PV-VENT systems and Heat Recovery Ventilation (HRV)

A nev type of counterflow heat recovery ventilation systems has been developed in Denmark with very impressive advantages compared to common ventilation systems:

# It has a electro/thermal ratio (electricity use compared to heat saving) of at least 1:8 and when PV-modules are applied it grows to about 1:14.
# The rate of heat recovery is at least 80-90%
# The power consumption required per housing unit is maximum 30 W
# The noise level from the installation is less than 25 dB
# The installation is equipped with a separate power consumption meter so it is possible to controll the electricity consumption

The background and perspectives of this development are as follows:

Ventilation systems are usually of a rather poor standard in the present house building in Denmark. Both out of consideration for the indoor climate and to reduce the energy consumption it is therefore important to test new designs with electricity saving mechanical ventilation and use of heat recovery, where fresh ventilation air can be preheated and utilise up to 80-90% of the heat in the used exhaust air.

The traditional solution to air problems (dust, moisture, mould fongus, etc.) is often to increase the exhaust ventilation. But in this way the heating bill will also be increased as the extracted air has to be equal to the supply of fresh air, e.g. by use of gap valves in windows. However, these have the disadvantage that they result in cold draught and therefore they will often be closed, resulting in poor ventilation. An energy saving solution is here to use counterflow heat recovery. It is, however, important to look at the entirety and estimate how much additional energy is going to be used compared to the heat saving.

On the basis of the work with building integrated PV-modules and new and more energy efficient ventilation systems with counterflow heat recovery, Cenergia obtained in 1997 funding from the EU-Joule programme to a research and development project in this field, the so-called PV-VENT project. This was in cooperation with the housing association FSB, the ventilation company AirVex Denmark (former TermoVex Denmark), the Technological Institute in Denmark Fortum in Finland, NTNU in Norway and Ecofys in Holland. Subsequently the PV-VENT project obtained supplemental grants from the Danish Energy Agency.

PV-VENT ventilation systems utilise for one thing electricity saving ventilation systems based and a new type of DC-ventilation unit with very low electricity consumption, as low as 20-25 W per dwelling for systems with both air supply and exhaust. Compared to this, ventilation systems with just exhaust ventilation must according to the Danish Building Regulations have an electricity consumption of maximum 87 W, which is considerably more.

The special thing concerning PV-VENT ventilation systems is that building integrated PV-modules are utilised to cover a considerable part of the already reduced electricity consumption. Already at present, a PV-VENT system can in many cases be installed with an acceptable economy and it has been calculated that the ratio between electricity consumption and heat saving will be 1:15, whereas it will be approx. 1:3 for usual ventilation systems, which is considerably poorer.

Calculations that have been made in connection with the PV-VENT project show that when ventilation with heat recovery is used in renovation projects, where the windows and the heating system are going to be replaced, it can at present be done with a very good economy.

The additional expenses compared to use of ordinary exhaust ventilation systems are typically between DKK 9,500 and 11,500 per dwelling. At the same time, experiences from completed projects show that it ought to be possible to save at least DKK 4,000 per dwelling when the heating system can be sized to approx. half the usual heating effect and there are no demand to place radiators below the windows. The actual additional investment will therefore only be between DKK 5,500 and 7,500 per dwelling. This is going to be compared to a possible saving of DKK 1,000 on the heating bill per dwelling, excl. the latest increase in oil prices. At the same time it ought to be possible to save DKK 400 per dwelling per year due to reduced electricity consumption compared to usual solutions that just meet the requirement in the Danish Building Regulations. In this way a much improved indoor climate and a very good economy are obtained, with a simple payback time between 4 and 6 years.

Even though a good economy can be obtained, it has, however, turned out to be difficult to introduce these designs in a number of projects, partly because it is something new, where one would be cautious about sizing the heating system on the basis of the reduced heating demand and partly because no model for the financing has been developed yet, which ought to take into consideration that a considerable part of the energy saving will be in the tenants' favour so a small rise in the rent will be acceptabel.

Already in 1993, Cenergia received funding for the first EU-funded demonstration project with PV-modules in Denmark. 200 m² PV-modules should have been installed in Ballerup municipality, but instead they were installed in Valby and at Vesterbro in Copenhagen. Subsequently, an additional project with 200 m² PV-modules has been initiated with EU-funding, this time at Frederiksberg.

As a partner in the EU research and development project PV-VENT, the housing association FSB must be said to have been very progressive. Now they have shown in a practical renovation project both how to integrate ventilation systems in connection with renovation of concrete building from the 1960'ties and how this can be combined with use of building integrated PV-modules.

Within the PV-field, the team behind the PV-VENT projects has been very innovative as we in 1998 arranged the first architectural competition in Denmark that was directly directed to using building integrated PV-modules in practice.

The Danish Energy Agency has subsequently also given PV-modules priority from the beginning of year 2000 in connection to the new development and demonstration programme for building integrated PV-modules, where DKK 30 million are going to be spent during three years. At the same time work is going on to initiate the new Solar 2000 implementation plan for PV-modules, where it is intended to install approx. 25,000 m² PV-modules in 3-4 years.


A photo with built-in, specially developed only 25 cm thick counterflow heat recovery units from TermoVex Denmark before installation of PV-modules to operate the ventilation fans and the temperature dampers to avoid overheating.


The solar wall with built-in thin counterflow heat recovery units in the corner block in Tøndergade 1, Sundevedsgade 14 (Copenhagen) is here shown both before and after the installation of the PV-modules to operate the ventilation unit and the air damper.


Example of a common PV-assisted ventilation system for three apartments. A shaft in connection with a common ventilation system is going to have space for both exhaust ducts and air supply ducts. Leading sizes are shown.

Fig. 3. Ventilation chimneys at the housing estate Lundebjerg in Skovlunde equipped with PV- modules which are supplying the electricity for the fans of heat recovery and exhaust ventilation. Theese are prototype systems for 27 appartements.

Indsættes: "Lineagaarden"
Fig. 4. Solar ventilation towers at Lineagaarden, Copenhagen with solar assisted heat recovery ventilation

Indsættes: "Ventilationsenhed på fladt tag&."
Fig. 5. PV-VENT system for flat roofs installed at citicens common house in Valby, Copenhagen.