Increased standards of living coupled with dwindling supplies of fossil fuels have forced investigators to focus on the issue of energy use in buildings while maintaining necessary thermal comfort. The HVAC systems, which play an important role in ensuring occupant comfort, are among the largest energy consumers in buildings. Performance enhancements to traditional HVAC systems, therefore, offer an exciting opportunity for substantial drops in the building energy consumption. Almost around 41 per cent of the total building energy demand is used to support indoor thermal comfort conditions in commercial buildings. Furthermore, as most people spend more than 80 per cent of their time inside a building, the development of energy-efficient HVAC systems that do not rely on fossil fuels will play a key role in reducing energy consumption. A closer look at worldwide energy consumption by HVAC equipment shows noticeable values as shown in Figure 1.
The growing reliance on HVAC systems in residential, commercial and industrial environments has resulted in a huge increase in energy usage, particularly, in the summer months when ambient conditions were severed. Developing energy-efficient HVAC systems is essential, both to protect consumers from surging power costs and to protect the environment from the adverse impacts of greenhouse gas emissions caused by the use of energy inefficient electrical appliances. With rapid changes in science and technology today, there are several methods that can be used to achieve energy-efficient HVAC systems which can prove to be energy efficient over conventional standard systems as shown in Figure 2.
In order to develop efficient systems, however, a clear understanding of building comfort conditions is necessary. Thermal comfort is all about human satisfaction with their thermal environment. The design and calculation of air conditioning systems to control the thermal environment in a way that also achieves an acceptable standard of air quality inside a building should comply with the ASHRAE standard 55-2004. According to this standard, thermal comfort conditions are acceptable when 78 per cent of the building’s occupants are satisfied. In order to predict appropriate thermal comfort conditions, an index called a
predicted mean vote (PMV), which indicates mean the thermal sensation vote on a standard scale for a large group of people, is used. PMV is defined by six thermal variables for an indoor environment, subject to human comfort: air temperature, air humidity, air velocity, mean radiant temperature, clothing insulation and human activity.
Energy-saving strategies in HVAC
Different techniques need to be implemented on HVAC systems to improve their energy efficiency and reduce their environmental impact. In recent years, different automatic control and optimisation strategies have been used to improve the energy consumption rates of these systems.
Energy-efficient HVAC installations
Substantial energy savings can be achieved by optimising heating, ventilation and air-conditioning (HVAC) system and by upgrading it with energy-efficient technology while improving the safety of a building. HVAC systems are subjected to more misuse than any other type of equipment in both residential and commercial sector. Poor maintenance, lack of knowledge on how to use them efficiently, overuse, and the large number of old and inefficient systems at work in the sector make HVACs a significant contributor to the demand for energy for built environment within buildings.
The important step in energy savings on HVAC systems is to reduce the cooling load. The amount of electricity air conditioning systems use also depends on the cooling load – the amount of heat the system has to remove. There are several steps for reducing cooling load.
Insulate the cooled space
This involves implementing various measures such as ceiling insulation, window glazing, blinds, awnings and door sweeps. All will contribute to creating a thermally efficient shell that can dramatically reduce the cooling load on HVAC systems while ensuring that comfortable internal temperatures are maintained. Reduce warm air filtration into the cooled space by keeping windows and doors closed when HVAC systems are in use.
Minimise the use of appliances and lighting
All lights emit heat, so lights and equipment that are not required at any particular time should be switched off to help to reduce the cooling load. Replacing conventional boilers with condensing boilers reduces the heat load, and replacing standard motors with high-efficiency motors results in lower losses and less emitted heat. Investing in variable speed drives (VSDs) for motors to match speed with output demand results in lower energy usage and heat load.
Ensure that controls are in place and HVAC operation reflects a demand.
HVAC loads vary at different times and in different parts of a building throughout the day. Well set time and occupancy controls should ensure that systems only operate when and where required during core business hours. It is also crucial to check settings regularly – many systems are set incorrectly because of forgotten short-term adjustments. Installing a building energy management system (BMS or BEMS) which offers close control and monitoring of building services performance, including HVAC, allows automatic control of the HVAC system. BEMS can reduce energy costs by allowing system performance to be monitored and settings to be changed.
Variable speed drives on HVAC fans and pumps
This allows motor-driven loads such as fans and pumps to operate in response to varying load requirements instead of simply operating in on/off mode can save 30 per cent annual energy approximately as shown in Figure 3. In addition, the VSD will include soft start and possibly soft stop algorithms which save energy and reduce the stress on components.
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 per cent speed will consume 50 per cent of the energy at 100 per cent 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 airflow 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.
Variable volume air system
The variable air volume system has advantages over the constant air volume system, but 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 airflow. This provides the fan motor with a constant load irrespective of the airflow rate. Using a variable speed drive varies the load on the fan motor with variations in airspeed and achieves energy savings as a result as shown in Figure 4.
Central plant optimisation 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 airflow and the rate of water flow. The water temperature will depend on the evaporator settings, which also depend on the compressor and condenser settings. The optimisation will require adjustment of the operation of all these units to achieve the best efficiency. Optimising energy usage in the HVAC system involves optimising every element and the system as a whole. The operation of the system as a whole can be optimised to ensure further energy savings even once the individual items have been set for maximum economy. Central plant optimisation can achieve further gains after equipment and motor drive upgrades. Up to 60 per cent saving is claimed versus the existing plant before equipment and VSD retrofits. Some 15 – 20 per cent 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.
Use of the evaporative cooling integrated hybrid cooling system
The evaporative cooling (EC) systems integrated hybrid cooling system has low set-up and running costs, and have been proven Figure 3. Comparison between constant drive and variable speed drive for energy saving. Figure 4. Saving in power consumption by optidrive control. energy efficiency to significantly improve a building’s cooling and ventilation capacity with minimal energy use. Using water as the working fluid, one can avoid the use of ozone-destroying chlorofluorocarbons and hydrochlorofluorocarbons. Other benefits from this system include easy maintenance, easy installation and operation as well as obviating CO2 and other emissions. Evaporative cooling integrated hybrid air conditioning systems can provide thermal comfort via the conversion of sensible heat to latent heat (desiccant cooling system); however, the lowest temperature DEC systems can reach is the wet-bulb temperature of the outside air. Therefore, the temperature of the supply air after cooling would be just on the edge of comfort and could rise a few degrees in passing through space, taking the temperature beyond the comfort zone. Therefore, the idea is to investigate both the possibility of increasing the utilisation potential of the evaporative cooling system by combination of different components with this system and the capability of improving the performance of other HVAC systems when integrating with an evaporative cooling system.
Variable Refrigerant Units
In conventional systems, one condensing unit is connected to one evaporator, providing conditioned air to one area of a building. If the system is to supply air to more than one area, ductwork must be added, along with zone controls. While this configuration works, it is not the most flexible or energy-efficient, and often results in complaints from building occupants.
VRF systems offer an alternative. In these systems, a single outdoor condensing unit is piped to multiple indoor fan coil units. Refrigerant is circulated in the system through either a two or three-pipe system. In two-pipe systems, all fan coil units or zones must be in either heating or cooling mode. Three-pipe systems have the ability to simultaneously heat some zones while cooling others. Because the load on the system’s compressor constantly varies based on the sum of zone loads, an inverter-driven motor is used to power the compressor. As zone loads decrease, the inverter decreases the frequency of power to the motor, decreasing the compressor’s speed and the flow of refrigerant. As the speed of the compressor decreases, there is a significant decrease in energy use. Each fan coil unit connected to the system has its own refrigerant metering device, which is regulated by the fan coil’s control system. As the load within that space changes, the metering device regulates the flow of refrigerant needed to meet that individual load. Reduced energy use is not the only advantage of VRF systems. With the ability to provide individual zone temperature control as well as simultaneous heating and cooling, better climate control is provided to all areas. With a single outdoor condensing unit, these systems require less installation space than conventional constant flow systems.
Use of ground-coupled HVAC systems
Ground-coupled cooling or heating technology relies on the fact that, at depth, the Earth has a relatively constant temperature that is colder than the air temperature in summer and warmer than the air temperature in winter. In this system, undercooling mode, operation heat is discharged to a ground loop that provides a lower temperature heat sink than the ambient outdoor air temperature. During winter heating operations, heat is extracted from a source that is at a higher temperature than ambient outdoor air. This system has been used on a different residential and commercial scale.
Use of thermal storage systems
Thermal storage systems (TSS) shift the energy usage of the HVAC systems from on-peak to off-peak periods to avoid peak demand charges. TSS is also able to rate variance between energy supply and energy demand to conserve energy. In this system, energy for cooling is stored at low temperatures normally below 20C for cooling, while energy for heating is stored at temperatures usually above 20C. As compared to conventional HVAC systems, TSS offers various advantages for heating and cooling systems, such as energy and capital cost savings, system operation improvements, system capacity extending and equipment size reduction, resulting in a technology that is widely used.
Effect of building behaviour
The energy consumption of an HVAC system depends not only on its performance and operational parameters but also on the characteristics of the heating and cooling demand and the thermodynamic behaviour of a building. The actual load of the HVAC system is less than it is designed in the most operating periods due to building behaviour. Therefore, the most important factor that contributes to HVAC energy usage reduction in a given building is proper control of the heating and cooling demand. Integrated control of building cooling load components, such as solar radiation, lighting and fresh air, can result in significant energy savings in a building’s cooling plant. It is estimated that around 70 per cent of energy savings is possible through the use of better design technologies to coordinate the building demand with its HVAC system capacity.
Energy-efficient HVAC system designs depend greatly on new configurations of traditional systems that make better use of existing parts. One effective way of achieving energy efficiency has been the design of HVAC system configurations that combine a range of different traditional HVAC system components. Recent research and development have demonstrated that a combination of existing air conditioning technologies can offer effective solutions for energy conservation and thermal comfort. Each HVAC discipline has specific design requirements and each presents opportunities for energy savings. It must be understood, however, that different configurations in one area may augment or diminish savings in another.
Dr. D.B. Jani
Government Engineering College,
Dahod, Gujarat Technological University – GTU, Ahmedabad