PVT readiness of heat pumps – definition and market overview

PVT collectors can make highly efficient heat sources for heat pumps, but they also create special technical challenges. Unlike conventional heat sources such as ambient air or shallow geothermal energy, PVT collectors deliver a much wider and more dynamic temperature range. In summer, the brine temperature can rise to 70 °C, while on cold winter nights it can fall below -15 °C. Heat pumps must therefore operate reliably under strongly fluctuating conditions. Not all devices on the market are designed for this. To address this issue, the German research initiative IntegraTE-XL developed a definition for PVT readiness of heat pumps. The consortium also analysed which products already meet these demanding requirements through adapted control systems or refrigeration circuit designs.

To address this issue, the German research initiative IntegraTE-XL developed a definition for “PVT readiness” of heat pumps. The consortium also analysed which products already meet these demanding requirements through adapted control systems or refrigeration circuit designs.

The researchers distinguish between two system configurations. In the first, PVT collectors act as the sole heat source for the heat pump. In the second, they work together with a geothermal source. In both cases, the consortium assumes that the electric heating element should only operate in emergency situations. The goal is mainly monovalent operation or bivalent-parallel operation during extreme weather conditions.

What makes a heat pump PVT ready?

The consortium has agreed on the following definitions.

Brine/water heat pumps are PVT ready with PVT collectors …

…as the sole heat source if the approved source temperature range of the heat pump is designed for the temperatures supplied by the PVT collectors under typical local operating conditions. The maximum thermal output of the heat pump for the lowest permissible temperature of the heat source and a typical operating temperature (e.g. 55 °C for domestic hot water heating) must be specified in addition to the standard operating points according to DIN EN 14511. In Germany, the source temperature range is between -15 and 70 °C. The maximum inlet temperature of the evaporator (e.g. 30 °C) can be maintained, for example, by a special design of the refrigeration circuit or by a mixing valve controlled by the control system integrated in the heat pump.

… as an additional heat source, if the heat pump allows the connection of several heat sources. In addition, the heat pump control system must be able to choose the heat source with the higher temperature while maintaining the approved source temperature range of the heat pump.

Nine suppliers joined the survey

The IntegraTE-XL consortium sent a detailed questionnaire to 18 heat pump manufacturers. Nine companies eventually participated and submitted products for evaluation: ait-deutschland, Ecoforest, Heim AG Heizsysteme, Heliopac, M-TEC, Nibe, Ratiotherm, Triple Solar and Waterkotte. Since ait-deutschland and Waterkotte belong to the Nibe Group, the survey effectively covered several device families from one corporate group.

The nine suppliers are listed in Table 1 and 2 with one or two selected heat pump types each. The selected heat pumps ranged from 5 to 56 kW thermal output and covered applications from single-family homes to larger residential and commercial buildings. All suppliers confirmed that using PVT collectors as a heat source does not affect warranty conditions.

Large differences in allowable temperatures

One key criterion for PVT readiness is the allowable inlet temperature range. Most suppliers already support very low source temperatures between -12 °C and -20 °C. One exception is the Solerpac model from the French company Heliopac, which is only approved down to -5 °C. Heliopac therefore positions the unit only for systems with an additional heat source (see table 2).

At the upper end of the temperature range, the differences are even larger. Depending on the device, the maximum approved source temperature varies between 20 °C and 55 °C. Some suppliers – including Heliopac, Ratiotherm and Triple Solar – even allow short-term inlet temperatures of up to 70 °C.

To protect the heat pump from excessive temperatures, some systems require a mixing valve in the brine circuit. The important question is whether the heat pump controller can manage this valve automatically. Heim, M-TEC and Triple Solar’s PVT heat pump 10s already offer this function as standard and therefore meet the consortium’s definition of PVT readiness. Ecoforest uses a thermally driven mixing valve that operates without active control.

Other manufacturers solve the issue directly inside the refrigeration circuit. Ratiotherm, for example, uses an internal intermediate circuit that works like a built-in mixing valve. Triple Solar’s heat pump 5.0 relies on an electronically controlled expansion valve and a dynamic superheat controller that can start the refrigeration circuit at low compressor speed at temperatures above 50 °C. The refrigeration circuit then circulates at minimum compressor speed until the refrigerant has cooled to 50 °C. This process has only been tested with the company’s own Triple Solar PVT elements.

Table 1: Overview of the functions that are required for PVT ready heat pumps with PVT as the sole heat source. Six products meet the PVT ready definition as set by IntegraTE-XL.
Source: Manufacturer’s specifications

PVT and geothermal energy: a promising combination

For older buildings or large properties with limited roof space, combining PVT collectors with geothermal energy can be an attractive solution. In these systems, solar heat from the roof can regenerate the ground source during sunny periods, allowing the geothermal boreholes to be significantly smaller. Depending on the system design, reductions of up to 40 % are possible.

In this setup, intelligent source management becomes especially important. The heat pump must be able to switch between the geothermal source and the PVT collectors depending on which provides the higher temperature.

According to the survey, only three products currently meet all criteria defined by IntegraTE-XL for this operating mode (see table 2): M-TEC’s BHPA412, Ecoforest’s ecoGEO Pro 2-10 and Triple Solar’s PVT heat pump 10s.

Some other manufacturers offer partial solutions. Ratiotherm, for example, provides an optional control unit that enables regeneration of the geothermal source through the PVT collector circuit. Heim, Heliopac and Waterkotte rely on external PVT controllers for source management. However, the consortium did not classify these solutions as fully PVT ready because the control is not integrated into the heat pump itself.

Table 2. Overview of the functions that are required for PVT ready heat pumps with PVT as additive heat source. Three products meet the PVT ready definition. Source: Manufacturer’s specifications

Market still at an early stage

The survey shows that several manufacturers have already adapted their heat pumps to the demanding operating conditions of PVT collectors. However, the study also highlights that further development is still needed, especially for systems using multiple heat sources. According to the IntegraTE-XL researchers, a growing market for PVT systems could help accelerate innovation and encourage more manufacturers to optimise their products for this application.

Websites of organisations mentioned in this news article:
IntegraTE-XL: https://websites.fraunhofer.de/integrate-xl/
ait-deutschland: https://www.aitgroup.com/de
Ecoforest: https://ecoforest.com/de/
Heim Heizsysteme: https://www.heim-ag.ch/heizsysteme/
Heliopac: https://heliopac.fr/
M-TEC: https://www.mtec-systems.com/
Nibe: https://www.nibe.eu/de-de
Ratiotherm: https://ratiotherm.de/en/
Triple Solar: https://triplesolar.de/
Waterkotte: https://www.waterkotte.eu/