AIR CONDITIONING So, can a healthy building be a sustainable
building too? The good news is yes, healthy buildings
can be – and should be – energy efficient and sustainable too. The not-so-good news is that there is not a one-fits-all solution to make this happen. Here, however, lies the opportunity. Each building has a unique set of needs and varying types of use. There may be waste energy within the building that can be re-purposed, therefore offsetting the additional energy required due to increased ventilation and/or air-conditioning.
For air-to-air systems, a typical opportunity is heat recovery. As discussed before, increasing fresh air rates comes hand-in-hand with the need to extract indoor air to maintain proper air balance. The opportunity lies in the heat contained in that exhaust air. Recovering the energy contained within that air and using it to pre-heat or pre-cool the incoming fresh air will reduce the need to oversize the HVAC plant. Such heat recovery technologies are widely known and have been available on the market for decades. The plate heat exchanger and enthalpy recovery wheel are the most common, with the latter offering much higher efficiency ratios, plus the ability of providing humidity control. Some European HVAC manufacturers can already provide fully packaged rooftop solutions which combine ventilation, cooling, heating and heat recovery in a single unit, with embedded controls that adapt the machine’s behavior according to the building need. Packaged rooftop units offer another feature which can help tackle the challenge of healthy and sustainable buildings – their ability to intelligently use ventilation as a way to offset the mechanical need for cooling. This feature is commonly known as free cooling (although not to be confused with free cooling chillers). The principle is simple: using temperature and humidity sensors, the unit controller
Case-study L
et’s take an example of a 100 kilowatt rooftop unit serving a logistics center located near London. The rooftop is commissioned to provide 20,000 m3/h of airflow with 50% fresh air, and a constant indoor temperature set point of 21°C. For simplicity, we’ll consider that the logistics center runs 24/7. During the heating season, recovering and re-purposing waste heat in winter by using an integrated energy recovery wheel avoids the rooftop unit having to turn on all (or any) of its compressors. This results in an estimated annual net energy savings of approximately 31,644 kWh – and a carbon footprint reduction of 9.4 ton CO2e (based on a UK conversion factor of 0.296).
For the cooling season, the benefit of free cooling can be quantified using the annual electric energy consumption (kWh) and Seasonal Energy Efficiency Ratio (SEER) as metrics, based on simulation models developed by Trane engineers and tested in a R&D facility. Results showed that the rooftop with free cooling had an SEER of 4.74, which is 11% more efficient than a rooftop unit without free cooling operation. This translates into an annual
energy saving of 1,674 kWh and a carbon footprint reduction of 0.5 ton CO2
e.
The largest benefit in energy efficiency is achieved by combining heat recovery with free cooling, with a total 16% reduction in energy consumption, and an almost 10 ton avoidance of CO2
e. Free
cooling usually comes with a minimal investment cost (some manufacturers even include it as standard), which results in a very attractive ROI for building owners.
In conclusion, improving indoor air quality does not necessarily mean building owners need to take the hit on their energy bills. There are technologies available on the market that allow us to be smarter about how we re-purpose and use energy. Some rooftop manufacturers can even provide simple and cost-effective system controllers that help manage these features in a user-friendly way, without the need for a complex Building Automation System. Nevertheless, adequate and regular service and maintenance play a critical role in keeping buildings both healthy and efficient. Even here, building owners have the opportunity to jump onto the wagon of innovation, as smart controls and remote services disrupt the HVAC services industry. But we’ll leave that for another time.
monitors the outdoor air enthalpy and compares it to enthalpy parameters measured inside the building. When the delta between the two falls below a certain pre-determined threshold, the unit outdoor damper will gradually open, increasing the intake of fresh air, while simultaneously turning down compressor
stages. In simple terms, it is like turning off the air-conditioner and opening all the windows to allow the outdoor air to naturally cool down the building. The combined use of heat recovery during the heating season and free cooling during the cooling season can truly deliver significant energy savings.
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