Hybrid Heating Systems: a Logical Step in Decarbonization

Decarbonization of building heating systems is a huge challenge, due in large part to the sheer scale of the endeavor, but also due to technical challenges and at a very basic level, typical purchase patterns for heating system components. Creating retrofit hybrid heating systems greatly reduces the magnitude of these problems, and while decreasing on-site greenhouse gas (GHG) emissions by 90% or more.

What are hybrid heating systems? Hybrid heating systems are heating systems that consist of more than one heating technology. Sometimes hybrid systems share system components, like a distribution system or emitters, but they don’t have to. Ideally, hybrid heating systems are integrated via one control. A typical hybrid heating system would consist of a legacy boiler or furnace that runs on natural gas, heating oil, or propane, and a heat pump of some sort (e.g. mini-split, air-to-air, etc.). Choice of operation of the heat pump or its fossil fuel alternative can be automated; whichever happens to be optimal at any point in time.

Thinking broadly, hybrid systems are seen in a wide variety of contexts and they are used to solve a problem. In silviculture, grafting branches from one type of apple tree onto the stem of another variety can create a productive and long-lasting hybrid. Hybrid cars provide drivers with better fuel economy and the ability to drive long distances conveniently. Sailboats have hybrid propulsion when they are outfitted with motors to help with navigation in harbors and on windless days. Even drying laundry can loosely be considered hybrid when outdoor drying on sunny days is combined with machine drying on rainy days. Hybrid heating systems have similar appeal as the two main components can complement each other, and actually solve several problems at the same time.

Advantages of Hybrid Heating Systems for Decarbonization

There are actually several good reasons to consider hybrid heating systems for decarbonizing buildings.

1. Efficiency – Hybrid systems can operate much more efficiently than single-source fossil fuel systems, provided a heat pump is one of the components. Higher efficiency leads to reduced GHG emissions.
2. Flexibility – Hybrid systems exercise the relative advantages of each heat source, including the ability of a fossil fuel boiler to operate during peak electricity demand periods*, or when the heat pump would be operating at it lowest efficiency.
3. Proactive – Because planning is easier when adding a second heat source, a heat pump can be added to a system at any time of year, which gets around the replacement cycles that center around component failures.
4. Additional weatherization (e.g. insulation, air sealing, etc.), which are highly desirable (even necessary) when heating with a heat pump only, can be accomplished over a longer time frame.
5. Additional system adaptations, such as retrofitting for “low temperature” distribution (i.e. 120F maximum) can be accomplished over a longer time horizon if necessary.
6. Financial benefits – Setting up a hybrid heating system is easier, and less costly than a complete system changeover. Financial incentives for heating retrofits are never unlimited. More GHG emissions can be eliminated by shifting more buildings to hybrid systems, than by shifting half as many buildings to 100% heat pump.

*This is not a huge advantage at this moment, but with increased electrification of heating systems, electricity demand peaks in winter will surpass the level of demand peaks in the summer.

Other Advantages of Hybrid Heating Systems

1. Hybrid systems are redundant. If a boiler or heat pump fails, the building can continue to stay heated.
2. Hybrid systems can heat and cool, so they can replace central a/c and window air conditioners.
3. As long as proper weatherization is in place, hybrid systems can be switched over to single technology/100% heat pump at any time.

More on Flexibility

The ability of a hybrid heating system to be operationally flexible is one of the most important benefits in the race to decarbonize. The reasons why have to do with two things; clean electricity generation and the capacity of the power grid.

Hybrid systems have the ability to reduce peak electricity demand. There is value in having that capability. Much more clean electricity will be generated from solar and wind in the years to come. These are intermittent energy sources. Because of the intermittency, long term battery energy storage systems will be utilized to help provide power when solar and wind aren’t available. Even so, there will be times when supply is limited and demand from electric vehicles and building systems outstrips the supply. So the value of being able to temporarily reduce electricity demand, such as is possible with hybrid systems will grow as more intermittent energy sources feed the grid.

Grid capacity is the other limitation, especially during peak weather events like very cold mornings in winter. As more building heating systems shift from fossil fuels to heat pumps, this will become a bigger issue. Just as described for intermittency issues, buildings with hybrid systems have the ability to temporarily reduce electrical demand by switching to a different energy source, which directly reduces demand on the grid.

More on Financial

Heat pumps operate at very high efficiency, particularly when outside temperatures are not near their winter lows. So building owners (or those paying the energy bills) can save money over combustion-based alternatives that can’t achieve above 100% efficiency.

With regard to asset and installation costs, home and commercial building owners will need some financial assistance to retrofit a system that reduces or eliminates GHG emissions. In what form, and at what level the support takes are worthy of debate but in any case, at any one time, any available support is likely to be limited. Investments in retrofits could be made on hybrid systems, or a smaller number of full changeover heat pump systems where the previous heat source is eliminated. The full changeovers can more than double the initial cost of an add-on heat pump because of the system and weatherization adaptations that are necessary at the start.

Increasing the number hybrid systems will have a greater decarbonization effect than would be the case with fewer transitions to stand alone heat pump systems. This is true at least for the initial period where speed of decarbonization has a high priority.

Summary

Using hybridization of heating system retrofits as a logical step for transitioning buildings with fossil fuel systems will result in the fastest cuts in GHG emissions. Where building owners have the resources to make a full changeover, they should probably do so, as long as they make other changes to reduce demand at the same time. But where external financial incentives are needed, hybrid systems make sense.

New buildings are different. They should be built with no accommodation for fossil fuels, and should be built to minimize the need for heating and cooling through building to passive house standards or similar.

What are the Next Steps?

Clearly hybrid systems can play an important role as a bridge to the future of 100% GHG-free heat. Questions that remain include:

1. what policies and incentives make the most sense to maximize near term GHG reductions?
2. what carrots and sticks make sense in areas like demand charges, energy storage, and variable energy charges?
3. where should subsidies come from and who should benefit?
4. at what level are subsidies or grants needed to achieve GHG reduction goals?
5. what controls are necessary for hybrid systems to operate optimally for GHG reductions?
6. what data should be collected on systems operations to help optimize or balance GHG reductions and end-user costs?
7. should operational data collection be required in return for a grant or other incentive?
8. how are environmental justice and other communities going to be equitably served?
9. how does the workforce need to change to ensure that this will happen?

These and other questions need to be explored, debated and pursued.

July 30, 2020October 1, 2020

Hybrid Model: Combining Geothermal Heat Pumps (GSHPs) with Air Source Heat Pumps (ASHPs) for Maximum Efficiency and Cost Effectiveness

Although uncommon, systems composed of a hybrid, or mix of heat pump technologies are a better solution than any single type of heat pump, or heat pump with a supplemental fossil fuel combustion heat source. Well designed hybrid heat pump systems balance upfront costs with low operating costs, resulting in maximum system efficiency, cost effectiveness, and the potential for net zero emissions.

The market for heat pumps is experiencing strong growth. The growth is propelled by recent advances in technology that make heat pumps efficient over a wider range of input and output temperatures. Also, they also can be used for both heating and cooling; a desirable feature. There are several types of heat pumps, the two main ones being ground source heat pumps (GSHPs) and air source heat pumps (ASHPs). These heat pumps are usually deployed as a single type, or in combination with a fossil fuel heat source. Hybrid heat pump systems are those that use both ASHPs and GSHPs.

Heat pumps are “all electric”. All electric heat pump systems in new buildings will soon be the norm as efficiency, sustainability, and emissions reduction goals are prioritized. This is also true in retrofits of buildings with older heating systems like steam, oil-fired warm air furnaces, and similar gas fired equipment . As power from renewables continues to grow market share, and as fully electric systems have the potential to be dominant market leaders in efficiency, the greenhouse gas emissions footprint of these buildings will be greatly reduced. Appropriately designed heat pump driven heating and cooling systems have the efficiency potential to help achieve net zero emissions goals.

Heat Pump Principles

Heat pumps move heat; from a heat source to a heat sink. For heating applications, the principal energy source, be it sub-surface ground or ambient air, is on site, sustainable, and free. Heat pumps take the freely available thermal energy from the air (ASHPs) and from the ground (GSHPs), and boost it to a higher temperature with a compression/expansion cycle. The process is reversed in summer for cooling by taking heat from inside and expelling it to the air or the ground. Heat pumps are most efficient when the difference in temperature between the source (e.g. air or ground) and the output are close. In winter, as the source temp declines, the system loses efficiency. Likewise, higher output temperatures also lower efficiency.

Leveraging Heat Pump Strengths

Here in New England, the temperature in the ground 6 feet below the surface is a steady (approx.) 55°F year round. During the heating season, removing thermal energy from the ground reduces the ground temperature around the heat exchanger. It’s not unusual for the ground temperature around a geothermal borehole or horizontal loop to drop from 55°F to below 30°F. The ground temperature gets slowly replenished, from moving groundwater for instance, but it doesn’t get back up to its starting temperature until after the heating season is over.

With ground temperatures dropping by 20°F or more, system efficiency can easily drop by 30% or more. So if a stand-alone GSHP system is being designed for high efficiency, more boreholes are required to keep the system efficient. Boreholes are expensive to drill and fit up, and they can’t be located too close together. Also, sizing for 100% of the heat load with GSHPs doesn’t make good sense economically, as the heat load only approaches 100% for a matter of hours or days in the year. The answer is to include another heat source to supplement the GSHPs. As you will see, air source heat pumps make a lot of sense to be that supplemental source of energy.

Mixing Heat Pump Types to Make Hybrid Systems

As shown in the image below, most of the time that buildings need heat, the ambient air temperatures are in the 30s, 40s and 50s°F. In eastern Massachusetts, when the temperature is below 60°F, the air temperature is above 30°F for 77% of the time!

When air temperatures are in the 50s, efficiencies between GSHPs and ASHPs are roughly equivalent because the source temperatures are the same. When outdoor air temperatures are in the 40s, the GSHPs have a marginal efficiency advantage over the ASHPs. The difference in efficiency becomes more significant when temperatures drop to the 30s and below.

It’s worth examining the charts above. Most of the heating season, air temperatures are above 30°F. So if the key objective of running a heating system is optimized seasonal efficiency rather than moment-to-moment efficiency, there is good reason to continue to use ASHPs at these above 30F temps. By not using, or by using the GSHPs only sparingly at these relatively mild temperatures, in-ground temperature remains in the 50s so that the GSHPs can be used very efficiently when air temperatures are frigid.

Planning a Hybrid GSHP ASHP System

The key to implementing an efficient and cost effective heat pump installation is with thorough technical and financial planning up front. A good plan should encompass expected energy use and cost analysis, and include monitoring and performance benchmarking over time. Up front costs for GSHPs is much higher than for ASHPs. And as shown, ASHPs are just as efficient for parts of the year. So the plan needs to identify the right balance of costs and benefits.

A feasibility study and design analysis is the best way to approach this problem. The analysis should include a site evaluation, a building evaluation, and a parametric analysis of data related to u–pfront costs, hardware efficiency estimates, and operating costs. A full design plan should also include system control recommendations for balancing the operation of the hybrid system.

To truly optimize the system performance, tracking and benchmarking must be included. Every site is different, so GSHPs will not operate exactly the same at different sites. System performance data matched with weather data, time-of-day and time-of-season data will help to fine tune the system optimization.

Contact CIMI Energy for information about prioritizing energy investments, and planning for your hybrid heat pump applications. [CIMI Energy uses computer modeling and a range of data (including heat load calculations, building occupancy patterns, hardware costs, installation costs, energy costs, manufacturers efficiency data, etc.) to determine the best system plan.]

May 3, 2018September 3, 2020

Advancing Beyond Outdoor Reset by Using a Building’s “Unique Energy Fingerprint”

It’s now possible to lower energy costs by applying bundled technologies that find and use each building’s “Unique Energy Fingerprint” (UEF). Our partner Leanheat has created energy intelligence software that bundles artificial intelligence (AI) with the Internet of Things (IoT) to determine each building’s UEF. The technologies integrate seamlessly with existing building assets. Building energy use is reduced through improvements in system efficiency.

Before this technology became available, this level of efficiency improvement was not achievable in a cost-effective way using current assets. Improvements that have been made have been limited by the available technology. The potential for improved control technology has expanded greatly. Leanheat has been able to focus on realizing that potential for the purpose of lowering building energy use and costs.

There are two major differences between using a building’s UEF and today’s common control optimizer, outdoor reset (OR). First is that the UEF is based on more variables (as opposed to just one or two with OR), giving a more precise picture of the heating and cooling load at any given time. Second, the technology that determines the UEF is able to anticipate loads, and thus to proactively optimize settings so that energy use is reduced.

Proactive beats reactive.

Outdoor reset is reactive. It operates on a curve tied to the outdoor temperature. Outdoor reset depends on finding a heating curve that matches up with the output with the load. Because outdoor reset is reactive, there’s an inherent uncertainty with the load matching. To compensate, outdoor reset curves must provide some extra buffer, and that extra buffer creates inefficiency.

On the other hand, Leanheat proactively anticipates the heating and cooling needs of the building. It does this in two ways. First, it factors in more data related to weather and climate. Factors include:

1. Present and anticipated outdoor temperatures
2. Present and anticipated solar irradiation (sunshine) on the building
3. Sun angle (latitude, time-of-day, and time-of year)
4. Wind direction and speed (“building wind chill”)

Other factors are indirectly (and automatically) factored in based on how the building responds:

1. Capacity of the heating and cooling system assets
2. Building mass & orientation
3. Insulation, windows, etc.
4. Air infiltration

Together, these are the variables that make a building’s UEF. Because it uses more variables, it’s able to get a more complete picture of the building’s energy load.

The second way that the Leanheat technology is proactive is that it monitors how building data correlates with naturally occurring changes in weather. As a result, the technology finds the UEF and its algorithms keep the building systems matched appropriately to current and incoming weather. The result is a typical cost savings of 10-15%.

Proactive heating and cooling control is especially important in buildings that react slowly to changes in weather and climate factors. Of course, any building with traditional controls can react slowly to a cold front with wind and rain or snow. Or when the cloud cover breaks up and suddenly the building is in full sunshine. Proactive technology controls the water temperature (or steam) with greater precision (and “less cushion”) than is possible with a reactive-based system like outdoor reset.

March 5, 2017September 3, 2020

Building Energy: Evaluate, Plan, and Act

An evaluation of energy use, energy assets and systems is a worthwhile undertaking for any large user of energy. Whatever the end use of the energy in question, a thorough evaluation provides a foundation or baseline for any planning and action that takes place after. Although the focus of this post is on building energy, the takeaway is equally valid for energy used to support industrial processes.

The goal of an energy evaluation is have enough information to create a strategy that upon implementation leads to lower costs and equal or better reliability. To find the best course of action for reducing energy costs and increasing reliability, an organization should make that evaluation comprehensive. To be comprehensive, think broad and deep. It should cover all aspects of power and heat consumption, including historical and anticipated consumption data, energy pricing, equipment condition, maintenance requirements, reliability, location, and so on.

How to Begin

Choose an Evaluation Plan

It may not be clear at first, but a decision will need to be made about whether the plan will include outside help. An internal team may be sufficient for when the broad set of energy assets are still in good operating condition, and if energy costs are satisfactory and a relatively small part of the budget. On the other hand, when energy costs are significant, and/or assets are unreliable, and causing extra maintenance or risk of failure, it’s probably worthwhile to bring in an external team. The external team adds some costs, but also brings expertise, and leaves the home team with the time to focus on their jobs.

In any case, an evaluation will start by evaluating energy assets as deployed at present. Look at the condition of those assets and factor in the amount of maintenance that’s required to keep them in working order. Connect those energy assets in the evaluation with energy consumption and pricing data.

Also consider your building’s energy needs at present, and anticipate what they will be in the future. Consider the expected remaining life expectancy of the equipment. Prioritize which needs are most urgent, and which ones are not.

Power Review

Have a professional evaluate your power consumption and the price you’re paying for it. Are there demand charges? At what point do they kick in? Are there other energy providers worth looking at?

Cooling & Refrigeration

How does cooling factor in? Is cooling something that’s used year round, or just at certain times of year? If cooling is a significant expense, a cooling demand estimate should be calculated. Chart it over time. Does cooling cause a spike in demand? Is that spike in demand causing expensive demand charges?

How does refrigeration factor in, if at all? If it does, an evaluation of refrigeration assets is probably worthwhile.

Heat Review

Review the demand and uses of heat. Space heating is typically seasonal. A heat loss calculation can be made to estimate demand for space heat over time.

Process heating is usually more consistent over time rather than tied to the heating season. Still, process heating demand may ebb and flow based on time of day, or with some periods peaking, and others where it’s non-existent. The more you know, the more useful the evaluation will be.

When you have all this information and data together, you can then make a judgement on the best course of action.

Nonlinear Effects of Efficiency Upgrades on Energy Use and Cost Savings

Care must be taken to factor in the nonlinear effects of asset upgrades. That is, when selecting efficiency upgrades, the energy reduction will count for more than the amount of efficiency increase. The cost savings will be greater. The following chart illustrates this using the examples of two sets of pumps; Pump #1 and Pump #2.

The example shows the nonlinear effects of an efficiency upgrade. Two choices are available in this example: an efficiency upgrade for two types of pumps, and the effects of an efficiency upgrade on each. With a much bigger drop in energy use connected with the efficiency upgrade of Pump #1, the example shows that cost savings are 3 times higher. This can seem to be surprising because the efficiency of the upgraded Pump #1 is still lower than the efficiency of Pump #2, even before an upgrade. To summarize this example, the graphic shows that the best upgrade, when faced with an either/or choice, is the one that decreases energy use the most, even though the increase in efficiency is much smaller.

A good example of this effect is the large but short-lived cash4clunkers program described below.

Taking Action on Findings

Your completed energy use evaluation should include a set of next step recommendations. The recommendations should be weighted with regard to level of urgency, amount of expected costs savings, and expected increase in energy security, if any. Those recommendations should be actionable in some way. The recommendation could be direct, such as recommending an investment in CHP. Or the recommendation may be to look more deeply into the calculus of an issue before significant capital is deployed or obligations are entered into.

Decisions will have to be made on how best to actuate changes. An internal team may be sufficient to manage the process if the fixes are simple. Alternatively, hiring an outside team will bring expertise to the process, and allow internal resources to be deployed in other ways.

Keeping Evaluations Up-to-date

If you’re a very large energy user, the energy evaluation should be templated and updated yearly. For moderate consumers of energy, the original evaluation should be updated with new data every few years at least.

Paying for Energy Upgrades

Investments can be made in a standard way; out of free capital, a lease, or with loans. As a further option, the equipment and installation can be obtained through an energy service agreement (ESA). With an ESA, there is no capital or lease required. You purchase power and heat from the 3rd party investor but you don’t own the assets at the start. Once the equipment is fully depreciated, you take full ownership.

Contact CIMI Energy for more information.

The Federal government’s “Cash for Clunkers” program from 1999 was a prominent example of this effect. The program was one where “clunkers”, low-efficiency cars and trucks were permanently scrapped and replaced by cars with significantly higher efficiency. Although the primary purpose of the program was billed as a stimulant for the economy, the government also recognized that a relatively small pool of older automobiles was providing CO2 emissions far beyond their numbers. A lot of money was already being spent on increasing automobile efficiency, but those marginal gains were outweighed by the smaller numbers of old gas guzzlers people were driving around. An incentive was set up pay people to scrap their “clunkers” and buy a new automobile with higher gas mileage. The program was controversial as it was mainly judged on the cost vs benefit of the economic effects, where the economic returns from higher new car sales were judged to be less than the $3 billion spent on the program. Nevertheless, the program undoubtedly had an outsized effect on reducing auto emissions nationwide.