An Experimental Analysis of Heat and Energy Recovery Ventilator Systems for Building Energy Applications

Abstract

Heat and Energy Recovery Ventilators (H/ERVs) use outgoing stale air to pre-treat incoming fresh air from the outdoors, effectively reducing the energy demand associated with air conditioning. The objective of this study is to evaluate the performances of Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs) under identical airflow rates and temperature conditions in the cooling mode, which coincides with hot and humid outdoor air conditions. This study also conducts an economic and carbon-footprint analysis with the latter providing insights into the environmental impact.

Thermal performance testing of H/ERVs was conducted using an experimental testing facility recently designed and built at the RELLIS Energy Efficiency Laboratory (REEL). In a series of tests, the HRV and ERV units were subjected to varying airflow rates and temperature differences between the indoors and outdoors for the purpose of investigating their thermal performances for a range of outdoor temperatures. In type 1 tests, the airflow rate was varied from 110 to 210 CFM while keeping a constant temperature difference of 20℉. In type 2 tests, the temperature difference was varied from 12 to 33℉ at a constant airflow rate of 210 CFM.

The resulting measured data was used to determine and analyze key performance parameters such as heat transfer rates, first law effectiveness, second law efficiency, as well as the economic and environmental impact. For example, with an increase in airflow rates, from 110 to 210 CFM, the sensible effectiveness of the HRV decreased slightly from 61% to 59% while the decrease of the total effectiveness for the ERV over the same flow range was significantly larger, being 59% to 45%. An increase in temperature difference from 12 to 33℉ increased the sensible effectiveness of the HRV from 56% to 62% while the total effectiveness of the ERV increased from 27% to 51%. Even though the ERV outperformed the HRV, it should be noted that the measured performance of the ERV may not represent its full capability, because the test facility did not have the ability to control humidity.

The first law effectiveness is characterized by different parameters for the HRV and ERV, in that energy transfer is heat for the HRV, and heat and water vapor for the ERV, making effectiveness an unsuitable metric for comparison. Second law efficiency serves as a standard measure for comparing the performance of both the HRV and ERV with a common metric. The second law efficiency has an inverse relationship with both the airflow rate and the temperature difference. With the increase in airflow rate from 110 to 210 CFM, the second law efficiency of the HRV decreased from 82% to 30% while for the ERV it decreased from 95% to 49%. The increase in temperature difference from 12 to 33℉ led to a decrease in the second law efficiency of the HRV from 39% to 29% and for the ERV from 58% to 40%.