Over the past year, it seems like everyone involved in California energy policy has started talking about heat pumps–even if they’re not 100 percent sure what heat pumps are. What is it about heat pumps that make them so interesting, and why are they suddenly getting so much attention?
Most of the interest is driven by the state’s aggressive push towards a zero-emissions future.
As a policymaker looking to cut greenhouse gasses (GHGs), your task is to identify the largest sources of emissions, and then figure out how to reduce them. Adopting heat pumps for space and water heating could be a significant step.
It should not come as a surprise that the majority of GHG emissions (45 percent) in California are from the transportation sector. The next largest source, approximately 24 percent, is buildings. Building emissions are primarily from “thermal loads,” such as heating, cooling, and making hot water. In California, the vast majority of buildings rely on the combustion of natural gas to generate heat.
The Problem with Natural Gas
A furnace is a pretty simple device. Imagine a fire burning inside a metal box; now put another box around it and blow some air through. The flame inside the box heats the air, and a blower circulates it through a duct system. The warm air from the furnace keeps your house warm. A gas water heater is even simpler. Take a large pot, fill it with water, and use a small flame under it to heat it. When the water runs out, add some more water and heat it again.
The problem is that according to the 2nd law of thermodynamics, combustion can never be 100 percent efficient, and what is left over is not nice stuff. As far as fossil fuels go, natural gas burns relatively clean compared to fuel oil or other options. Burning natural gas produces some not-so-lovely byproducts, including nitrous oxide, a key component in smog, though at much lower levels than burning the gasoline in our cars does. One thing about gas that is often overlooked are the leaks in the transmission and distribution system. Natural gas leaks release methane—an even more potent greenhouse gas than CO2—into the atmosphere before it even reaches your home. It turns out the leaks in the distribution system are much more significant than has long been assumed.
The storage of natural gas is tricky. When a pipe at the Aliso Canyon storage facility in Southern California broke, it released over a hundred thousand tons of natural gas directly into the atmosphere, creating the largest gas leak in US history—the greenhouse equivalent of burning 1 billion gallons of gasoline. It took over four months to cap and stop the leak. Thousands of people were made sick and were displaced from their homes.
Natural gas explosions are mighty and extremely dangerous–it’s not uncommon for a natural gas explosion to lift an entire home off of its foundation. The natural gas distribution system is vast and old—it’s only a matter of time until we have another San Bruno level explosion somewhere in the state.
Gas burning appliances in our homes are not perfect. Earlier I described a furnace as a box with fire inside a box with air. When the box with the flame gets a hole or crack, combustion byproducts mix with air and circulate throughout the house. These types of failures happen more often than you might think. While performing energy audits, I have found more than one home where a natural gas appliance was poisoning the occupants with carbon monoxide. In two cases, the appliances were brand new. One had damage from shipping, and the other was installed incorrectly. Carbon monoxide, a colorless and odorless gas, is often referred to as the silent killer, producing flu-like symptoms (headaches, fever, achy joints, lethargy) and causing people to fall asleep before they fully succumb to the effects.
Cooking with natural gas releases all kinds of pollutants into your home and is a significant contributor to indoor air quality issues. Recent studies have linked the use of natural gas cooktops to increased rates of asthma and other respiratory illnesses.
If natural gas appliances have so many issues, why are they so prevalent? The simple answer is that they are cheap, easy to install and have the potential to last a long time without significant service. But they are not the only option.
The Case for Heat Pumps
Heat pumps don’t rely on combustion to operate. Instead, they use electricity and refrigerants to move heat from one area to another. Heat pumps are not new or revolutionary technology—we depend on them every day. That beer you just pulled from the fridge is cold because a heat pump moved the hot air out of your fridge.
Does your home have air-conditioning? Air-conditioners are heat pumps. Heat pumps are everywhere: grocery stores, cars, airplanes, shopping malls, and the International Space Station all depend on heat pumps for regular operation. It is a time-tested and reliable technology.
Heat pumps are much more efficient than their gas counterparts. Because heat pumps move heat rather than generating it from combustion, they can be more than 100 percent efficient, as long as the electricity used by the pumps and compressors is less than the amount of energy the heat pump can transfer. Today’s heat pumps in the US are typically around 300 percent efficient. Japan, which has a more mature market with different refrigerants, is achieving 500 percent efficiency. Because heat pumps are powered by electricity, you can use solar panels to heat and cool your home and make hot water. Imagine the idea of cooling your home with the sun’s energy. It’s not science fiction; it’s a reality today.
Heat pumps also don’t produce combustion byproducts, as there is no combustion. As a result, heat pumps are much safer than their gas counterparts, especially if they operate inside a home or building.
Most of the energy policy conversation in California today revolves around replacing gas water heaters with heat pump water heaters and replacing gas furnaces with heat pumps for space heating.
Heat pump water heaters are relatively new technology. They look similar to gas tank water heaters; the difference is how they function. A gas water heater has a flame located at the bottom that creates heat by combustion. The heat is transferred to the tank by conduction and heats the cold water. A flue runs up the center of the tank to vent the exhaust gases outside. As hot water leaves the tank, cold water fills it. The flame in a water heater is comparatively small and heats the water over a time, which is why you can run out of hot water. There are variations on this theme, but all rely on combustion and convection to heat the water.
A heat pump water heater has an electric compressor and some controls on the top. It uses the surrounding air to transfer heat to the refrigerant. The refrigerant circulates in the storage tank via a copper pipe running inside. The refrigerant flows through the copper pipe and heats the water by conduction. There are limits to how much heat it can transfer from the air to the refrigerant. In some cases, the heat pump water heater might rely on electric resistance heat strips to supplement the heat from the refrigerant.
Heat pump water heaters are taller than their gas counterparts, and sometimes the plumbing is routed differently due to the top-mounted compressor and controls. Heat pumps require a certain amount of air around them to transfer the heat from the air to their refrigerant. Many heat pump water heaters have the option of ducting the air from another space to provide adequate airflow. A side benefit/feature of a heat pump water heater is that as they remove heat from the air, the output is colder air. This colder air can help cool the space around the water heater. Heat pump water heaters work well in garages, as there is plenty of air to transfer heat from, and they can cool the space slightly at the same time.
Heat pump space heaters have been around for decades. They work similarly to heat pump water heaters, moving heat rather than creating it with combustion. In a heat pump space heater, refrigerant cycles through a coil, similar to the radiator in your car. The coil resides in an air handler–basically a metal box with a blower. Air is blown through it and circulated through the home via a duct system. The process is similar in a “ductless” heat pump system, but the coil is in a head unit mounted on the wall. Heat pump space heaters are essentially air conditioners that can be reversed to provide heat. Heat is transferred from the outside air or, in some cases, from the ground. They are a reliable and established technology.
Heat pump space heating has had some challenges. In the 1970s, there was a push towards heat pump space heating. Unfortunately, many systems did not work very well. I blame the industry, not the technology. Heating and cooling contractors had been slapping in oversize gas furnaces for years with no regard to the duct systems. Leaky or poorly sized ducts were no big deal as the gas systems were 3 to 4 times larger than they needed to be, so they still produced heat. Heat pumps have less excess capacity, which means if you don’t size the units and the duct systems carefully, you will have less than optimal results. Today’s systems are much more sophisticated and can make up for some deficiencies in installation. Regardless, it pays to use an experienced contractor who sizes the system correctly and configures the duct system to match. A properly designed and installed heat pump space heater will outperform a gas furnace all day long, providing steady, even temperatures, with little or no noise at a much higher efficiency than any combustion-based system can. The real key is a high-quality installation from a knowledgeable contractor.
Anything that removes a combustion appliance from a building is a good idea, in my opinion. Today’s heat pump space heaters work very well, even in cold climates without back up resistance heat. They are a perfect solution in regions with small heating loads like most of California, something most furnace manufacturers have failed to address. They are quiet, efficient, and safe, as there is no combustion. They are over 100 percent efficient, and they use electricity, the cleanest fuel choice we have today. Heat pumps have been around for decades and are poised to be the heating solution for buildings in the future. Perhaps this helps to explain why they are front and center in our efforts to reduce GHG emissions.
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