Sustainable Heat Energy?
In the fight against rapid climate change one of the key areas of attack is heat and power generation. Globally, heating and electricity production accounts for 31% of greenhouse gas emissions. The rest of the emissions come mostly from agriculture, transportation, and industry. We have made huge strides in figuring out renewable electricity generation. Think solar PV panels, wind turbines, hydroelectric dams, or geothermal plants. While grid integration challenges remain and energy storage technology still has far to go before we can have an all-renewable grid, with the proper scale such a grid would be feasible. At a smaller scale, it is already economically feasible to have an off-grid residence with just a few solar panels, trackers, and batteries. But what about heating? The status quo for heating plants and combined heat and power (cogeneration or CHP) plants in the US is to burn coal, natural gas, or heating oil. So what are the alternatives?
There are many different forms of biomass energy. Think wood and agricultural byproducts, municipal solid waste, biogas, landfill gas, or alcohol fuels. The cogeneration plant at UNH is burning a blend of natural gas and processed landfill gas from a nearby landfill at Rochester. It is important to note that renewable does not necessarily mean carbon neutral. The carbon emissions from biomass combustion are similar to and often higher than emissions from fossil fuels. However, biomass fuels can be considered carbon neutral because, in the case of waste products, it is assumed that the carbon would have been released anyway (even if not combusted for direct heat use), and in the case of wood, it is assumed that in a sustainably managed lot trees would be planted to offset the carbon emissions. Globally, ~2.6 billion people rely on biomass fuel to heat their homes, but much of this biomass, through renewable, is harvested unsustainably and burned at a very low efficiency with high levels of air pollution. So remember, renewable doesn’t necessarily mean better than fossil fuels from an environmental perspective. In the further discussion of biomass energy, I’m going to focus on wood products, since that is the most common form of biomass energy and the most relevant to my fellowship. Sustainably harvested wood is a great fuel alternative for heating in forested, cold regions like New England, especially because it supports the local economy. However, it has several challenges:
- More expensive than natural gas (but cheaper than heating oil or propane).
- Requires lots of space, often external to the building (fuel storage silos, boiler houses, etc).
- Cannot be ramped up or down quickly to track demand the same way as fossil fuels can.
- Often requires backup burners (fossil fuels) to meet peak demand on the coldest days in order to keep the size of the system economical.
- Regional resource, cannot be sustainably scaled. Not feasible for large district heating.
Larger systems require expensive air quality control apparatus (electrostatic precipitators, cyclones, etc.).
More complicated fuel delivery process than oil or propane and especially more complicated than natural gas piping.
Wood chip boiler house for High Mowing School in Wilton, NH.
The large tanks are chip storage silos (Courtesy of Froling Energy).
So from a strictly economic point of view, biomass heating is a great option for individual buildings or small clusters of buildings where there is ample space for fuel storage, sustainably harvested wood is available, but natural gas isn’t. When considering carbon emissions, the negative environmental impacts of fracking (used to extract the natural gas), and the instability of fossil fuel prices, biomass could even win out over natural gas, but it’s harder to monetize those factors.
Most of us are familiar with photovoltaics, a use of semiconductors to turn energy from the Sun into electricity. However, there are also technologies that take advantage of solar energy directly for heat, building on the common sense idea of placing a pot of water in the sun and waiting for it to heat up. Solar collectors can be used to warm up water for domestic use, or in some cases even for space heating or cooling. Large mirrors can be used to concentrate solar power on a fluid to generate steam and drive a conventional thermal power plant. This can be done on the utility scale in deserts, though the mirrors can have a negative impact on wildlife.
Challenges associated with solar thermal include:
- Higher upfront costs than natural gas or electric water heaters.
- Intermittent, poor storage capacity, supplemental energy source required for long sunless stretches.
- Not feasible for space heating in cold winters.
Distributed solar heating can be a nice option to provide low and medium grade heat, but it’s not feasible as the sole space heat provider in a cold climate like New Hampshire’s.
Heat pumps (air-source or ground-source)
Heat pumps use electricity to drive a cycle of compression and expansion to move heat from one side of a building wall to the other. They are much more efficient than electric resistance heaters, and can deliver 3 times more energy than they use. The same unit can function both as a space heater in the winter and as an air conditioner in the summer. Heat pumps can be air-source (ASHP) or ground-source (GSHP). GSHPs often get confused with geothermal energy. While GSHPs simply exploit the year-round constant temperature of the soil a few feet below ground, a geothermal power plant uses the latent heat of fusion deep in the Earth near active volcanoes or fault lines to generate electricity and in some cases provide direct heating (the western US, Iceland, or New Zealand are good example locations).  Heat pumps work well for individual buildings, and there are even district-scale GSHP systems popping up. Heat pumps eliminate the space requirement for fuel storage and reduce utility bills in most cases (except when burning natural gas onsite). The following challenges are associated with heat pumps:
Schematic of an air-source heat pump (Source: Efficiency Vermont)
- High startup costs; for GSHP excavations are required; difficult and costly retrofits
- Cheaper heat pumps with lower Coefficients of Performance could raise the electric bill more than the heating bill is reduced.
- Can experience trouble on particularly cold days, especially for low performance, poorly insulated, older buildings, so a less efficient backup system is usually required (i.e. electrical resistance heat). However, there are plenty of new, ductless air-source pumps supposedly perfect for cold New England winters.
- Older GSHPs sometimes overheated or overcooled the ground, rendering them useless over time. This is not an issue with new systems. 
- In older open loop systems lots of water is wasted and refrigerant leaks can contaminate the ground. New closed loops systems take care of this issue. 
- If the electricity used to run the heat pumps comes from fossil fuel sources, this cannot be considered a carbon-neutral or fully renewable technology.
If the new cold climate ASHPs are what they are promised to be, heat pumps could have a huge potential for space heating and cooling in New England and also globally. With future installations of district GSHP, heat pump technology could be scaled sustainably, unlike biomass. The key to making heat pumps sustainable is to ensure that the electricity used to run them come from renewables. At this point in time, this can be accomplished by using solar panels or wind energy to offset the grid load of the heat pumps. In the future, if an all renewable global grid is to become a reality, heat pumps would likely be the most efficient and most sustainable form of space heating.
Hydrogen Fuel Cells
I don’t want to dedicate much space and time to this technology here, because it is still several decades of R&D from being economically viable, but I just wanted to put it on your radars as a likely future source of heating energy, a zero carbon alternative to natural gas.,
Between biomass boilers, solar collectors, heat pumps, fuel cells, and very low natural gas prices I think it is clear that we currently do not have a clear roadmap to a sustainable future for heating energy. In the short term, I foresee an expanding natural gas infrastructure unless fracking and carbon emissions receive their proper price tag based on costs to society and the environment. In the meantime, biomass will continue to be a small scale solution in forested, cold regions where natural gas is not readily available. In the long run, however, I think that to have sustainable heat we will need to rely on a combination of solar thermal, fuel cells and heat pumps powered by a renewable grid, with electrical baseboard heaters or small scale biomass boilers as backup wherever necessary.
Me at the processing plant at the landfill in Rochester, NH,
where UNH gets their landfill gas for the campus cogeneration plant
(Photo credit: Dave Bowley)
A big part of my fellowship work with the UNH Energy Task Force is to evaluate fuel switching opportunities on campus that would lead to reductions in the university’s carbon footprint. I’m looking at energy consumption data and utilities distribution maps for building clusters that are either not heated by the cogenerations plant’s hot water distribution system, or have extensive, non-looped hot water piping that will have to be replaced soon. I’m using life cycle cost and carbon reduction potential to analyze the different options available at each site. The alternatives I’m evaluating are to a) stick to the status quo, b) switch to a central biomass heating mini-district, c) switch to distributed wood pellet boilers, d) switch to a central natural gas mini-district, and e) switch to distributed natural gas. In the case of buildings heated by electrical baseboards, I’m also looking into offsetting the related carbon emissions using off-site solar array purchases. Though a combustion-based system would provide the most straightforward retrofit for the buildings under consideration, I might also look at heat pumps as a possible alternative.
Due to its current cheap price and wide availability on campus, natural gas seems like the winning option in terms of life cycle costs. However, sustainably harvested wood chips or pellets would diversify the university’s energy portfolio, insulate them from fluctuating fossil fuel prices, and reduce carbon emissions by 40-80% more than natural gas would, depending on the original fuel source . In the end, it will be up to the university to decide where it finds the most value for its current as well as future students.
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