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Highly efficient heating, ventilation and air conditioning

Heating, ventilation and air conditioning (HVAC) systems supply fresh air and condition the indoor air temperature and humidity of a building. HVAC is reported as the key energy user (37%) in US buildings (WBCSD, 2008), accounting for 59% of the energy used in China commercial buildings in 2000 (Levine et al., 2007). Therefore, HVAC is a key component of climate change mitigation potential in the building sector.

Introduction top

HVAC systems normally consist of components to supply, filter, heat, cool and distribute the conditioned air into targeted interior spaces. In an HVAC system, the principle: ‘the whole is more than the sum of its parts’ is applied. This means the high-efficiency of one component can operate at expense of the others. As an example, take two categories of HVAC systems: high- and low-pressure systems. Highpressure systems allow high-velocity air to flow through the duct system in the range of 10 to 25m/s. These systems have smaller ducts and require less space to house the duct system, but require more fan energy to drive the air. The high-velocity airflow has its speed reduced at the terminal outlets to avoid a strong flow of air that creates discomfort for occupants, known as draught. Low-pressure systems conduct supply airflow at low velocities and require larger duct spaces. In this case, HVAC system efficiency depends on the selection and integration of the key components that suit a specific building and its context.

As highlighted, highly efficient HVAC systems can be achieved through the best-fit integration of HVAC’s key components. These key components, or sub-systems, are heating, cooling and ventilation. These components have constantly undergone technological upgrades to improve their efficiency.

Heating systems

Boilers are usually used to generate hot water or steam using coal, diesel or natural gas. Conventional boilers – i.e., cast iron boilers or water-tube steel boilers – have combustion efficiencies between 78 to 86%. The newer generation of condensing boilers achieve up to 96% combustion efficiency. Condensing boilers are often fired with natural gas – a less polluting energy source. They are more efficient at removing the heat from the flue gases, and can be operated more efficiently than the conventional boilers at part-load (Graham, 2009).

Heat pump technologies are developed as an alternative to fossil-fuel-based boilers. The technologies extract heat from warmer underground earth, air or sub-surface water during winter months, in temperate regions, to condition the temperature for indoor usage. Reversing the above cycle during summer months, a heat pump extracts heat from indoors to outdoors to provide cooler indoor temperature.

Cooling systems

Chillers are used to produce cool water, which is then pumped to air handling units to cool the air. Chillers use either mechanical compression or an absorption process. Among mechanical compression chillers, centrifugal chillers are the most efficient for large-capacity operation, such as in large office buildings or retail complexes. Absorption chillers, on the other hand, produce cool water through heat sources, i.e., gas burners or high-temperature water, instead of using electricity to run compressors. In this way, absorption chillers enable the use of hot water tapped from solar thermal systems for air conditioning.

Condensers are required in chiller systems, which reject heat to the environment and allow chillers to continuously remove heat from indoor conditioned spaces. They can be air-cooled or water-cooled. Aircooled condensers are used for small-scale application, whereas water-cooled condensers are more costly but much more efficient for large-scale systems and are usually seen in large building complexes. Water-cooled condensers require cooling towers, usually located on the rooftops of buildings, to reject heat from condensers into the environment.

illustration © climatetechwiki.org

Figure 1: Diagram of a typical conventional cooling and ventilation system.

Energy recovery installed in the mechanical ventilation system can help save energy. Air conditioned air fume cupboards can be use to cool incoming replaced air through a heat exchanger instead of being discharged directly outdoors. This can pre-cool incoming replaced air to a temperature of approximately 25°C in tropical regions, thus reducing energy use for cooling (BCA, 2007). Desiccant wheels have the ability to dehumidify the air while carrying out heat exchange, and are also suitable for hot and humid regions in the tropical belt.

An automatic condenser tube cleaning system allows water-cooled heat exchange type chillers to maintain good performance through constant cleaning of the condenser tubes. The system circulates cleaning sponge balls into the condenser tubes, which are then rinsed in a ball receptacle through swirling motions (Hydroball, 2007).

illustration © climatetechwiki.org

Figure 2: Diagram of a typical energy efficiency cooling and ventilation system.

Ventilation systems

Variable Air Volume (VAV) systems vary the amount of air intake to a room while keeping the air temperature constant. This strategy is different from the Constant Air Volume (CAV) systems, which supply a constant rate of air intake while varying the temperature of the supply air. As the supply air is centrally cooled to meet the coldest temperature demand, CAV systems may lead to rooms/zones with a lower temperature demand to be over-cooled resulting in energy being wasted. VAV, instead, allows for better room temperature control, and when used with variable speed drive fans, can save up to 15% on energy use (BCA, 2007).

Displacement ventilation uses the principle that ‘warm air rises’ to provide ventilation in an air conditioned room. Displacement ventilation typically supplies conditioned air from a raised floor system through a series of adjustable floor-mounted registers. The room’s air is stratified: lower temperature air stays in the bottom portion of the room (where people are located and cool air is needed) and high temperature air rises towards the ceiling (Graham, 2009). As a result, displacement ventilation helps reduce energy used for higher fan speed to drive cooled air down from the ceiling like conventional ceiling-mounted air outlets do. Furthermore, displacement ventilation can provide the same level of comfort with a significantly higher supply air temperature, i.e., about 18°C compared with about 13°C in a conventional ventilation system (Levine et al., 2007).

illustration © climatetechwiki.org

Figure 3: Displacement ventilation.

Feasibility of technology and operational necessities top

Capacity building, local building codes related to HVAC and supporting the growth of energy services companies, ESCOs (see section 4.16) are three key elements to make highly efficient HVAC systems and their sub-systems more feasible for large-scale implementation, especially in the context of developing countries.

One of the major barriers to implementing highly efficient HVAC systems is the installation of oversized systems that result in inefficient part-load most of the time. In order to break the vicious circle created by this conventional practice, training workshops to upgrade professional knowledge will be necessary. Furthermore, well designed demonstration building projects that are equipped with highly efficient HVAC systems and show a proven records of energy saving and good thermal comfort performance, will be a good catalyst.

Setting minimal performances in building codes provides an institutional setting for the design and implementation of more efficient HVAC systems in buildings. An example of good standards is the American Society of Heating, Refrigerating and Air conditioning Engineers (ASHREA), which also provides guidelines on how to achieve highly efficient HVAC design and installation.

As HVAC systems are seen as the main energy-consuming component in buildings, improving the energy performance of HVAC systems is the main business area of many ESCOs. Therefore, supporting the development of ESCOs and energy performance contracting business will indirectly nurture the implementation of highly efficient HVAC systems.

Highly efficient HVAC systems require great efforts during the design stage for coordination, selection, and design for best fit integration of HVAC components to be suitable for a specific context and unique parameters of a building.

Zone control is the first and easiest strategy for a highly efficient HVAC (including heating and cooling) system. Wherever possible, spaces/rooms in a building should be divided into smaller enclosed rooms, each equipped with own thermostat, motorised damper and control system. This way, users are able to adjust the room temperature independently to suit their thermal comfort level. It is estimated that the application of zone control in a commercial building in Singapore can cut energy consumption by up to 25% (DLS, 2009).

Proper sizing of components. This is a simple concept but is hard to achieve. The conventional practice of mechanical and electrical engineers is to base sizing on the worst-case scenario for simultaneous load demands e.g., worst-case weather, lighting loads, equipment’s load, full occupancy and so on. However, in recent years, empirical data from building science research has proven that oversized equipment operates less efficiently and costs more. It is suggested that it is better to “plan for expansion, but do not size it” (Graham, 2009).

Location of fresh air intake has to be carefully considered and placed away from any (potential) pollution and odour, such as from basement garage floor or directly facing garbage collection points. It is also not desirable to locate an air intake close to an air exhaust outlet. In this way, incoming air to the HVAC systems is fresh and of good quality.

Shifting peak load in cooling systems to utilise off-peak electricity at night or solar energy during the day to generate thermal energy, e.g., in the form of ice or chilled water. This thermal energy will be stored and used for air conditioning during peak cooling/heating times. This will result in lower electricity peak demand and will reduce energy costs.

Heat delivery in heating systems to the occupancy spaces includes two common methods, hydronic heat and forced hot air. In a hydronic heat system, heated water from a boiler is pumped through pipes running in floor slabs and/or walls around the building. Heat is radiated from the hot water to warm the occupancy spaces. The advantages of these systems is quietness, and that heat can be distributed evenly.

In forced hot air systems, heated water is circulated through a fan-coil unit to warm the heat exchanger. Air from inside the building is then circulated and is passed through the warm heat exchanger. Finally, the warmed air is delivered to the occupancy space(s). The warm air outlets are recommended to be located on the floor or lower wall of the occupancy spaces. Ceiling mounted outlets work against the natural buoyancy of warm air, and thus requires additional energy for higher fan speeds to drive the warm air down to the human level.

Status of the technology and its future market potential top

Global demand for general HVAC equipment has been reported to increase by 6.2% per year up to 2010 to US$93.2 billion. In the Asia-Pacific region, the demand growth will outpace the global average with China’s market growth contributing to about 40% of global demand growth (Freedonia, 2010).

With the increase in construction expenditures and higher per-capita income, India’s high HVAC market demand is also projected to grow at a faster pace than the global average. With growing worldwide demand, highly efficient HVAC systems stand to enjoy good market prospects. Moreover, rising oil and electricity prices, coupled with wider public awareness of being energy efficient, will help to push the demand toward the highly efficient section of the market.

The IPCC also highlights the trend of higher demand for individual apartment and home air conditioning in developing countries, reaching even higher levels in developed countries (Levine et al., 2007). This trend is evident in the production trends of such air conditioning units – from 35.8 million units in 1998 to 45.4 million units in 2001, which is an increase of 26% (IPCC/TEAP, 2005).

Although higher investment costs are required, the market penetration of absorption chillers is estimated to be one-fifth for China’s central HVAC system market. This is much higher than that in the US, which is about 1%. This is because in China, many buildings and factories already have diesel generators and fuel storage tanks to address blackouts. Therefore, it makes more economic sense for building owners to install absorption chillers than to install those that run on electricity (Bradsher, 2010).

Displacement ventilation is reported by the IPCC’s Fourth Assessment Report to have high take-up rate in Northern Europe, i.e., 50% of the new industrial buildings and 25% of new office buildings in the Scandinavian market. However, the take up rate of displacement ventilation in North America has been much lower, i.e., less than 5% of new buildings.

How the technology could contribute to socio-economic development and environmental protection top

Due to the high percentage of energy consumption, highly efficient HVAC systems contribute to both economic and environmental development. Firstly, they are reported to have the potential of energy saving of 30 to 40% in overall building’s conventional HVAC energy consumption, which contributes greatly to GHG emissions reductions in the building sector. Furthermore, such savings can be translated into significant savings in electricity bills for the building owners and/or tenants. With the right government regulations and support, ESCO businesses could prosper in this area, which in turn stimulate more market demand and the adoption of highly efficient HVAC systems.

Highly efficient HVAC systems deliver cleaner and better quality air to the indoor environment – i.e., through the carefully located fresh air intake, the installation of automatic condenser tube cleaning systems and UVC emitters. This, in turn, contributes to better indoor living and working environments, reduction of sick building syndrome, and better living comfort and productivity.

Financial requirements and costs top

As highly efficient HVAC systems can be achieved in many ways, depending on the nature of the buildings, their financial requirements vary. If highly efficient HVAC systems are designed during the design stage, additional investment costs may be minimal in many cases, thanks to that the equipment cost is reduced from proper sizing (instead of oversizing) of the equipment.

Additional investment costs are sometimes required for additional HVAC subsystems for example, from installation of automatic condenser tube cleaning systems, larger piping areas or ice storage systems. In general, the increased investment costs for highly efficient HVAC systems will be recouped from energy savings and reduced maintenance costs. For example, the typical payback period for a system with 30% energy reduction is about 3-5 years in North America (Graham, 2009). The following are some indicative investment costs for highly efficient HVAC sub-systems in Singapore:

  1. Absorption chiller with capacity of 1mW costs approximately S$315,000; 2mW costs S$501,000; 3mW costs S$783,000; and 4mW costs about S$1,061,000.
  2. A 1.5kW variable speed drive chiller costs approximately S$922, 5kW costs S$1,500, 10kW costs S$2,000, 22kW costs S$3,200, and 30kW motor costs S$3,600. The payback period is about one year or less.
References top

BCA (2007). Green Building Design Guide – Air-conditioned Buildings. Singapore: Building and Construction Authority.

Bradsher K. (08 December 2010). China’s green-centric tycoon. The Bulletin. [Online]: http://www.bendbulletin.com/apps/pbcs.dll/article?AID=/20101208/NEWS0107...

DLS. (2009). Green Building Products and Technologies Handbook. Singapore: Davis Langdon & Seah Singapore Pte Ltd.

Freedonia (2010). World HVAC Equipment: Industry Study with Forecasts for 2014 & 2019. Clever land, USA: the Freedonia Group.

Graham, P. (2003). Building Ecology: First Principles for a Sustainable Built Environment. Oxford: Blackwell.

Hydroball (2007). Product Specification. [Online]: www.hbt.com.sg/design_spec.htm

IPPC/TEAP (2005). Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons. Special Report. Cambridge: Cambridge University Press.

Levine M., Urge-Vorsatz D., Blok K., Geng L., Harvcey D., Lang S., Levermore G., Mongameli Mehlwana A., Mirasgedis S., Novikova A., Rillig J. & Yoshino H. (2007). Residential and Commercial Buildings. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Metz B, Davidson O. R., Boshch P. R., Dave R. & Meyer L. A. (eds)]. United Kingdom & United States: Cambridge University Press.

WBCSD (2008). Energy Efficiency in Buildings Facts & Trends. World Business Council for Sustainable Development’s Report. Switzerland: Atar Roto Presse SA. [Online]: http://www.wbcsd.org/DocRoot/ JNHhGVcWoRIIP4p2NaKl/WBCSD_EEB_final.pdf