Written: Prakash Maiya & team
Refrigeration and Air-Conditioning Laboratory, IIT Madras
The gases used as coolants and refrigerants in our air-conditioners, compounds known as hydrofluorocarbons or HFCs, have very bad environmental impacts. These HFCs have several hundred to several thousand times more global-warming potential than carbon dioxide, which is the most common greenhouse gas. If allowed to be used unrestricted, it is estimated these HFCs will cause an additional temperature rise of about 0.5°C by 2100. Not surprisingly, therefore, a global effort is on to reduce and eliminate the use of these gases. Last month, countries reached a major agreement in Kigali, Rwanda, to phase down use of all HFCs by almost 90 per cent by 2050.
But we also need to find substitutes for these HFCs. We already have a few available. Unlike the HFCs, the substitutes currently being worked on are not universal in their use. A substitute that works well in the cooler climates of Europe, for example, might not work in India when ambient temperature is high.
There is also an attempt to look for natural compounds to serve as coolants and refrigerants instead of synthetic molecules like in the HFCs.
Ironically, carbon dioxide is seen as a very viable alternative refrigerant for a variety of cooling requirements. Carbon dioxide was used as refrigerant in air-conditioners long ago, but was discarded when more efficient, low-pressure fluids like CFCs (chlorofluorocarbons) came along. But CFCs were found to be eroding the ozone layer and these got replaced by HCFCs (hydrochlorofluorocarbons) and HFCs. Now, there is an attempt to go back to carbon dioxide as it is a natural compound.
Carbon dioxide has already started being used for some cooling applications in Europe, particularly in commercial refrigeration/supermarket refrigeration plants. But it is not considered suitable for warmer countries like India where ambient temperatures can be anywhere between 30° and 45°C. The efficiency of carbon dioxide as a coolant drops drastically with rise in temperature. However, this can be improved by incorporating work recovery devices in the system layout.
Our Norwegian collaborators and we are trying to design carbon dioxide-based refrigeration and air-conditioning plants that can work with high efficiency in Indian conditions. Our partners in Norway are global experts in carbon-dioxide-based refrigeration and air-conditioning and are convinced such a system can work in India. There are several reasons why a carbon-dioxide-based system makes sense. Broadly, there are three kinds of cooling requirements — air-conditioning (cooling to around room temperatures), refrigeration (cooling to about 0°C), and deep refrigeration (cooling to sub-zero temperatures). Carbon dioxide can be deployed for all three purposes inside a single centralised plant, unlike conventional systems in use in which different HFC gases have to be used.
Carbon dioxide is also naturally available and would not need to be manufactured. Carbon dioxide emissions from any industry can be collected and used for refrigeration. In this manner, carbon dioxide-based refrigeration can also be seen as a small carbon sink.
But there is certainly a risk involved. Carbon dioxide-based air-conditioning systems work at high pressures, at least four to five times higher than HFC systems. If not handled well by mechanics during repair, there is a possible hazard. This was one of the reasons for carbon dioxide being discarded when CFCs were introduced. Mechanics, therefore, would need to be properly trained. But the high-pressure operation also means these systems can be much more compact than the conventional systems we use now.
As part of our project, which is still in its early stages, we are demonstrating the use of carbon dioxide in an integrated refrigeration and air-conditioner system for a mid-size supermarket in Indian conditions. We have added a few new components to this system to enable it to work more efficiently in high temperatures. The system will be fabricated in Europe, encased in a container 6m long, 2.5m wide and 2.9m high, and shipped to IIT Madras for us to work on.
We will then deploy it in a simulated environment in our laboratory, collect data, evaluate its performance, hold workshops, demonstrate to stakeholders and make design changes if necessary. The carbon dioxide pilot system will also be applied to perform practical training to gain know-how on carbon dioxide refrigeration technology. The target group is local students and other refrigeration staff of partner companies.