Heat from the Cold: Flameless Heating for Buildings

Efficiency is always a hot topic in discussions of energy. Heat loss is the cause of most of the inefficiency when combustion powers transportation or generates electricity. One might think then that the use of combustion for heating would be the most efficient, but it’s not even close to the best option.

The calculation of efficiency for fossil fuels is easy. Here’s a lump of coal, how much of the heat can you use when it’s burned? Furnaces that burn fossil fuels range from around 56-98.5% efficient. Typical mid-range models average around 80% efficiency. Electric resistor heaters can be closer to 99% since the electricity is converted directly to heat and temperatures can be controlled more precisely.

There’s an amazing technology, though, that uses the properties of fluids to reach efficiencies well above 100%. This might sound like a perpetual motion machine or some kind of sci-fi a concept, but it’s not. Heat pumps can provide both heating and cooling with the same system, moving heat instead of generating it. The coefficient of performance is measured as the heating energy out divided by the electrical energy in, and heat pumps can reach coefficient of performance over 5 (500% efficiency). Heat pumps do this by extracting heat from the cold outside, or cold from the heat outside. Sound impossible? Let’s get into a little more detail on how this works.

Have you ever deflated a bike tire and felt the air rushing out? It feels cold, right? The air within the tire before deflation is not cold; it’s the same temperature as the ambient air. It’s the deflation that causes a drop in temperature. When any gas is decompressed it expands, and cooling happens. This is the same process that makes it cold on the top of mountains.

The opposite is also true, so when gases are compressed, they heat up. The heating and cooling can be cranked up even more by using fluids that can change phase, from gas to liquid and vice versa. If a liquid evaporates when expanded, it can provide even more bone-chilling cold. Similarly, condensation can be a huge source of heating.

All these thermodynamic principles can be put to use by compressing a fluid called a coolant in locations where heat is needed, and decompressing it where cooling is desired. Air conditioners and refrigerators decompress coolants inside, providing cooling. Expansion happens in coils outside. If you’ve ever walked past the coils on the other side of a big air conditioning unit of a building, you’ve felt the huge blast of hot air occasionally coming off of the unit. That’s the air conditioner working to extract cold from the air outside.

This process can be run backwards as well. Heat pumps are the most efficient way to heat up buildings in weather that’s not incredibly cold. Expand the coolant outside, allow the frigid coils to be heated by the outside air, and then compress the coolant inside, creating warmth.

This process doesn’t work very well if it’s extremely cold outside. But there are ways to set up high efficiency heat pumps to work even better. Ground-sourced heat pumps have their coils underground, where temperatures typically don’t change as much. Compared with the standard air-sourced heat pumps, which have coefficient of performance ratings (under standard test conditions) between 2 and 4, ground-sourced heat pumps have coefficient of performance ratings between 3 and 6. The real-world values depend on the system design, the weather conditions, the temperature needed, and the type of ground, among other factors.

Heat pumps can be used in unique ways that maximize efficiency. Ductless mini-split heat pumps move the coolant into individual rooms before compressing or expanding. This means that air need not be moved within ducts, where much of the loss within buildings can occur. Different rooms can be heated or cooled individually, with separate thermostats.

There are a variety of ways to reduce heating and cooling needs within buildings, through the use of both new technologies and traditional methods. Insulation is a first step, so drafts of cold air don’t sneak in in the winter. Building structures into the ground can help to add thermal mass, so temperatures stay more consistent throughout the seasons.

The Sun provides such a remarkable resource for heating on Earth. Properly oriented windows can be used to allow in light when it’s cold or to reduce illumination needs, and shades can be drawn to lower temperatures in hotter times. Overhanging shading can be constructed to block sunlight when the sun is higher in the sky during the summer. The Sun can also be used to heat up water in rooftop solar heating, which can be used directly or to heat floors, a common method in Asia.

All in all, combustion within buildings leads to a carbon dioxide release of 3.5 Gton per year, a little less than 10% of total emissions. Most of the combustion is for heating, either of spaces or of water. Across the world, fossil gas, oil and coal are all used for heating, as well as biofuels. Buildings use electricity too of course, but this is not included in the 3.5 Gton per year figure. Electrification of space heating and hot water heating with heat pumps, and increasing efficiency with passive solar and other techniques are the cornerstones of eliminating building emissions.

Another source of emissions in buildings is fossil gas cooking stoves, which is a major source of indoor air pollution as well. There are many electric options for cooking, including induction stoves and resistance burners. The spiral coils of resistance burners, the cheapest option, make chefs turn up their noses due to their poor cooking quality. Induction burners, on the other hand, are quickly becoming the cooking method of choice for top chefs worldwide. Induction stoves are powerful and efficient, and can boil water in half the time a gas stove can. Induction cooking requires metal cookware because the pots and pans are directly heated by magnetic induction. This cooking method means that there is less heat wasted around the sides of the pots, adding to energy efficiency and keeping the entire kitchen cooler. While many induction stoves cater to luxury buyers, it is possible to purchase single induction burners for as low as $50-60. This is the choice method of Chef Dargan, whose induction cooking tips follow.

Extreme heat

Heat waves are quite dangerous, and are increasing worldwide as the planet gets hotter. Extreme heat is especially dangerous to the elderly, children, people with disabilities, and people working outside. They’re the most deadly type of weather disaster worldwide, over tropical cyclones or extreme rainfall. In addition to heat stroke, cardiovascular issues are more common during heat waves. Many types of air pollution, like ozone, can be worse in heat waves as well.

Our bodies have the natural ability to fend off certain types of heat. When we sweat, the water evaporates, resulting in cooling. This cooling is less effective if there’s water vapor, or humidity, within the air. The difficulty of cooling by sweating is why humid heat feels so much more stifling. Heat index is the measure of how hot it feels, given the temperature and humidity.

Humid heat waves are dangerous for more than just the heat index. Since water vapor is a heat-trapping gas, night stay warmer when it’s humid. Mortality and hospital admissions in heat waves is especially correlated with nighttime temperature. When people have time at night to cool their bodies back down, there are less severe consequences on health. Community cooling centers, with air conditioning and adequate sitting space for families and the elderly, are a proven way to keep people safe during temperature extremes.

“Water, rest, shade” is the mantra for workers outdoors in extreme heat. It’s also often necessary to have a buddy system, when co-workers can monitor each other for early signs of heat stroke. Often long before heat stroke sets in, there are increased chances of injury, as workers can become slightly disoriented in ways that affects their performance on the job. It’s extremely important that workers across the world have safe working conditions as we face increased weather extremes over coming decades.