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Advances in Ventilation Heat Recovery

An assessment of peak loads shaving using renewables

Time: Wed 2022-05-25 13.00

Location: B1, Brinellvägen 23, Stockholm

Video link:

Language: English

Subject area: Civil and Architectural Engineering

Doctoral student: Behrouz Nourozi , Hållbara byggnader, Fluid and Climate Technology

Opponent: Professor Carey Simonson, University of Saskatchewan, Canada

Supervisor: Docent Sasan Sadrizadeh, Hållbara byggnader; Dr. Adnan Ploskic, Hållbara byggnader

QC 20220422


The building sector accounts for approximately 40% of total global energy usage.In residential buildings located in cold climate countries, 30-60% of this energy isused for space heating, 20–30% is lost by discarded residential wastewater, and therest is devoted to ventilation heat loss.Sweden experienced a construction boom during the so-called Million Programme(MP) in the 1960s and 1970s. A retrofit requirement of buildings constructed duringthis era shifted from pure exhaust ventilation to mechanical ventilation with heatrecovery (MVHR), which peaked in Swedish dwellings between 1990 and 2000. It isestimated that 43% of Swedish multi-family buildings built during this decade wereequipped with MVHR systems. A common problem with efficient MVHR systemsis frost formation during cold winter hours when cold outdoor air and humid, warmreturn air exchange heat in the air handling unit. Outdoor air preheating usinglocally available renewable heat sources has been an alternative solution to preventfrost formation in the heat exchanger.The main objective of this work was to investigate the solutions for improving theperformance of MVHR systems during the coldest periods of the year. The primaryfocus was frosting, a critical problem in MVHR units that operate duringcold periods. The recovered heat from discarded wastewater and the local geothermalenergy were the two investigated renewable heat sources used to preheat theincoming cold outdoor air to the MVHR in order to prevent frost formation on theheat exchanger surface.The performance of the suggested outdoor air preheating systems and the impactof air preheating on the entire system’s thermal efficiency were evaluated by TRNSYSdynamic simulations. Temperature control systems were proposed based onthe identified frost thresholds to efficiently use the limited thermal capacity ofwastewater and maintain a high heat recovery of MVHR. Two outdoor air preheatingsystems configurations with temperature-stratified and -unstratified tanks weredesigned and compared. A life cycle cost analysis was applied to further investigatethe cost-effectiveness of the studied systems.Detailed heat transfer simulation of the ventilation heat exchanger revealed thatwhen condensation occurred in the heat exchanger, the heat transfer rate betweenthe return airflow and the plate increased significantly. This was reflected by asharp increase in the plate temperature, increasing the supply air temperature tothe building. Monitoring the relative humidity of the airflow at the inlet of theheat exchanger and using the onset values of frosting/condensation suggested in this work will allow a more precise and proactive prediction of freezing and moreefficient utilization of outdoor air preheating resources.The results obtained from the simulation of building energy usage indicated thatresidential wastewater had sufficient thermal potential to reduce the defrosting needfor MVHR systems (equipped with a plate heat exchanger) in central Swedish citiesto 25%. For colder regions in northern Sweden, the defrosting time was decreased by50%. The suggested temperature control systems ensured high MVHR temperatureefficiencies above 80% for most of the heating season, while the frosting period wasminimized. LCC analysis revealed that outdoor air preheating systems equippedwith temperature stratified wastewater tank and an unstratified storage tank couldpay off their investment costs in 17 and 8 years, respectively.