Current ventilation guidelines recommend maintaining ventilation with 100% outdoor air 24/7, or as much as possible if that is not feasible, as an effective way to keep airborne pathogen concentration under control. Obviously, outdoor air needs to be filtered and conditioned to guarantee the indoor temperature and humidity set points; this, in turn, implies a higher HVAC-related primary energy consumption, because more outdoor air needs to be conditioned compared to the pre-COVID-19 situation, when the flow of outdoor air was typically 20% of the total supply air entering the space.
Is it possible then to minimise primary energy consumption to reasonable levels, and even to values that are similar to the pre-COVID-19 situation?
It appears so, based on simulations carried out in four different European climates: Athens, Berlin, Madrid, and Milan.
In order to save energy, recovering as much as possible from the exhaust air is of paramount importance. Therefore, a heat exchanger between the exhaust and the supply air flows is a big plus in the AHU used to simulate the four cities.
Evaporative cooling, both direct and indirect, helps to reduce the energy consumption related to cooling.
Modulation of all devices in the AHUs (coils, evaporative coolers, blowers, etc.) brings an additional advantage, as operation of the devices is modulated so as to provide no more than necessary, i.e. exactly the required heating/cooling, or the supply air flow based on actual occupancy.
The AHU used in the simulation is illustrated in the following three images. The AHU is the same; what differs is the way the supply air is generated:
- Pre-pandemic scenario: ventilation is on duty 12 hrs/day; supply air (constant flow) is composed of outdoor air (20%) and recirculated air (80%); humidification is by steam (electric); the heat exchanger is not bypassed; no indirect evaporative cooler is installed.
- Pandemic scenario: ventilation is on duty 24/7; supply air (constant flow) is composed of 100% outdoor air; humidification is by steam (electric); the heat exchanger is bypassed as per ventilation guidelines; no indirect evaporative cooler is installed.
- Post-pandemic scenario: ventilation is on duty 24/7; supply air (variable flow) is composed of 100% outdoor air; humidification is by direct evaporative cooler; the heat exchanger is not bypassed and it is assumed there is no cross-flow contamination; an indirect evaporative cooler is installed and running.
Figure 1 - Pre-pandemic scenario. Click to enlarge.
- Air flow Q= 31,600 m3/h
- Heat recovery ζ= 73% sensible, bypass on/off
- Isothermal humidification from electrical source
- Percentage of outside air = 20%
- Percentage of recirculated air = 80%
- Constant air flow management (CAV)
- Operation 12 hours/day
Figure 2 - Pandemic scenario. Click to enlarge.
- Airflow Q= 31,600 m3/h
- By-passed heat recovery
- Isothermal humidification electric source
- Percentage of Outside Air = 100%
- Percentage of Recirculated Air = 0%
- Constant airflow management (CAV)
- Operation 24 hours/day
Figure 3 - Post-pandemic scenario. Click to enlarge.
- Air flow Q= 31,600 m3/h
- Heat recovery ζ= 73% sensible, bypass modulating
- Adiabatic humidification
- Percentage of outside air = 100%
- Indirect evaporative cooling with heat recovery unit dampening
- Percentage of recirculated air = 0%
- Variable air flow management (VAV)
- Operation 24 hours/day
In all four cities, the simulations generate similar results:
- Pre-pandemic scenario: baseline.
- Pandemic scenario: there is a large increase in primary energy consumption, as the amount of outdoor air to be conditioned increases from 20% in the pre-pandemic scenario to 100%.
- Post-pandemic scenario: primary energy consumption is reduced dramatically compared to the pandemic scenario, reaching values similar to those in the pre-pandemic situation, due to modulation, heat recovery and evaporative coolers.
Simulation of primary energy requirements in the three AHU configurations for the climatic conditions in the city of Athens. Click to enlarge.
Simulation of primary energy requirements in the three AHU configurations for the climatic conditions in the city of Berlin. Click to enlarge.
Simulation of primary energy requirements in the three AHU configurations for the climatic conditions in the city of Madrid. Click to enlarge.
Simulation of primary energy requirements in the three AHU configurations for the climatic conditions in the city of Milan. Click to enlarge.
Once the current pandemic is over, we will return to normal. Yet, depending on the climate, it will be possible to have ventilation with 100% outdoor air, but with primary energy consumption close to the pre-pandemic scenario; and the same will be true for running costs. This will be possible thanks to fully-modulating ventilation systems, with energy recovery, evaporative coolers, and control and monitoring systems capable of getting the most from the devices they drive.
About the Author
Raul Simonetti joined CAREL INDUSTRIES S.p.A. in 2000 as a Product Specialist for humidification. He dealt with sales support and training for several years, making presentations at specialist events.
Following a period as Application Manager at the Climate Business Unit (humidification and evaporative cooling systems), since 2014 he has been HVAC/R Corporate Business Manager for all CAREL businesses. His role involves active participation as CAREL’s representative at ASHRAE Conferences in USA; he is also directly involved in associations that represent the industry in dealings with the institutions, either directly (via EPEE and EUROVENT in Europe) or indirectly through other colleagues (EUROVENT again, AHRI in USA, CRAA in China).
About CAREL INDUSTRIES S.p.A.
CAREL is a world leader in control solutions for air-conditioning, refrigeration and heating, and systems for humidification and evaporative cooling. Our products are designed to bring energy savings and reduce the environmental impact of machinery and systems. Our solutions are used in commercial, industrial and residential applications. Founded in 1973, in 2020 CAREL had a turnover of more than 331 million euros, with over 1700 employees, 29 subsidiaries and 9 production sites, in addition to partners and distributors in a further 75 countries. Research & Development are the heart of our commitment. A total of approximately 6% of consolidated sales is regularly reinvested in Research & Development, in order to anticipate customer needs and supply advanced solutions. CAREL also operates two research laboratories, one focused on thermodynamics and the other on humidification, true points of excellence in their respective fields.
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