How to improve Energy Efficiency in HVAC Systems
Conditioning fresh air in HVAC systems increases the load on the system. Generally, air is transported through ductwork, while water and refrigerants are distributed through pipework. The entire process is energy intensive since the main users of this energy are the HVAC plant, fans and pumps. System consists of various components and there are some very good opportunities to improve overall energy efficiency of HVAC systems.
A typical Heating, Ventilation and Air Conditioning (HVAC) system consists of plant equipment (chillers, boilers etc.) which transfer energy via air, water or a refrigerant to air distribution systems consisting of a series of fans and coils. These distribution systems are usually called air handling units (AHUs). The AHUs then use this energy to warm or cool the air that is supplied to the office space.
Air is warmed or cooled as it flows over the heating or cooling coils in the air distribution system. Heat rejection is also required at this point, to reject excess heat collected from the space, to the atmosphere. Heat rejection can also occur through plant equipment such as cooling towers or evaporative coolers.
The main thermal loads in a commercial building are a combination of:
i. Heat produced by people
ii. Heat generated by computers and equipment
iii. Solar radiation through windows
iv. Heat conduction through walls, windows and roof
v. Heat generated by lighting.
To provide comfortable indoor conditions, an amount of fresh outdoor air must be supplied to the building. The quantity is proportional to the number of people in the space. Air from outside is usually warmer in summer and cooler in winter than the desired indoor conditions; hence this air must be conditioned.
Conditioning this fresh air increases the load on the system. Generally, air is transported through ductwork while water and refrigerants are distributed through pipework. The entire process is energy intensive - the main users of this energy being the HVAC plant, fans and pumps. Pumps and fans require energy in the form of electricity to distribute water and air through the building. Chillers and boilers can run on either electricity or gas.
Figure 1 shows the schematics of a HVAC system and the interactions between plant, medium, systems and loads. All systems in a building are linked via a Building Management System (BMS), which monitors the systems and provides a point at which issues can be diagnosed, and systems tuned and optimised.
Energy Efficiency Opportunities in HVAC
Figure 2 shows the typical energy consumption breakdown in an office building, with HVAC systems consuming the greatest portion (39%)
A further breakdown of the typical energy consumption associated with these HVAC system components is shown in Figure 3.
Figure 3: Typical Energy Consumption of HVAC Systems
It is important to reduce overall energy use and increase energy efficiency, while delivering a comfortable environment. Optimum comfort for sedentary work is between 20°C and 26°C, depending on the time of the year and clothing worn.
Generally the temperature in office buildings should vary according to the outside temperature and should be changed month to month.
In order to maintain a comfortable temperature and humidity level within a building, the HVAC system must overcome all of the different loads in the building that work against the desired conditions. The methods to improve energy efficiency in a building’s HVAC system can be broken down into two categories:
1. Reduction of the loads within a space that the HVAC system has to overcome
2. Improve efficiency of HVAC equipment and systems.
1. Reduce Loads within Space
Reduce Equipment Load – Reducing usage of heat-producing equipment such as computers, printers and lights will reduce the need for HVAC systems to condition a space.
Simple things such as turning off monitors, computers or lights when they are not in use will reduce conditioning loads.
Improvements to Façade (Walls) – Increasing the insulating properties of the building fabric will generally decrease the amount of cooling or heating required, and hence reduce overall energy usage.
Improvements to Façade (Windows) – Window loads occur in the form of solar radiation and conduction. Solar radiation refers to the heat created as direct sunlight comes through a window and hits a solid surface in an internal space absorbing the electromagnetic radiation. Conduction refers to the movement of heat from the hotter side of the window to the cooler side. Shading devices minimise the solar load from the windows and reduce loads in the space. A typical double-glazed window will conduct significantly less heat into a room than a typical single glazed window. Another aspect to be considered is air sealing, a lack of which can lead to increased conditioning requirements.
Demand Based Ventilation – Conditioning fresh air requires considerable energy as it is often at a significantly different temperature to that desired in the space. A minimum quantity of fresh air is required in a space based on occupancy in accordance with Australian Standards. Carbon dioxide (CO2) sensors can be used to determine the minimum amount of fresh air required and lower the fresh air supplied, saving energy.
2. Improve HVAC System Efficiency
System Selection – Significant energy savings can be realised through optimal system selection. For example, a Variable Air Volume (VAV) AHU regulates the volume of the supply air to the spaces depending on the amount of heating or cooling required. This system allows for greater control and reduces airflow rates, which reduces overall fan energy consumption. This greater control may also reduce energy wastage associated with re-heating, where the heating system counteracts the cooling system.
Plant Selection – Plant selection can greatly affect the overall energy consumption of a building. For example, condensing boilers - used for the provision of heating hot water - can have gross efficiencies over 90%.
Evaporative pads can be used on air cooled chillers to pre-cool air prior to reaching chiller condensers, improving efficiency. Accurate plant sizing also has a large impact on energy efficiency. Chillers generally work most efficiently at a particular load, so they should be sized to operate at or near this load most of the time.
Building Tuning and Commissioning – Energy savings can be achieved through the commissioning and tuning process. After a HVAC system has been installed, commissioning and on-going tuning of the system will allow the system to function as per the original design intent, optimise its operation and obtain maximum energy savings.
Energy Recovery – Energy recovery involves capturing waste energy and recycling it. Exhaust air is generally closer to the desired space temperature than the outside fresh air being introduced into the system. Heat or ‘coolth’ can be drawn from the exhaust air and used to pre-warm or cool the fresh air entering the system prior to it reaching the AHU. This lowers the energy used by the AHU to condition the fresh air.
Smart Control Strategies – There are many examples of smart control strategies achieved through optimisation of the BMS and building information available to it. One option is to install smart control software that allows the BMS to determine optimum operating conditions for the entire HVAC system to achieve the lowest energy consumption. To achieve this it is important to optimise consumption information for separate parts of the HVAC system. This can be achieved through installation of sub-metering and sensing.
Economy Cycle – Economy cycle involves using 100% outdoor air to supply air to the space. This occurs at times of the year when the outdoor conditions are cooler than the return air temperature in cooling mode. This allows the plant to turn off the cooling coils and reduces chiller energy consumption.
A ten year strategy under the National Strategy on Energy Efficiency should be framed that aims to drive long term improvements in energy efficiency of HVAC systems Nation-wide. Under the Energy Efficiency Working Group, the Buildings Committee manages the implementation of the HVAC Strategy. This committee is comprised of representatives from Govt, State and Local Self Govt. The Strategy takes a whole of life perspective in targeting HVAC efficiency improvement, encompassing the design, manufacture, installation, operation and maintenance stages of the HVAC lifecycle. The Strategy consists of a number of complementary measures that fall under three broad initiatives – People, Practices and Systems. This Basics of HVAC Energy Efficiency factsheet relates to all three categories. It is one of a suite of factsheets developed to provide a quick overview and reference to inform, educate, and encourage energy efficiency in the HVAC industry.
7 Easy Tips to Improve Your HVAC Efficiency
Residential and commercial buildings account for 40% of total energy consumption. According to CSE Magazine, HVAC systems use between 40-60% of the total energy consumed in buildings, based on data from the Department of Energy (DOE). Since we normally don’t interact directly with our business’s heating or cooling systems, we often don’t think about the energy consumption and money wasted through inefficient HVAC systems. The following will provide some simple tips to keep your building’s HVAC more efficient.
Schedule a professional energy audit
First things first, have a professional conduct an energy audit on your HVAC system. Most utilities in New England offer complimentary energy audits, so there’s no reason not to have your systems evaluated for potential energy efficiency upgrades. Depending on the age of the unit and the parts, and the condition the unit is in, there may be some simple fixes you can make without fronting the cash to purchase a new system. Let’s discuss some of the upgrades you can make.
Install an economizer
One of the simplest and lowest-cost solutions, economizers take advantage of cooler temperatures outside your building by bringing in that air, instead of mechanically cooling warmer air by powering the compressor, to cool your building. Thus, the only energy needed is to move the air throughout the building. Buildings often produce a high amount of energy and heat during normal business hours, making the air outside cooler than the air inside the building. This is why we sometimes need to use the AC on cool days. Utilizing this simple solution, on average, can save businesses up to 30% in energy costs.
Controls allow the user to set preferences to how the heating and cooling systems operate and make them run more efficiently behind the scenes. Controls include devices such as programmable thermostats, timing automation systems, demand and occupancy sensors, and more. Simply put, these optimize the heating and cooling functions during peak hours, and for when the building is unoccupied—which is especially important. Why heat or cool your building more than necessary?
Reduce load capacity
Load capacity refers to the total amount of heating and cooling your building uses. Reducing load capacity helps your existing systems run less frequently. There are several important steps you can take to reduce your building’s load capacity, including:
Insulation and air sealing are commonly overlooked components of building energy management. There are many ways to seal and insulate your building, including roofs, walls, pipes, and ducts. Over time, these components wear down, allowing heat to escape through cracks or holes. You want to heat your building, windows, and doors, not “the whole outdoors,” as the expression goes. Proper insulation, building envelope sealing, and maintenance can increase the efficiency of your building dramatically.
Installing energy-efficient windows
For small businesses, installing storm windows can help your business save up to 15% of your annual building costs. For larger, multi-story businesses with many windows, the project is a lot more complex, but the concept and savings are similar. In some cases, larger businesses can reduce overall energy consumption by up to 50%.
Business owners should notice an immediate difference on their next energy bill, but those savings will decline without proper maintenance. Installing window film (discussed in the next paragraph), cleaning, caulking, and replacing components all ensure the windows stay at peak performance during their lifetime. For instance, cleaning ensures buildings take advantage of solar heat, while caulking helps stop air leakage.
Installing window film
Another overlooked component of energy management is solar heat—sunlight coming in through the windows. According to FacilitiesNet, “roughly one third of an average building’s cooling load is due to solar heat gain through windows” (Zimmermann, 2006). Window film can block up to 80% of solar heat by absorbing and reflecting the heat back outdoors. According to Jim Mannix, business development manager at 3M, “From a building management perspective, you can reduce kwh, make tenants more comfortable and reduce demand charge” (Zimmermann, 2006). In winter, the sun can be used to offset heating costs, as well, provided your windows are properly sealed.
Installing energy-efficient lighting
As a core component to your entire building’s energy management, lighting, especially older, less efficient models, generates a lot of heat. Upgrading to more efficient fluorescents or LEDs gives off less heat and reduces the strain on your HVAC system. This is one of the easiest and most cost effective measures for energy savings and maintenance.
Upgrading to Energy Star certified office equipment
Similar to lighting, old office equipment generates a lot of heat. Upgrading to more energy-efficient equipment, especially those that are ENERGY STAR certified, are easy ways to reduce your building’s load capacity. Better yet, you can receive rebates on many ENERGY STAR appliances.
Like all equipment, proper maintenance is essential to ensure your HVAC system is running as efficiently as possible. We recommend checking and tuning up your HVAC before the start of every winter season. Check for leaks and defective equipment in your pipes, ducts, coils, and unit fittings to make necessary repairs. Chillers, boilers, and other heating and cooling equipment all wear down or break over time. Cleaning debris and dust that accumulates in the coils and ducts in the system can prevent your heat transfer from being compromised and your system requiring emergency repair.
It is also critical to change your HVAC filters regularly. You wouldn’t run your dryer without changing the lint trap often, right? We recommend changing your filters at least once a month. Filters are cheap, ranging from only a few dollars, and can improve system performance and air quality. Plus, this is important to keep people working in the space healthy.
Educate your workforce
As a business owner or facilities manager, you know the importance of energy savings and how they affect your bottom line. Your company’s other employees, however, may not share the same knowledge. It is important to train your employees on the best ways to increase your building’s efficiency. Little things such as turning off computers after hours and making sure to turn off lights, even with a motion sensor, affect your building’s energy efficiency. We recommend a brief training session to educate your workforce on energy efficiency best practices to reduce load capacity.
Purchasing a new HVAC system
After you’ve had your HVAC inspected and you’ve determined it makes more sense to upgrade, we recommend you check the system’s Seasonal Energy Efficiency Ratio (SEER) rating. The SEER rating, developed by the Department of Energy, evaluates the efficiency of an HVAC system. Older models generally have SEER ratings of 6 or less. Today, the minimum required SEER rating for all commercially sold units is 13. According to Energy.gov, “SEER 13 is 30% more efficient than the previous minimum SEER of 10.” Upgrading models with SEER ratings of 6 or less will see even higher energy savings.
The Author wishes to thank to Norman Disney & Young for their valued contributions as well as various sources from the Internet.
AUTHOR CREDIT & PHOTOGRAPH
Dr. Omprakash G. Kulkarni
Scientist, Mentor, Adviser,
Technology Provider and
Renewable Energy and Others
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