Heat Exchanger Configuration - Estimate Energy Savings

Good morning everybody.

I am a third year mechanical engineering student doing an internship in a heavy truck garage.We have an air exchanger equipped with a rotary heat exchanger and en electrical heating element. The thing is the heating element (we do not have air conditionning, we only use the exchanger for heating purpose) is located in the fresh air duct before the heat exchanger. This results in a loss of effectiveness (well power gain is lowered) of the heat echanger since it is now feeded with no so cold air.

I was asked to estimate the savings of putting a heating element after the heat exchanger. To do so, I was given output air temperature in various situations (heating element only, exchanger only, both) for a given outside temperature of -8°C. From this data, I calculated that 13.5kW of potential heat recovery is wasted by air preheating. Furthermore, this mounting method limits the fresh air maximum temperature below room temperature (since if it's hotter than room before entering the exchanger it will be cooled there). My problem is when comes the time of estimating the savings. My first try was supose the given condition of 13.5kW saving is my average saving for december, january and february when the 24 hour average temperature is about -8°C. I then calculated the working hours. Multiplying the 13.5kW of savings by the working hours of the system and by the price of electricity I calculated a potential saving.

Do you think this is a sound way of doing this? How do you think I should deal with the rest of the year when the temperature is not the one where I know the working conditions knowing that in this region we have about 3900 heating degree-days?

Thank you all,

Lova Andriamanjay