cover story >>> SEAN McGOWAN REPORTS Thermal energy systems — what you need to know Increasingly, building designers are turning to thermal energy to help deliver sustainable outcomes. Thermal energy systems come in many forms, from thermal mass to chilled water storage and phase change materials — and are delivering results in a range of applications. With today’s focus on more responsible building design, including sustainability, occupant health and comfort, indoor air quality and importantly, low energy consumption, thermal energy systems are emerging to play an important role in delivering these outcomes. While some forms of thermal energy have been used by man to provide thermal comfort for hundreds, if not thousands of years, more recent developments are delivering both measurable and sustainable benefits. HVAC&R Nation takes a closer look at five thermal energy systems, and discovers what you really need to know. Thermal mass Thermal mass is the ability of a building material to absorb and store heat, before releasing it at a later time. Therefore when used correctly, it can provide cooling during warmer months, and heating during cooler months. Thurgoona Campus, Charles Sturt University “Thermal mass is the quiet According to Arkins, this process is called the best, but in areas where the diurnal temperature range achiever of the building world,” “thermal flywheel” effect, where the greater the time is small, thermal mass can make internal conditions says Chris Arkins, associate lag of external construction the greater the effectiveness worse by radiating all the heat at nighttime during with Steensen Varming of the mass. a heat wave, and absorbing all the heat you produce on a winter night. Australia. Thermal mass influences the internal temperature within a building by providing heat sink surfaces for Thurgoona Campus, Charles radiation, conductive and convective heat exchange; by “It has characteristics that when understood and applied providing a time lag between peak external and internal Sturt University, Albury Australia in an appropriate manner can provide opportunities not temperatures and by providing a reduction in heat Experiencing hot windy summers, the Thurgoona commonly available through more traditional systems transfer across an external wall. Campus of Charles Sturt University in Albury has used and design approaches.” thermal mass to hold warmth in winter and coolth in Thermal mass can also be used in mixed mode to summer. Rammed earth exterior and internal walls Building materials of high density, such as concrete, reduce air conditioning loads, with the most common between 300mm and 600mm have been employed, bricks and tiles are said to have a high thermal mass, application being a nighttime purge cycle to precool along with concrete floor slabs and suspended slabs while those which are lightweight such as timber have the building mass and remove overnight residual internal to upper floors and ceilings. a low thermal mass. Therefore by using materials with a heat gains. high thermal mass, and positioning them in such a way The campus’s main lecture theatre is buried under that they make the most of the sun’s radiation at the “Energy savings are then realised through reduced a grassed mound to take advantage of the thermal right times of the year, thermal benefits can be exploited. morning pull down loads plus the convective cooling properties of the earth around it, moderating heat offered by the storage capacity of the mass,” says Arkins, losses and protecting it from extreme ambient heat, As the summer sun is positioned high in the sky, and the adding that in commercial CBD buildings, this can while many of the buildings use stack-induced natural winter sun is positioned lower, shading techniques can typically only be achieved through mechanical means; ventilation created by thermal chimneys. moderate the thermal effects of the summer sun, and with the fan power required often offsetting the energy maximise the heat gain in winter when the sun is lower. savings achieved. Thermal mass in a building acts as a thermal battery, Decoupled thermal mass or thermal sink – whereby in summer it absorbs heat, Where it works Decoupled thermal mass storage is a term used to describe keeping the space comfortable, while in winter the In areas that experience a large diurnal temperature situations where thermal mass is accessed outside the same thermal mass can store the heat from the sun and range (that is, a large difference between daytime building envelope, such as the earth below the building, release it during the night. and nighttime temperatures), thermal mass works or depending on location, water from a river, lake or ocean. www.hvacrnation.com.au September 2008 HVAC&R Nation 19 Federation Square, Melbourne The two most common applications of decoupled around the world, where it can reduce first costs in some thermal mass are labyrinth systems which consist of cases, as well as increase lettable space. tunnels built from a high thermal mass material that “The use of low temperature water from ice storage use air to transfer the thermal storage capacity of allows savings in piping, pumps and air handling the labyrinth to the building; and geothermal heat equipment,” explains Ramez Naguib, international exchangers that provide access to substantial thermal marketing manager for Baltimore Aircoil Company. mass, with water used as the medium to transfer energy to and from the building to the earth. Ice thermal storage systems generally produce ice at night taking advantage of off-peak electrical tariffs, and Labyrinth designs are particularly effective in exploiting therefore helping to reduce peak demand on electrical the diurnal temperature variations of a location to supply. This ice is then stored until cooling is required, at Federation Square labyrinth provide low energy cooling. For example, Australia’s which time water is circulated through the ice storage largest labyrinth lies under Melbourne’s Federation area before being distributed to the system to provide operational hours to cool the Atrium via a low velocity Square and is 1600m² in size and 1.2 kilometres in length. space cooling. displacement system at floor level, as well as provide These systems work by flooding the labyrinth with environmental climate control and act as a pre-cooling An ice storage system offers two major benefits. Firstly, cool night air, which in turn cools the high thermal system for other areas of the complex. it can take advantage of off-peak electrical tariffs, mass material. During the day, air is drawn through the and secondly, it can reduce the chilled water flow “In peak summer conditions, the labyrinth is capable labyrinth where it is cooled before entering the building. requirements by half. In winter, the process is generally reversed. of delivering air to the Atrium at up to 12°C below the external ambient temperature, equivalent to “An ice storage system with supply and return water Where it works conventional air conditioning, but using one tenth of the temperatures of 2°C and 13°C versus a traditional system energy consumption and generating less than one tenth using 7°C to 13°C requires almost half the water flow, Again, decoupled thermal mass systems take advantage of the CO² emissions,” explains Arkins. pump BHP and smaller pipes,” says Naguib. of large diurnal temperature ranges, so areas that experience this type of climate are prime candidates Other installations “In addition, when low temperature air distribution for this system. is also used, additional savings are realised from • The Earth Centre, Doncaster U.K. smaller ductwork, smaller fans as well as less energy Federation Square, • National Museum of World Cultures, Gothenburg consumption and increased usable space.” Melbourne Australia Sweden The fact that this system has traditionally been an open Below Melbourne’s Federation Square lies a 1600m² loop has delayed its widespread adoption, however new concrete labyrinth which provides energy efficient Ice storage ice-on-coil technology allows designers to have a closed cooling and heating to the complex’s Atrium and other loop system. areas. Using ice for thermal comfort has been used for In Australia, however, such systems have not proven centuries, and not just by the Eskimos. It works by pulling the cool night air off the adjacent popular, with only handful of older buildings using the Yarra River through the labyrinth, which in turn cools In modern times, ice thermal storage has been system. While electricity suppliers prefer the system down the thermal mass walls, and is used during successfully implemented in many building projects because of its use of off-peak electricity, its high 20 HVAC&R Nation September 2008 www.hvacrnation.com.au energy requirements mean it’s unlikely to be popular anytime soon. Where it works Ice storage has been used effectively in major projects around the world, including the U.S.A. and Japan where its use in district cooling projects is both popular and effective. In Australia its use is limited. Centex Building, Dallas Texas, U.S.A. The Centex Building in Dallas is one of the United States’ most energy efficient buildings, thanks largely to its ice storage system. By combining the use of fourteen ice tanks with two high efficiency electric rotary water- cooled chillers, the normal peak electrical load of a building such as this has been dramatically reduced. The chillers cool a glycol solution to – 4°C to produce ice in the storage tanks, which are then used for eight hours starting from noon to provide efficient cooling to the building while using only one chiller. This partial ice storage system provides up to 7400kWh of cooling capacity, with the melting ice providing 40% of the building’s cooling requirements on a 38°C day, and a much higher percentage during cooler months.