Cool buildings by using heat?

Published on Tuesday, 16 August 2011 14:00

	By Robin Whitlock Follow @RobinWhitlock66

Imagine an air conditioner that cooled houses using excess heat.

Such devices have been employed in large industrial buildings for years but it may now be possible to adapt them for use in smaller buildings such as domestic properties and shops. Researchers at Pacific & Northwest National Laboratory (PNNL) have developed new porous materials that improve adsorption chillers thereby bringing closer the day when air coolers could utilise excess heat from solar water heaters and generators in small buildings.

Conventional air conditioners operate by first evaporating and then condensing a refrigerant. The substances used have good thermodynamic properties, are un-reactive and have a high heat of vaporization (the energy required to transform a quantity of a substance into a gas at a particular pressure). Refrigerants have traditionally consisted of harmful substances, such as CFC’s, ammonia, sulphur dioxide and methane. The evaporation of the refrigerant removes heat thereby inducing the cooling effect. The refrigerant is then condensed using an electric compressor which pressurises the vaporized refrigerant.

An alternative to conventional air conditioning and refrigeration is adsorption chilling. Adsorption (not to be confused with absorption) is a process which involves a molecular reaction between solids and gases enabling gas to be adsorbed on to the surface of a solid. In essence, particles of a gas, liquid or other dissolved substance adhere to a surface creating a film known as an adsorbate.

This occurs because unlike the particles within the interior of a bulk material, the particles on its surface are not totally surrounded by other particles and are thus free to attract other particles. The solid then acts like a sponge. Adsorption is common in natural physical, chemical and biological systems but in industrial use adsorbates commonly consist of oxygen compounds, carbon compounds or polymers.

An Adsorbtion Chiller. Image courtesy of Thermax. 

Adsorption chillers typically employ a special adsorption chamber which is filled with a solid, usually silica gel, thereby eliminating moving parts and the noise that accompanies them. The process uses excess heat, normally from hot water, to condense or ‘desorb’ the refrigerant within the solid. It is the hot water that drives the adsorption process and in industrial buildings this is normally sourced from gas turbines or water heaters. Prime heat from solar thermal equipment can also be used. When heated the solid releases refrigerant vapour which liquefies when cooled. The liquid refrigerant provides its chilling effect by subsequently absorbing heat and returning to a gaseous state whereupon it is readsorbed into the solid.

Pete McGrail leads the team at PNNL and is quick to stress the advantages of the new adsorption chiller over other air forms of air-conditioning. “The adsorption chiller runs off low-grade heat instead of electricity” he explains, “so provides a means to cool buildings while simultaneously reducing electricity consumption. The heat can be derived from any convenient source including solar or waste heat generated in industrial processes.”

Most adsorption chillers, despite being relatively cheap, are too big and expensive to be employed in anything other than large, usually industrial buildings and they are also far less efficient than compressors. Nevertheless, because they use excess heat they have obvious environmental benefits and as a consequence they are proving to be very popular in many countries including the USA, Japan and within Europe.

The team at PNNL started working on advanced adsorption chillers back in 2009. They built their compact adsorption chiller research on the back of earlier work involving nano-structured metal-organic heat carriers (MOHC’s). These are special nanostructured materials that have been developed for heat transfer operations.

“We design these materials at the molecular level to adsorb and desorb refrigerants” Pete says, “such that higher efficiencies can be obtained in heating, cooling and power generation devices.”

The MOHC’s self-assemble into complex three-dimensional shapes which are more porous than the silica gel thereby providing a greater surface area for the refrigerant to cling to. The new material can trap three to four times more water than the silica gel which means that the size of the chiller itself can be drastically reduced. It is also far more efficient at releasing the water molecules than silica which will also help to reduce the size of the device. Other refrigerants besides water can be employed which means that the chiller could be used within a wider range of temperatures at which the refrigerant can be cooled.

“Prior to this work on adsorption cooling, we had been designing MOHCs for improved efficiency in geothermal power production” Pete continues. “The adsorption chiller idea was an offshoot of that earlier work.”

The US Department of Energy (DOE) Advanced Research Projects Agency for Energy (ARPA-E) announced a funding opportunity in the spring of 2010. The team submitted a proposal and were lucky enough to be selected out of hundreds of submissions. They awarded the lab $2.54 million in research funding. Pete is confident that they can make the new adsorption chillers 75 percent smaller than existing devices and much less expensive. This means they could also be operated using heat from domestic household solar panels.

“The intended use is for commercial building cooling,” Pete explains. “But the higher efficiency and smaller size of the chillers we hope to enable with MOHC technology is expected to open up potential for residential applications and even industrial applications such as chemical separations that were never possible before.”

This has important implications for countries such as India and other countries in tropical and sub-tropical zones, especially so considering rising global average temperatures caused by climate change. The growth in demand for air conditioning units also provokes increased electricity demand with accompanying rises in carbon emissions. This also increases peak electricity demand with 40 to 50 percent of residential energy demand due to air conditioning in India alone.  There are further considerations to take into account. Conventional air conditioning is less efficient in warm and humid climates at dehumidifying the atmosphere.

“Current adsorption chillers using silica gel and water also have reduced efficiency in tropical climates due to limitations of operating conditions for the condenser in the chiller,” Pete says. “With MOHCs, we are looking beyond silica gel water systems and examining refrigerants that can operate more efficiently in hot/humid climates because of their much different vapor-liquid condensation curves.”

Research on the new chiller is an ongoing project for the moment. A potential problem to be overcome with respect to heat sourced from solar power is the possibility that supply of hot water from solar water heaters may be restricted. Therefore, it may be that some kind of heat storage system may have to be devised for use after nightfall.  Professor Yunho Hwang of the Centre for Environmental Energy Engineering at the University of Maryland however points out that thermal storage tanks can be used to solve this.

“A thermal storage tank can be used with solar thermal panels,” Yunho Hwang explained. “And then stored heat can be used to operate heat activated systems during the night time.”

An Adsorbtion Chiller. Image courtesty of Thermax