Dipl. Eng. July Armianov

It seems the discussion for the necessity of energy recovery systems is over not only in Western and Northern European countries, but also in Bulgaria. For the people who made serious effort for convincing and proposing legislative changes in the last 20 years, it is a real success. Today it is rare to see a ventilation or air conditioning project without energy recovery (recuperation or regeneration).

The economic pressure (rise of energy prices) surpassed the legal regulations. Designers and investors reacted more adequately than the legislators.

The next important step is the choice of the right energy recovery system. The right choice means a cost-effective choice that doesn’t compromise the microclimate.

In this study is used a method in which to the initial investment are added energy costs and maintenance for the next 10 years.

There are many factors in the choice of energy recovery systems:

• The working hours of an installation (hours per day) is the main factor.
• External climate conditions (different approach has to be taken whether considering Norway or Southern Italy).
• The price of energy depending on the source (diesel fuel, electricity, gas, pellets).
• Initial investment.

The presented cases are produced using the following data:

• Climate data for Sofia during the winter: Temperature in the premise 22°C, Relative humidity – 25%.
• Primary energy source – electricity: 0,2lv/kWh (0,1€/kWh)
• Annual electricity price increase: 3%.
• Annual service costs: 5% of the initial investment.
• Three models for working hours of the system:
  • 24 hours
  • 10 hours
  • 6 hours
• Air volume of the ventilation (air conditioning) system – 4000m3/h.
• Initial investment cost – according to real systems of same producer
• There are four types of HVAC systems (Fig. 1) working with 100% fresh air and supplying it at 22°C to the premise. The first two systems do not have an option for cooling because it isn’t necessary or it is already done by the AC installations.

Fig. 1: Energy recovery ventilation and air conditioning systems (heat recovery)

• Fig. 2 demonstrates the energy costs referring to the above four types of installations. The green zone shows the saved energy through heat recovery. The red zone – the energy needed for heating the fresh air to temperature of the premise (not to affect the internal thermal balance). The yellow zone shows the energy needed to prevent freezing of the high efficiency heat exchanger.

Fig. 2: Annual energy costs for ventilation systems with heat recovery

• Fig.3 represents the financial results of the four types of HVAC systems, including initial investments, energy expenses and maintenance for the next 10 years.

Fig. 3: Expenses for initial investments and accumulated expenses for energy
and maintenance for 10 years in different working hours of the system

The conclusions from the above analysis are summarized below: 

• The longer an installation is working, the advantages of high efficiency heat recovery ventilation increase.
  Preparing economical/technical analysis has to be a standard practice.
• If cooling is not required or the AC system could cope with the additional load, the most efficient and economical solution system is the S2. Because of that, in recent years, the high efficiency heat recovery ventilation TANGRA EVB – HiE prove to be more popular choice for office and commercial buildings as well as private residences.
• Regardless of the working hours an S4, air handling units with external heat pump module, have more advantages than an S3, air handling units with heat pump incorporated, due to the higher energy saving from the relatively cheaper and passive element (fig.2). In the same time the heat pump capacity can be reduced.

The described here is valid if the systems work with equal mass flows (supply and exhaust), the supply temperature is equivalent to the temperature of the premises and condensation is not expected.
If the systems supply air with temperature higher than this of the premises, the systems with heat pumps would have additional advantages over the conventional heat recovery ventilation.
The higher relative humidity of the exhaust air could increase the energy saving effect of all systems because of the condensation in the heat exchanger and heat pump module of the S3.
Unfortunately, sometimes the sale contrivances are taken into consideration more than the professionalism and brochures, demonstrating efficiency higher than the theoretically possible are available. Due to this reason, we would like to advice investors to ask and pay for technical/economical analysis, which will help them make the right decisions.