Heat Pumps, Carbon Emmissions, and Carbon Dioxide: a Love Story.

I love heat pumps. The refrigeration cycle is an elegant and beautiful thermodynamics hack: by manipulating the temperature and pressure of a gas/liquid phase changing fluid, heat can be forced to move from a cold space to a warmer one. A neat trick on it's own, but better is the fact that the energy required to drive this process is significantly less than the energy that is actually moved - by a ratio of up to 6:1.

Best is the fact that the energy used to drive the cycle (which turns into heat via friction and ends up being picked up by the phase change fluid) is itself transferred to the high temperature zone. So in contrast to a fossil fuel heater, where 81% - 97% (at the very most) of the energy available in the fuel is transferred to the heated space, a heat pump moves between 200%-400% more energy than it uses in electricity.

However, heat pumps use 'freons' - the outlawed CFCs like R22, and the new HCFCs like R410A or R404A -- and these gases are expensive, were ozone depleting, and cause climate change. Furthermore, commercial HCFC heat pumps have historically been limited to outputs of 55C - considered due to bacterial growth dangers to be to low for domestic hot water use.

However, there is good news: one of the up and coming heroes of the refrigerant world is the world's leading climate changing gas.

Carbon dioxide (CO2) in the atmosphere causes climate change, and it is released in large volumes due to our civilization's fossil fuel addition. It's important to remember that carbon dioxide emissions resulting from disruption of natural carbon sinks is harmful to the planet's atmospheric carbon dioxide levels, but in previously occurring quantities of atmospheric CO2 (prior to about 200 years ago) this life-giving gas formed (and still forms) the building block plants use, in combination with water and sunlight, to grow.

CFC refrigerants, on the other hand, are synthetic chemicals with no origin or purpose in nature. I'm not intrinsically opposed to such chemicals unless they have negative properties - but CFCs, and HCFCs, the environmentally preferable replacements which do not cause ozone layer depletion - cause global warming at a rate 1500 - 4100 times the rate of an equivalent volume of CO2. And because refrigeration cycles require that the CFCs function in a high pressure environment and in a cost-effective piping system, even factory sealed systems can lose their refrigerant charge over time. In large grocery refrigeration systems, leakage rates of 5%-15% are not uncommon.

This environmental consideration, along with the very high cost of HCFC gas - is driving a current industry quest for cost effective refrigeration solutions utilizing natural refrigerants - that is, non-synthetic, naturally occurring chemicals which have phase changes from gas to liquid at temperatures and pressures suitable for food refrigeration or space conditioning.

Ironically enough, CO2, the 'problem' greenhouse gas, turns out to be a fantastic refrigerant. Here are some of the best features about CO2 refrigeration systems:

• low material costs in comparison to HCFCs, due to a high heat of vaporization, which allows lower mass flow rates through the system, and smaller tubing.
• the phase change properties of CO2 are such that much hotter condensing temperatures can be achieved in CO2 systems: products available in the marketplace today can heat water to 60C - 90C, while still operating with a coefficient of performance of up to 4.
• in some systems, this high output temperature can be created concurrently with very low evaporation temperatures; in fact, COPs increase with higher temperature differences between the low and high sides of the system. This is very unusual and creates opportunities for heat pumps where they would typically be unable to perform.
• very inexpensive: CO2 costs between .80c - $2.00/lb, versus $22-$30/lb for HCFCs.
• available: CO2 is used in the food industry to carbonize fountain drinks; easily purchased in large quantities anywhere in North America where fast food can be found.
  
  Japan's in this secret. Over the last 10 years, over 1.5 million 'EcoCute' heat pump domestic hot water heaters have been sold to homes throughout the country. These units cost 66% less to operate than an electric water heater. More information can be found at http://en.wikipedia.org/wiki/EcoCute.
  
  The only caveat to this love-in for heat pumps is that, depending on what electrical grid the heat pump is supplied by, the overall climate change impact of it's operation will change dramatically - and isn't always a win over a natural gas appliance.
  
  In Alberta, Canada, most electricity is generated by burning fossil fuels like natural gas in remote power plants to generate heat, and produce steam. This steam runs a turbine, which generates electricity (30-45% efficient process). This electricity is run through high voltage wires (7-8% line losses), through several step down transformers (each of which has 1-5% losses). Even with a COP of 3, it's probably better just generate the heat on site (3%-20% losses) than to bother with this.
  
  However, for many other grids, such as next door neighbour British Columbia, much of the electrical generation is sources from hydro, wind, and nuclear power. Without fossil fuel origins, there are only secondary environmental impact greenhouse gas emissions (i.e. methane from flooded lands for hydro) and line losses. The use of electricity to pull heat from surroundings and use it effectively makes sense for these areas, and supports the development of clean alternative electricity sources.