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A/C future is ancient: heat-pump & evaporative cooling hybrids

They can look a bit like an air conditioner unit. In the winter, the appliance extracts ambient heat from outdoor air, pumping it inside, while it reverses the process in the summer to pump heat out. The new models are so efficient in doing so that heat pumps make sense now almost anywhere.

There’s one crucial transformation taking place that gets less attention than electric vehicle adoption and renewable energy capacity (or share of energy production): the rise of heat pumps as climate systems for houses, offices, commerce, and industry.

A heat pump is an HVAC device that can heat a building by efficiently transferring thermal energy from the outside using a refrigeration cycle (or the classic thermodynamics principle of heat transfer). They move heat rather than generate it, consuming much less energy, especially in places that rarely stay below-freezing temperatures over extended periods of the year.

There’s no mystery in this proven technology: to heat a space, a liquid refrigerant in a copper coil extracts heat from the atmosphere as warm air moves toward the cold; the heat the refrigerant into a cold gas:

“A compressor then pressurizes the gas, raising its temperature and heating the air inside the house. In the summer, the appliance cools a home by absorbing heat from the inside and transferring it outside. Heat pumps are up to four times as efficient as natural gas furnaces since they merely move heat from one place to another rather than burn fuel.”

Improving things without reinventing the wheel

Heat pumps are not that different from air conditioners. However, their few divergences explain heat pumps’ reduced impact and superior efficiency: in an AC system, the refrigerant absorbs heat in the air and dumps it outside, whereas heat pumps reverse the process, finding the heat outside and using it inside. To function, heat pumps need to stay on all the time; their system automatically adjusts to outside temperatures, ramping down when it’s warmer.

A water heater that works with heat pumps uses the same heat interchange principle. It is also up to four times more efficient than combustion or electric water heaters: only solar-thermal heaters are more energy-efficient (when functioning correctly at peak hours of sun radiation, that is). Instead of creating heat, heat pumps move it around: even the most efficient combustion boilers generate as much heat energy as they consume, whereas air-source heat pumps deliver up to three times more heat energy than they consume.

There are three types of heat pumps based on the thermal interchange source: air, water, or groundmass. Air-source heat pumps use the energy in outside air (or air coming from a ventilation system) for heating, cooling, and heating water; water-source heat pumps take energy from water (ground, surface, sea, or sewage water) and are especially efficient thanks to the conductivity of water; and ground-source heat pumps tap into the energy stored as thermal mass in the ground, using either a vertical or horizontal collector to do so.

Ground-source heat pumps can be exceptionally efficient but also have a higher upfront cost associated with extensive land excavation; some run on the geothermal energy in the backyard thanks to buried plastic pipes that exchange heat with a given terrain. Or, according to Ryan Dougherty from the Geothermal Exchange Organization, geothermal heat pumps work as a giant battery:

“You can draw off that thermal battery in the winter and you can heat your house with the energy that’s right there in your yard. And then in the summer, the process is just reversed: You take heat out of your house and you put it back into the battery.”

The question of modern heat pumps under extreme weather

However, heat pumps are overall simple, relatively affordable, easy to maintain, and durable. They are also easy-to-install devices, and, despite their name, they don’t only provide heating and hot water but cooling as well. Relying only on electricity, they also have a smaller impact, using a third as much electricity as equivalent electric heaters. They create comfort way more efficiently than more pollutant gas, oil, or biomass furnaces.

Are these devices too good to be true? Experts and energy agencies agree on the advantages of modern heat pump systems as they rely only on electricity and avoid combustion to provide thermal comfort and clean indoor air, thus helping environmentally at both the macro and micro scales. They also include conventional air filtration systems designed —as in traditional furnaces— to clean the air from airborne irritants such as dust, wildfire smoke, traffic pollution, and pollen, potentially helping people manage allergies.

Heat pumps’ biggest caveat: they work in different types of climates, although they decrease their efficiency by creating hot or cold air when the weather outside is extreme (either too cold or too hot), and so they excel in the temperate climates of middle latitudes: heat pumps get progressively less efficient as the temperature divergence between the two environments (“outside” and “inside”) increases. They work hardest, in other words, during extreme weather situations when it’s dangerously hot or cold outside, yet they still manage to be more efficient than their counterparts.

Early heat pumps’ bad reputation derived from their traditionally weaker performance amid bitterly cold weather, but the situation has changed in the last years:

“Now, as long as there’s heat energy present in the air outside — and there’s always some, until we reach absolute zero (negative 273 degrees Celsius/negative 459 degrees Fahrenheit) — an inverter heat pump can pull heat energy into its refrigerant, pressurize it, and move it indoors. Modern cold-climate heat pumps have been shown to operate efficiently even at sub-zero temperatures.”

The situation has reversed to the point of finding today that most heat pumps are installed in some of the coldest climates (more than half of all households in Norway have a heat pump).

This recent improvement explains recent developments across regions with harsh winters, from the US Northeast and Canada to Germany and Scandinavia across the Atlantic. According to the European Heat Pump Association, EHPA, the market for these devices grew by 50% in Finland in 2022. The country has now installed 1.5 million heat pumps in a country with just 2.7 million households. In Germany, the market grew by 53%, with 236,000 units sold in 2022 and the beginning of exponential growth.

Heat pumps and increased self-reliance

Modern heat pumps also have a higher upfront cost than their more expensive-to-run electric or combustion alternatives, and since they work exclusively on electricity, they are susceptible (like traditional A/C) to power outages. Their dependence on electricity makes them also challenging in off-grid buildings that depend on intermittent renewable sources for power, hence increasing the installation price through the need for batteries to use stored energy as a backup.

Despite the bigger upfront cost, those thinking of replacing their HVAC system anyway can get incentives when replacing less efficient systems for a heat pump, including the US (up to $8,000 after the approval of the Inflation Reduction Act), the UK (with a £5,000 subsidy), Canada (up to $5000 for upgrades and $600 for energy efficiency evaluations), and some EU countries (several countries have recently promoted their installation as a national strategy to decrease their dependence from Russian gas).

Also, heating replacements can be less expensive than thought for homes with central heating, as heat pumps can reuse the existing ducts as long as they are in good shape and the house is not too drafty. Heat pumps seem to fit homes with dated electric panels or even off-grid electrical installs. Most models are high-voltage appliances, although several companies design lower voltage heat pump water heaters designed with 120 volts that don’t require installing a 240-volt circuit panel.

California Accessory Dwelling Units, small cabins, RVs, or tiny houses have two widely available commercial options designed for gas unit replacements: the 120-volt version of Rheem ProTerra and Ruud Ultra water heaters, both described as “plug-in heat pumps designed for easy gas unit replacement.” Electric, tankless water heaters designed for RVs can also function on 120-volt installations but can be more energy-intensive and are less efficient in heating water than their heat pump counterparts.

Those focusing on heating air and water in small spaces can also install a biomass stove (included in the tax credit approved by the US federal government). RVs, tiny houses or boats are also eligible for tax rebates as long as they are considered as a second home for tax purposes.

Swamp coolers and dry climates

Heat pumps may be the right HVAC system for almost any home, all things considered, but are there portable or temporary installs for those living in rental units, in places with complicated renovation rules, or unconventional dwellings? Several portable and window air conditioners include heat pump capabilities. In mild weather areas, such appliances with their inverter compressors will be more efficient than a 1500-watt electric heater. That said:

“Like traditional heat pumps, these temporary setups work best when maintaining comfortable temperatures between 60 and 70 degrees rather than actively heating or cooling a space. Once the outside temperature drops below 40 degrees or so, you might need to find an alternative heat source. But if you live in a place with a warmer year-round climate where that’s not a problem, then a portable or temporary heat pump could be a decent option.”

Heat pumps have a more affordable and perhaps flexible alternative in dry, warm climates, one HVAC low-tech (and low-cost) combination that’s better suited in an off-grid environment with intermittent renewables as a power source: in such cases, a combination of solar PV for energy generation, solar thermal panels for water heating, and a swamp cooler running on solar may be enough to reach comfort all year-round.

Also known as “evaporative coolers,” swamp coolers are simple, unsophisticated, low-maintenance appliances that blow outdoor air through water-saturated pads; as warm air goes through the soaked pads, water evaporates, causing the air to become cool. The blower pushes the cool air inside, and the warm air escapes through open windows. Some people improve such systems using homemade ventilation towers.

Swamp coolers are inexpensive, easy to install, and cheap to operate, though windows need to be partially open to let the warm air dissipate outside, requiring attentive supervision.

Modern-ancient systems: qanats-windcatchers of the future

Several ancient cultures used evaporative cooling methods to keep temperatures down by maintaining ambient moisture. Earlier forms of evaporative cooling, like windcatchers, were first used in ancient civilizations from Egypt to the Fertile Crescent and Persia thousands of years ago. Buildings would include wind shafts on their roof to channel hot wind that would be cooled down over subterranean water channels in a “qanat,” discharging the cooler air into the building.

Qanats transport water from subterranean aquifers and wells to the surface via underground aqueducts that prevent evaporation. The oldest working qanats predate biblical times and are mentioned in Abrahamic texts. Resilient under extreme conditions, they are built in a gentle slope that guarantees their flow thanks to gravity, including well-like vertical shafts (covered to minimize blown-in sand) that allow direct access to water and maintenance. They’re still used for irrigation, animal and human consumption, and ancient cooling systems, as well as ice—and food—storage.

Some qanats damaged or abandoned in the last decades are being restored. Some qanats still work in conjunction with wind towers (“windcatchers”) to generate an effective low-tech, easy-to-maintain climate system, perhaps the first HVAC technology deployed at a civilizational scale.

Heat pump water heaters inner workings: air intake (1), compressor (2), heating element (3) and digital controls (4) Source: New Buildings Institute

Windcatcher chimneys had 4 openings; the opening opposite the wind direction allows air evacuation from inside the house, whereas incoming air is pulled inside to regenerate its current as it cools down. The airflow across the vertical shaft opening recreates a lower pressure that draws cool air (heavier than warm air) back up; the remaining heat quickly dissipates as water evaporation around the qanat feeds the air stream.

Without heat pump technology, ancient refrigeration methods such as the combination of windcatchers and qanats allowed livability conditions and gardening inside cities whose houses and shady streets had temperatures up to 15 degrees Celsius inferior to the immediate surroundings. Could heat pump systems deployed at a big scale prevent some urban areas from becoming unlivable?

The future solutions look both at current efficiency improvements of heat pumps and at the effective marvel of ancient evaporative cooling methods.