Heat pump retrofit
This estate is typical for current requirements related to the old building's upgrade to renewable energy for heating and DHW preparation using a heat pump. A square courtyard with 5 separate units - office, main housing and 3 smaller annexes.
fish pond serves as a heat source, fitted with geothermal ring collectors at the bottom of the pond. A modulating 30 kW heat pump covers heating, cooling and DHW.
"Heat pumps love direct heat circuit charging"
"Technically speaking, heat pumps are high flow systems, they replace overtemperature with volume flow. But they don't want to push for higher temperatures than absolutely necessary. Every extra Kelvin between source and sink cost about 2.5% efficiency, so about 25% at 10K elevation", as explained by Ing. Arne Komposch of wp-plus.
The hydraulics-related challenge is to constantly ensure the required minimum flow for the serveral independent, mixed heating circuits of the heat pump. The simple variant, at cost of efficiency, is the hydraulic disconnect via the parallel attached buffer tank. This was not an option due to reasons stated above, and as such, the TA controller came into play: The heating circuit with the highest demand sets the heat pump's heating curve. It's set to constantly drive the mixer valve to fully open to ensure the minimum flow, enabling direct heating circuit charge by the heat pump, without a bypass of the heat flow past the heat sink.
The clever part of controller and hydraulics is that, as per future serial renovations or changing user behavior, the leading circuit and be switched freely, without needing new parameters on the controller.
A concrete example
Of the 5 circuits, 4 were already equipped with surface heating during earlier renovations, the fifth one still runs on radiatiors. That's why it currently still transmits the flow demand to the heat pump. Now, if in the near or far future a renovation of this unit too will switch it to surface heating, the demand/heating curbe of this circuit will sink automatically, and a different will probably be lie above it. The controller then automatically controls it as a modulating circuit with fully opened mixer valve. A future renovation of the last unit with radiators or a change in user behavior in a different unit, the controller automatically adapts to the new requirements - without additional changes to parameters being necessary.
Since the 30kW brine-water heat pump runs modulated for a rather small radiator heating circuit of an unrenovated annex building, a small row tank is spliced into the return line. This only increases the active water volume in the heating circuit and does not cause any blending of the medium. It increases the heat capacity that the heat pump can influence. During DHW charging operation, this row tank is bypassed using a 3-way valve.
The TA controller is not only used for controlling the system, but also for visualisation, monitoring and remote maintenance. Such complex systems always require initial support of operation beyond basic commissioning - monitoring and optimisation over the first winter are a standard package.
The demand-optimised requests at the heat pump occurs via the energy integral of the temporal change of the temperature difference between set and real value. The modularly enabled output range of the heat pump is set by the required heating load according to current outdoor temperatures (building curve). Within this control range, the output is matched by a PID controller which supplies the set value to the compressor.
A DHW tank is assigned to both main utilization units. The tank equates to a fresh water station. The heat is stored in the form of hot water in the tank, while tapping the hot tap water, the plate heat exchanger exchanges heat to the cold tap water in transit.
The heat pump and TA controller communicate via Modbus. For comfortable monitoring, remote control, data logging and visualisation of all controller instances, a CMI is in use.
Heat metering and energy cost calculation occur seperately for both main units. Energy load (thermal and electric), current load (thermal and electric), efficiency factor, annual COP are visualised in real time. By entering current power cost, the annual energy costs are displayed.
Using the geothermal collector ring as a storage independent of the current temperature, a sector coupled mode of operation (grid-lead in the deep winter, PV-lead during warmer temperatures) is possible for the future.
This project was realized by:
RGK e.U.
Ing. Arne Komposch
Radgrabenstraße 4
4611 Buchkirchen
Austria
https://www.ringgrabenkollektor.at
