Zero energy homes generate as much energy as they use over the course of a year. Typically, these homes reduce energy use through carefully designed energy-efficiency measures, and then satisfy the remaining need through on-site solar panels. Of course, solar panels generate only electricity, so most zero energy homes consume only electricity.
While it is possible to have a true zero energy home that uses natural gas for space heating, water heating or cooking, this is seldom the case. There are four main reasons for this.
Methane Generates Carbon
Natural gas is essentially methane, a fossil fuel that is often associated with petroleum fields. Like petroleum, natural gas is pumped from the earth and transported in pipelines from the extraction point to homes and other buildings. Burning natural gas releases carbon dioxide and water, just like burning other fossil fuels. The big advantage of burning natural gas is that 30% to 50% less carbon is generated when compared to other fossil fuels like coal or petroleum products. Burning natural gas is definitely an improvement over burning fuel oil or coal. However, it still creates carbon dioxide. Solar panel generated electricity produces zero carbon.
Methane is a Greenhouse Gas
Methane doesn’t have to be burned to affect the climate. The chemical itself is a potent greenhouse gas with a global warming potential (GWP) 28 to 36 times that of CO2 before it is burned. Of the natural gas used for electricity generation about 2.4% leaks from extraction sites, transmission pipelines, and equipment. It’s not known how much gas leaks from distribution lines as it travels to homes, businesses, and factories for direct combustion. However, in cities where The Environmental Defense Fund looks for natural gas leaks, they are commonly found. By contributing to greenhouse gases released into our Earth’s atmosphere, natural gas proves to be even more damaging when it isn’t burned.
Net Metering Discourages Combustion Fuels
The typical home in an urban area uses energy in two forms: electricity and natural gas. Some areas still use fuel oil or propane, but the issues below still apply. One of the most important features of zero energy buildings is that they are usually grid-tied and have the ability to receive credit for unused electricity. Through a process called net metering, electric utilities either purchase excess energy directly from consumers or bank credits to be applied to future utility bills. Net metering policies vary widely among utilities and state regulators, but most policies limit credits to the amount used in the building. Whether the cap is applied monthly or annually, it discourages the building owner from installing an on-site solar system that generates more electricity than is used in the building. Now imagine that your building uses a fossil fuel for space and water heating. It would be easy to calculate the amount of energy consumed for these purposes and generate extra solar electricity to balance it out, so the home would be net zero in theory. However, under the vast majority of net-metering policies, you would not be paid for that extra production. In other words, you would purchase fossil fuel from one supplier, but not be able to recoup the cost from the electric company to whom you have sent your solar produced electricity.
Electric is More Efficient
While the purchase price of natural gas is cheaper than electricity almost everywhere, the story doesn’t end there. Another important factor that must be considered is end-use efficiency. Natural gas is burned on-site in furnaces, ranges, and water heaters with an efficiency topping out around 96%. That’s certainly good, but not nearly as efficient as electric heat pumps for space and water heating, which range between 300% and 400%. Electric induction cooktops have been shown to be more efficient and just as capable as gas ranges. In the end, electricity’s greater efficiency more than compensates for its higher fuel cost.
Post-Carbon Economy
Natural gas may still play an important role as an industrial-scale energy source in the transition from a fossil fuel-based economy to a clean-energy economy. But the four factors mentioned above will ultimately drive zero energy homes and buildings to use clean, site-generated electricity as the fuel of choice.
Brad Lemmon says:
How do you calculate an efficiency of greater than 100%? Where I come from that is called a perpetual motion machine.
Bruce Sullivan says:
It seems like magic, but it’s not perpetual motion. A standard tank-style water heater with an electric resistance element is at best 95% efficient. That means that 95% of the electricity consumed comes out as usable hot water. All of the electricity is converted to heat in the water, but a small amount escapes through the tank insulation. A heat pump doesn’t burn the electricity in a resistance element. Instead, it drives a compressor that circulates refrigerant between two areas. One area is the source of heat that evaporates the refrigerant. As you know evaporation absorbs a lot of energy. It takes 1 btu of heat to raise a pound of water 1 degree Fahrenheit, but it takes 971 btus to make water at 212 degrees into vapor at 212 degrees. So evaporation consumes that much more energy. Refrigerant turns to vapor in the evaporator coil (absorbing all that heat), the compressor moves it to the condenser coil inside the tank where all that energy is released as the vapor turns back into liquid. It’s called the vapor-compression cycle. Becuase it takes a lot less energy to turn a compressor than to heat the resistance coil, the efficiency is greater than 100%. You get more hot water with the heat pump than consuming the same amount of electricity in a resistance coil. By the way, your refrigerator uses exactly the same technology and very similar equipment to keep the ice cream cold by blowing all that heat into your kitchen.
John G says:
Brad, the confusion is understandable. The proper term is Energy Factor, “EF” or the related Coefficient Of Performance “COP” used when energy is not transformed from fuel to heat, but is moved as a Heat Pump. A Solar Heat Pump can transform 1 kW input into 3 kW of heat, for a 3 EF or 3 COP rating.
https://www.sunpump.solar/net-zero-energy
Michael Blaney says:
Your analysis overlooks the obvious source energy cost. For example most Rankine Cycle generators are at best about 33% thermally efficient. The claim of 300% efficiency for the heat pump is deceptive and you should know better. Further these are optimal condition figures and do not include the winter months for air source heat pumps as an example.
Bruce Sullivan says:
I mention that the fuel cost of gas is lower. You make a good point that heat pump efficiency declines at lower temperatures. Performance reports written by the Northwest Energy Efficiency Alliance show energy factors ranging from 2.5 to 3.3 for northern climates that take seasonal temperature swings into account. I’ll admit that 300% efficiency was stretching it a bit, as I had the CO2 heat pump in mind when writing this. Nevertheless, 250% efficiency is enough and I stand by my statement that it costs less to deliver hot water with a HPWH than a gas tank. Thanks for your comment.
John G says:
I can add two more reasons for not using Gas in a low energy home.
1. An efficient home requires very little heat, often less than the hot water load with a Passive House. Finding a low capacity NG Furnace is difficult if you need less than 30,000 BTU/h – let alone when the need is under 10,000 BTU/h.
2. Super-insulated homes and modern Code require air-tight envelopes for super-high efficiency. The trouble is NG stoves expose occupants to higher toxic fumes like Nitrous Oxide, Carbon Monoxide, plus some Benzene that are a health concern in a tight home. Cognitive functions are significantly lower (~50%), in homes with low oxygen and carbon fuel burning indoors. Sleep is also better in a clean home.
An alternative for zero carbon added renewable heating and hot water is a Solar Heat Pump.
https://www.sunpump.solar/net-zero-energy
Bruce Sullivan says:
John, In my climate a modestly-sized zero energy home might have a design heat load between 15,000 and 20,000 BTUs/hr. Would your solar-assisted heat pumps be appropriate? Can you give us some idea of the cost? Thanks, Bruce