Energy consumption through heating, ventilation and air conditioning represents a significant portion of nationwide energy usage. The HVAC equipment accounts for nearly 40 percent of energy usage in both residential and commercial buildings. According to DOE, energy used by homes creates twice as much greenhouse gas emissions – recognized to contribute to climate change – compared to cars. HVAC usage in a single entity (e.g. commercial or residential building) can vary widely from one place to another. Heating and cooling efficiency is the most effective way that people can decrease their overall energy consumption. HVAC equipment that is currently available has different degrees of possible efficiency. However, this is only the beginning. Experts consider the spectrum of efficiency rates, factoring in differences in equipment, quality of installation and maintenance, as well as personal use. Based on all of these factors, the Department of Energy (DOE) and many professional organizations conclude that much can be done to improve the HVAC efficiency of virtually any single system.
Considering HVAC processes, improved or advanced HVAC equipment and components are available in the market may be purchased and installed, replacing existing inefficient components. The commonly used typical HVAC energy usage of HVAC components may be divided into five categories, these are; fans, cooling, heating, pumps and cooling towers. The energy usage of fans, cooling and heating account for 34%, 27% and 17%, respectively. Pumps and cooling towers are responsible for 16% and 6%, respectively. In hindsight, replacing existing equipment, responsible for these energy-intensive processes with higher efficiency equipment, may other substantial savings. Furthermore, existing systems are usually outdated and subjected to poor performance, due to degradation over time. The main contributor to the degradation of these systems may be a result of continuous operation, poor maintenance and prolonged exposure to environmental conditions.
Moreover, recently developed equipment may offer substantial improvements in terms of energy efficiency. For instance, the use of variable speed drives to control compressor systems in chillers, as opposed to standalone fixed speed compressors, may offer substantial savings in terms of energy consumption in HVAC. A demand response component is introduced recently, as opposed to the constant supply of compressed refrigerant being delivered by the conventional HVAC system. In addition, variable speed drives have similarly been introduced to water pumps and fans, to obtain further efficiency gains. Pumps are located in both the evaporator and condenser sections, while fans, responsible for heat extraction and airflow regulation, are located at the condenser section and the air-handling units, respectively. Other improvements in HVAC design may incorporate permanent magnet synchronous motors, rather than induction motors, offering average energy-efficiency gains of up to 10.4% across the total speed range. To put this into perspective, the overall energy savings in HVAC systems may be as high as 8.58% as a result of replacing induction motors. Permanent magnet synchronous motors may replace all existing induction motors in the HVAC system, so that overall energy efficiency may be increased. Additionally, the permanent magnet synchronous motors usually operates near constant efficiency of approximately 96%, at any given speed percentile.
Multi-stage heating and cooling systems conserve energy by adapting to different temperature needs in the HVAC. The word stage refers to the level of power the machine uses to heat and cool the indoor space. A single-stage unit provides only one level of heating and cooling regardless of temperature needs. In other words, the HVAC unit cools the indoor space with the same level of power regardless of how hot it is. This scenario isn’t always the best use of energy, and generally results in higher utility bills. Multi-stage units save energy by alternating power settings to conserve energy and run on low power settings most of the time. Under more extreme conditions, the units switch to higher modes of power.
HVAC models that are more than 20 years or older are needed to be replaced as they are likely to be less energy efficient. In addition, heating and cooling systems lose their efficiency over time. The average lifespan of these HVAC systems is about 15 to 20 years. If HVAC equipment is available more than 10 years, it needed to be replaced as the consideration of the cost effectiveness of the maintenance on an older system. Making repairs might improve its efficiency and offer some benefits, but ultimately the repairs will extend the life of an older system, especially with significant damage or wear and tear.
When considering an energy-efficient cooling or heating system, It is necessary to select the proper energy ratings of HVAC. Such as in Seasonal Energy Efficiency Ratio, SEER, for air conditioners and an Annual Fuel Utilization Efficiency, AFUE, rating for furnaces. Higher ratings indicate that the machine uses less energy to heat and cool your home. Heat pumps can come with both SEER rating and Heating Seasonal Performance Factor, HSPF, which measures heating efficiency.
Factors affecting energy efficiency in HVAC
When it comes to the operation of HVAC devices and machines, energy efficiency is the first thing people look for. It is no different for HVAC systems, which can take up a significant portion of energy costs in both residential and commercial applications. Different factors will make HVAC an system energy efficient. From the type of system to how it is maintained can all affect the energy efficiency. Because of this, even small changes to an existing system can help make it more energy efficient.
Efficiency in HVAC Design
Efficiency applies to design of HVAC equipment, as well as use. Experts suggest taking a holistic approach to decreasing energy consumption, by examining all the ways that each part of the HVAC system uses energy and looking for ways to improve it. Demand-controlled ventilation is key to reduce the cooling or heating load so that buildings are not cooled or heated regardless of the needs of the building’s inhabitants. Designers should aim to use renewable energy sources whenever possible. Since heating and cooling tends to produce a lot of waste, HVAC system designers ought to take advantage of natural conditions or by-products to more effectively heat and cool. For example, the system may be built to use heat exhaust to warm air or utilize natural moisture to cool air.
Assuming that the heating and cooling equipment is designed with the greatest efficiency in mind, the most significant impact on effectiveness comes from the installation, maintenance and use of the system. All equipment for the HVAC system must be expertly installed to ensure that the maximum amount of cooled or heated air will reach all specified areas of the building. Once installed, the equipment should be maintained regularly and repaired, as needed. This includes appliances such as furnaces and air conditioners, but also auxiliary equipment like ductwork, which can be a significant source of wasted energy. DOE recommends that people in all buildings use programmable thermostats efficiently to minimize energy consumption. It also suggests that building managers and home owners take a proactive stance toward a decrease in energy consumption. When HVAC users plan to reduce their energy usage, they are more likely to succeed. HVAC efficiency allows people to use their HVAC equipment to cool or heat buildings without wasting energy unnecessarily. Given the contribution of HVAC energy consumption to global greenhouse gases, improvements to these systems through design, installation, maintenance and use are vital to any environmental conservation plan.
Rating of HVAC system
When it comes to finding the most energy efficient HVAC system, it helps to have an understanding of the criteria behind them. Here are some of the different scientific ratings most energy efficient HVAC systems will be rated with the Season Energy Efficiency Ratio (SEER). The SEER is the measured ratio of cooling output, which is measured in British Thermal Units (BTU) and then divided by usage, measured in kilowatt hours. For the SEER rating, the higher the number, the more energy efficient HVAC ducting you have. The SEER HVAC rating uses seasonal cooling conditions rather than lab-created conditions. Energy Efficiency Ratio (EER) is the another rating most energy efficient residential HVAC systems will have is an EER rating. The EER rating does not use seasonal averages to get its ratios. Instead, it uses strict laboratory conditions for its testing. Like with the SEER rating, the higher the EER number, the more energy efficient the HVAC system is. Heating Seasonal Performance Factor (HSPF). The HSPF rating is the ratio for how efficient the HVAC’s heat pump is, which cycles in both directions to produce hot and cold air. The HSPF rating measures exactly how much total space heating is necessary, using the BTU measurement divided by the total amount of electricity used by the pump in kilowatt-hours. The higher the HSPF ratio, the more efficient the heat pump is.
Proportional integral-derivative (PID) programming and Variable Speed Drive (VSD) and Variable Frequency Drive (VFD)
Due to the complex relationship among the HVAC system operational parameters, it is necessary to suggest optimum settings for different operations in response to the dynamic cooling loads and changing weather conditions during a year. Proportional integral-derivative (PID) programming can effectively handle the discrete, nonlinear and highly constrained optimization parameters. Energy efficiency process has been made by controlling of alternative current (AC) drivers for ventilation and exhaust fans, according to supplied air flow capacity and differential air pressure between supplied and exhaust air. In addition, supervisory controller softwares were also developed by using programmable controllers and human machine interface (HMI) units. The new designed HVAC control system would have a saving potential of about 40% as compared to the existing operational settings, without any extra cost.
In the old conventional control system, exhaust and ventilating fans had been driven at constant speed. The differential pressure sensors, anemometers, and driver inverters for exhaust and ventilating fans have been added to the new designed control system. In this manner, the exhaust and ventilating fans’ speed have been controlled depending on the requirements of the plant conditions (Fig. 1). Fig. 1 shows the difference between a fixed speed and a variable speed compressor at different loads. The compressor motor control drive contains algorithms that enable the drive to operate efficiently and protect the compressor. These active protection algorithms incorporated into the motor control drive safeguard the compressor and drive from many adverse operating conditions.
In addition, the VSD will include soft start and possibly soft stop algorithms which save energy and reduce the stress on components. Many compressor manufactures offer a variable speed drive matched to the compressor and mounted on the same frame as the compressor. Alternatively, a separate VSD may be retrofitted to existing equipment. Variable speed compressors can operate in the range from very low load (≈10%) to full load. Other improvements include the use of permanent magnet synchronous motors, which are particularly suited for VFD systems these offer improved efficiency over induction motor-driven units (Fig. 2). Other compressor improvements include oil-free magnetic bearing drives that reduce oil usage, have a lower number of parts and, therefore, lower maintenance requirements.
Indoor Air Quality
Poor indoor air quality (IAQ) will cause HVAC unit to work longer which will make it less energy efficient. Testing the indoor air quality in air conditioned room will help keep occupants healthy and HVAC system more efficient. Sometimes, the process cleaning the indoor air handling units and outdoor compressors may help to make HVAC system working better. To tackle these factors that determine HVAC efficiency and healthier indoor environment are results into more comfortable home and lower utilities. To improve HVAC system’s efficiency, regular maintenance or service is required.
HVAC System’s Size (capacity)
Proper size (capacity) plays an essential role in HVAC system performance. If your system is too large for smaller cooling space, the system will spend excessive time in its power-wasting startup phase, which results in short cycling. If the system is too small for the large cooling load of the conditioned space, the heater or air conditioner will spend excessive time attempting to reach the thermostat’s threshold, which it never does. Either way, poor sizing wastes energy. The HVAC professionals can diagnose whether or not system fits the load requirement of conditioned space area.
Insulation of space to be cooled or heated
To have the most energy efficient residential or commercial HVAC system possible, it is to be ensured that the whole indoor space to be cooled or heated should be well insulated. By proper insulating the indoor space well and keeping the ducting of the HVAC unit properly, there is less of a chance of air escaping, so the installed HVAC unit will not have to work as hard to heat and cool indoor built environment overall.
When an energy efficient HVAC unit resulted to the leaky duct system, it completely defeats the purpose by allowing the air to escape before it reaches the indoor conditioned space. This is a costly issue that is often overlooked in HVAC maintenance and can costs a lot of money. By using a product like Aero seal, any cracks and holes will easily be filled, preventing any more air leakage (Fig. 3).
During summer months, keeping curtains closed during the day will help block out some of the summer heat, allowing conditioned indoor space to maintain a cooler temperature. Alternatively, opening the curtains during winter will allow even the smallest amount of sunlight to come through and naturally warm indoor space, so HVAC system works less.
Fans and fan controllers
Energy saving on fans is much greater than on other equipment. On fan loads, the power requirement varies as the cube of the speed, so the slower the fan speed, the less energy required. A fan running at 80% speed will consume 50% of the energy at 100% speed. Modern fan controls consist of much more than just speed controls and variable speed drives. Key to identifying the energy savings opportunities of VSDs in HVAC systems is an understanding of the operating cycle of the system versus the heating and cooling needs actually required. Most HVAC systems are designed to keep the building cool on the hottest days and warm on the coldest days. Therefore, the HVAC system only needs to work at full capacity on those days. For the rest of the year, the HVAC system can operate at reduced capacity. This is where a variable air volume system with variable speed drives (also-called variable frequency drives, or VFDs) can be used to match air flow to actual heating and cooling demands. The VSD can reduce the motor speed when full flow is not required, thereby reducing the power and the electrical energy used.
Air distribution systems
The variable air volume system has advantages over the constant air volume system (feed a constant flow of air and regulate the air temperature to the heat load), but in the basic version has several drawbacks. In a variable air volume system, the air temperature is kept constant and the flow is varied to meet the heat load requirements. The basic method of control is to use a constant speed fan and a damper to regulate air flow. This provides the fan motor with a constant load irrespective of the air flow rate. Using a variable speed drive varies the load on the fan motor with variations in air speed and achieves energy savings as a result.
The evaporator is used to lower the temperature of the chilled water returned from the water circulation system. The water is passed through the evaporator in pipes surrounded by condensed refrigerant. The heat from the water evaporates the refrigerant and the water is cooled in the process. The flooded evaporator (Fig. 4) was commonly used in the past. In the flooded type, the refrigerant covers the tubes completely and evaporation of the mass of refrigerant takes place when returned chilled water is passed through the tubes.
In the falling film evaporator, the surface of the tubes in the upper portion of the evaporator is covered with a thin film of refrigerant, giving a very effective heat transfer mechanism. In the mixed falling film type evaporator (Fig. 5), a thin film of refrigerant is sprayed over the top tubes. Some of this evaporates and the gas passes on to the compressor. The remaining refrigerant covers the bottom tubes and evaporates as well.
The FF-type (full falling film evaporator) offers higher heat transfer efficiency and requires a lower charge of refrigerant than the flooded type evaporator. In the full falling film evaporator (see Fig. 6), the film covers all the tube and the remaining refrigerant collects in the bottom of the evaporator. Only the evaporator bottom has a small amount of liquid refrigerant so that refrigerant charge is less, complying with environment protection.
Central plant optimization and energy efficient operation
HVAC systems consist of a complex arrangement of different components, all of which must be controlled to work together. In a manually controlled system, each of the systems is set to its optimum condition, which might not be optimum for the system as a whole. Take, for instance, the air handling unit: There are two flows that can be controlled, the rate of air flow and the rate of water flow. The water temperature will depend on the evaporator settings, which also depend on the compressor and condenser settings. Optimization will require adjustment of the operation of all these units to achieve best efficiency. Optimizing energy usage in the HVAC system involves optimizing every element and the system as a whole. The operation of the system as a whole can be optimized to ensure further energy savings even once the individual items have been set for maximum economy. Central plant optimization can achieve further gains after equipment and motor drive upgrades. Up to 60% saving are claimed versus the existing plant before equipment and VSD retrofits. Some 15 – 20% savings are claimed to be possible compared to performance with upgrades only.
Comfort Point Open (CPO) systems can work with any brand of equipment or plant that can interoperate with building management protocols. Most work on well-established proprietary algorithms and practices. CPO is essential in larger buildings where there is more than one chiller plant running, and the heat load in different sections of the building follows a unique pattern, with no correlation with the pattern in other parts of the building.
The maximum potential savings in the HVAC that has a large variance in the load requirements and climatic conditions has to be taken into consideration for optimum energy efficiency. Furthermore, terms of demand-side management of HVAC systems has also been taken into consideration particularly for the purpose of reducing maximum load demand. Moreover, additional quantifiable data is necessary for effective decision making, regarding the implementation of energy efficiency initiatives.
Dr. (Prof.) D.B. Jani
Associate Professor at GEC, Dahod,
Gujarat Technological University,
(Education Department, State of Gujarat, India).