank can also be used as a cistern.

Data: Our own system for room heating, water heating, and cooling is up and running since autumn 2012. We have monitored it in detail and provide detailed measurement data. Due to the combined heat source the seasonal performance factor is well above 4 [*]. This documentation of measurement data is being updated every few months:

[ ► Measurement data since 2012 - 2018 and systems' configuration details ]

[*] We use the European definition of SPF / COP - that is the same units for heating energy and electrical energy. 4 means 4 kWh heating energy per 1 kWh electrical energy.