Many organizations today are working to identify ways to reduce their dependence on carbon-rich fossil fuels, such as coal, natural gas, oil, and propane. Their goal: to dramatically reduce their emissions of the greenhouse gases implicated in climate change, while, hopefully, also decreasing their energy costs and bolstering organizational reputation at the same time.
For most organizations, the production of heat is the largest individual application of energy, and space heating for buildings is one of the largest components of that output, accounting for more than $20 billion each year expended by the commercial sector. With most of this heat generated by fossil fuels, building heating applications are therefore also among the largest components of the carbon footprint culpability of nearly any operation.
Efforts to reduce the carbon emissions associated with building heating has recently led organizations to consider replacing fossil-fuel fired furnaces and boilers with electric-driven options. For many, this is the next step in the decarbonization of heating, building on the prior step of replacing traditional boilers with the more efficient condensing-type. Now, with the growing use of renewables on the supply-side, there is additional opportunity for decarbonization on the demand-side by replacing the boilers and furnaces altogether with heat pumps for an even larger improvement in operating cost, emissions, and decarbonization performance. This electrification of building heating can provide significant benefits at the location, allowing operators to not only reduce carbon emissions, but also remove on-site fossil fuel flue-based NOx (nitrogen dioxide) and SOx (sulfur dioxide) emissions, improve energy efficiency, and reduce operating costs. This evolution also provides a building’s power providers with the opportunity and flexibility to meet increasing demand with carbon-free renewable sources rather than fossil fuels, thereby further developing the potential to exponentially improve their carbon footprint. This possibility, of course, is not available to utilities providing fossil fuels only.
Unfortunately, operators currently experience occasional barriers and limitations in taking these otherwise highly beneficial measures. Electric-driven boilers, perhaps the simplest option in terms of retrofitting infrastructure, utilize electric resistance heat, which is quite inefficient. Electric heat pumps — the more preferred choice, on the other hand, currently most commonly consist of equipment based upon constant-speed positive displacement screw compressor technology, but, while more efficient, often offer a return on investment (ROI) payback period that is somewhat longer than many organizations require to justify investment. Further, the more efficient heat pump technologies, based on operating temperature limitations, have previously required also changing out demand-side equipment to enable operation at significantly lower heating temperatures.
Fortunately, there is a new advanced compressor technology emerging that is specifically geared for heat pump applications that enables significant gains in efficiency and decarbonization while eliminating many of these former limitations. These oil-free, magnetic bearing centrifugal compressors are currently involved in several test projects and yielding impressive results. Modeling suggests that, when used in electric heat pump applications, oil-free, magnetic bearing centrifugal compressors can provide up to 40% greater energy efficiency and lower resulting emissions compared to constant-speed positive displacement screw compressor-based heat pumps, driving significant operating cost savings and substantial reduction in carbon footprint. When compared to variable-speed positive displacement screw compressors — a more efficient heat pump technology, modeling suggests that a significant 15%-20% reduction in emissions and 10%-15% percent reduction in energy costs can be realized. And, when replacing or as an alternative to a high-efficiency condensing boiler, the operating costs can be reduced by 40% and the CO2 emissions by 64%. This emissions reduction estimate increases as renewables are integrated into the power grid.
Further, these comparative improvements can further increase over time because oil-free, magnetic bearing centrifugal compressors maintain performance over the long term, whereas the performance of positive displacement screw compressor-based heat pumps can degrade as much as 10% in the first five years and 20% within the first 10. This degradation process is driven by a combination of the mechanical degradation of the positive displacement compression sealing process as well as oil-driven heat transfer degradation, which will be discussed in more detail below.
In short, this is an innovation that will be of significant interest to heat pump designers, manufacturers, energy management consultants, and, especially, any commercial or industrial facility interested in decreasing their carbon footprint, increasing energy efficiency and reducing heating costs.
Of vital note, although they deliver a quantum leap improvement in performance, oil-free, magnetic bearing centrifugal compressors do not represent a brand new, heretofore unknown technology. Actually, and perhaps somewhat ironically, oil-free, magnetic bearing centrifugal compressors represent a heating technology that borrows its foundational elements from a well-established, well-proven technology long used in electric air conditioning/chilling/cooling applications. In this article, I’ll describe this technology, how it works, and the technical/engineering challenges that needed to be overcome in order to make it practical for heating applications. In addition, we will discuss several strategies for stakeholders to put these new compressors to work.
Selecting the most efficient compressor option possible
The heart of most electric temperature conditioning equipment — both heating and cooling equipment from a/c cooling units to heat pumps and even refrigerators — is the compressor, a device that, by compressing a gas, reduces its volume or accelerates the gas speed and increases its pressure, mechanically driving the refrigerant used in both heating and cooling applications.
There are several types of compressors commonly utilized in HVAC applications, all fitting in the categories of either positive displacement or dynamic (centrifugal or axial) technology. In positive displacement versions, such as screw compressors, energy is used to generate pressure, and the refrigerant is squeezed to add energy to it. In the centrifugal/dynamic versions, refrigerant is thrown, increasing kinetic energy. The latter is frequently more efficient, driven by the inherent advantages of dynamic compression and the aerodynamic design optimized to the targeted temperatures. Further, due in part to the kinetic forces in a centrifugal/dynamic compressor, it is possible to move a higher volume of low-pressure refrigerants. These as a rule have lower global warming potential (GWP) because they break up and do not last in the atmosphere while at the same time having a limited flammability risk.
Unfortunately, however, due to the need to generate a higher differential (“lift”) temperature in heating applications, these more efficient centrifugal compressors, while common in cooling applications, have heretofore mainly been precluded from heating applications. Now, with the recent development of new features and design innovations, magnetic-bearing centrifugal technology is at last able to deliver these same benefits to heating applications.
Technology choices driven by decarbonization goals
Along the road to adapting this technology to heating applications, a number of enabling operational and technical decisions arose. All were made with the achievement of the challenging differential temperature capability at the optimum energy efficiency and carbon footprint reduction as the main criteria.
One choice to be made in designing a compressor is whether to specify oil-lubricated or oil-free bearings. As suggested previously, one of the “dirty little secrets” of the more common oil-bearing versions is that, in the normal operation of any oil management system, some amount of oil will leak into other areas of the system over time. This oil becomes held up in the heat exchangers, from which it has to be returned to the compressors. Inevitably, and more frequently today with heat exchanger tube enhancements, some of this oil becomes entrained in the enhancements and degrades tube heat transfer performance, resulting in a drain on capacity and energy efficiency. Additionally, with screw compressors there is mechanical degradation of the compression sealing process that occurs due to the metal-to-metal contact, which can erode the clearances, allowing high-to-low pressure leakage and creating even further capacity and efficiency degradation.
Unfortunately, unless the operator is constantly tracking power consumption over time, this degradation is unlikely to be identified. Knowing this, oil-free bearings, which avoid this issue and maintain more consistent performance over the long-term, were the obvious choice for the new centrifugal compressor when efficiency and lower carbon footprint — at design as well as over full operating life — were the paramount goals. Further, there are a number of additional benefits that accrue when oil-free bearings are applied on centrifugal compressors, including low noise operation and reduced maintenance requirements.
Of note, for those that are aware of and proactively work against oil encroachment in their systems, a complex oil management system can add up to $3,650 in annual maintenance costs in a typical 400-kW system. Since oil-free compressors do not have or need this complex oil management system, this cost would be eliminated. In selecting oil-free bearings, a decision needed to be made amongst three types of oil-free technologies: magnetic, ceramic, and gas. Magnetic was the obvious choice — well-proven in the chiller market, no metal-to-metal contact, less costly than ceramic, and more reliable today than gas, based on substantial installation and operation experience.
The evolution of compressor technology
From its first introduction over twenty years ago, the Danfoss oil-free, magnetic bearing, multi-stage, variable speed, direct-drive centrifugal compressor has grown to become an industry standard for cooling applications with more than 70,000 units installed. Danfoss compressor engineers were also well-versed in the technology, its benefits, and its limitations. They understood the need for increasing the lift to make it suitable for heat pump applications, and sought a solution for doing so, analyzing the situation to determine the key technology factors that required modification.
The analysis focused on the highest lift compressor in our existing magnetic bearing centrifugal compressor product line and found two key areas for innovation that needed attention in order to raise the temperature differential to the levels necessary for heating applications.
First was the fact that in all existing Danfoss centrifugal compressors, both stages of compression were occurring on one side of the unit. Although this created lift that was more than adequate for most cooling applications, it was found to cause additional axial (back and forth) forces, which proved to be a limiting factor if a significantly greater lift was desired. The engineers developed a method for moving the second stage to the opposite side of the compressor to put the resulting axial forces into balance, which in turn enabled a higher differential temperature/lift capability.
However, as is the case with most any mechanical system, when one portion is changed, supporting areas need to be modified in lock step. In this case, the impeller, the quickly rotating component of the compression process, and the rest of the process flow path needed to be adapted with a new aerodynamic design that was optimized to the newly raised differential temperatures it would experience. This adaptation was an iterative process that led to the increased differential temperature operating capability while also maximizing the energy efficiency and operating performance of the system.
The new compressor design, as is the case with its counterparts in the cooling arena, offer a number of additional benefits. For example, the compressor is approximately 20% of the cubic footprint of an equivalent capacity screw compressor, 20% of its weight, and has sound levels averaging 8dB lower than equivalent capacity rotary screw technology. These benefits have helped to make this compressor technology the go-to choice for retrofit applications in the cooling market, where space is at a premium and access to equipment minimal with other equipment often built up around the chiller since installation. Those same benefits are also helping make the new oil-free, magnetic bearing centrifugal compressors well-suited for new and retrofit heating projects in a commercial or industrial facility. Here, a smaller footprint heat pump could easily replace less efficient furnaces and boilers, enable easy expansion with a compact modular design, and help reclaim valuable floor space for more productive activities.
Retrofit and usage possibilities
With the development and ongoing validation of this highly efficient, oil-free, magnetic bearing centrifugal compressor technology, heat pump designers have a new tool to help customers achieve their goals of lowering operating costs and carbon emissions. And, end users, in industries of every kind, will have a new, highly superior heating choice as they strategically plan their HVAC methodology to improve profitability and maximize sustainability at their locations.
Through discussions with customers and partners, Danfoss has identified several potential ways to begin to benefit from this technology. Innovative heat pump designers will no doubt creatively develop new scenarios, but we offer this as a starting point.
First, it is important to understand that heat pump designs mainly follow one of three setups: air-to-air, air-to-water, and water-to-water.
Air-to-air heat pumps extract heat from ambient air. Because they extract heat from what can be very low-temperature ambient air, they are the lowest efficiency and generally the lowest capacity. While common in residential applications, they are not likely to be adapted to the higher demands of commercial or industrial applications. Perhaps, then, the fastest and easiest way for a building to begin to benefit from the higher efficiency of this technology is to replace an existing set or sets of old, inefficient fossil fuel-fired furnaces and boilers with an electric air-to-water heat pump that incorporates oil-free, magnetic bearing centrifugal compressor technology. This type of retrofit typically can be done very quickly, causes minimal disruption, and, with its compact size, would likely fit well within the existing footprint on-site. It is an evolution that would be quickly implemented, low cost, and yield increased energy efficiency, reduced emissions, and decreased operating costs immediately from day one.
However, when considering air-to-water heat pumps, it’s important to note that while these new compressors have greater operating temperature flexibility than their technology predecessors, they still would be limited in operating temperature flexibility due to the lower efficiency of air-to-water heat transfer as well as the high variability of climate-driven air temperature. This means that an oil-free, magnetic bearing, compressor-based air-to-water heat pump will be able to provide sufficient heat in mild-to-temperate climates but likely not at the coldest ambient temperatures in colder climates. If utilizing an air-to-water heat pump solution to replace fossil fuel heating, it is critical to ensure that it be applied in climates and for applications where it will meet desired heating requirements during the most challenging conditions.
For those end users who wish to accrue the most substantial benefits, a water-to-water electric heat pump, driven by the new oil-free, magnetic bearing, centrifugal compressor technology, would generate considerably greater benefits. For a water-to-water heat pump application, the operating temperature limitations at the lowest ambient conditions, as previously noted, are no longer a concern. The efficiency benefits accrue even more strongly because the heating process begins with water rather than air. Water can be a much warmer medium to start with; it results in more efficient heat transfer than air and, thus, demands less energy to boost the heat to the target temperature. River water, for example, can be dozens of degrees warmer than the surrounding air even on a cold winter day.
The setup logistics of a water-to-water electric heat pump, however, can be more ambitious and require a larger upfront investment. For example, a source of water must be nearby and piping infrastructure would need to be built to access it. But innovative users often find ways to get faster results. For example, it is also possible to operate these highly efficient heat pumps with nearby groundwater/ground energy using a geothermal loop. And, some could use the water-to-water concept to compound the benefits by reclaiming heat from an industrial process or other source which is otherwise rejected to ambient, if such is available.
However, from the current purview, perhaps the greatest immediate benefits can be gained from changing out a series of fossil-fuel driven boilers and furnaces with a series of electric heat pumps using oil-free, magnetic bearing centrifugal compressors — preferably water-to-water models, operating them at a central location, and piping the heat to a series of buildings in a “district heating” model. While the district heating strategy is prevalent in some U.S. cities, it is more common on American university, hospital, and corporate campuses, where related buildings are in close proximity. Yet, it is far more common throughout many European countries in municipal grids of all kinds. The use of this technology — with its high efficiency, scalability, and the additional component and system optimization strategies it enables — can be considered the “holy grail” for all those who truly desire to minimize their carbon footprint and energy usage.
While, for some, this would also require significant new infrastructure and licensure, the payback period can be surprisingly compact. However, for those with existing district heating infrastructure, gaining exponential benefits may be no more involved than it would be for others swapping out fossil fuel-fired equipment with electric heat pumps. In addition, it should be noted that many Northeast American cities, such as New York and Boston, were heavier users of the district heating strategy many years ago. This robust infrastructure still exists in many areas and, even where not in current use, may be at least partially reclaimable to form a cost-effective foundation for a modern, highly efficient, locally carbon-free district heating system. In any case, there are a number of knowledgeable organizations — Danfoss among them — that can help interested end users determine the feasibility and economics of initiating such a strategy. Danfoss also offers a portfolio of components utilized in district heating systems outside of the heat pump, which can further enable a highly-efficient, low emissions system.
Finally, it’s important to note that the previously-discussed “holy grail” of heating efficiency — district heating using a high-efficiency electric heat pump driven by an oil-free, magnetic bearing centrifugal compressor — only takes into account the supply side. While somewhat outside the scope of this article, it should be noted that there are opportunities for commercial entities to generate even greater carbon footprint reduction by combining district heating — or even high-efficiency individual electric heat pumps — with complementary demand-side strategies. For example, low-temperature radiant underfloor heating allows comfortable, even heating at significantly lower supply temperatures than that demanded by old, inefficient forced-air systems. It is a very effective enabling strategy when coupled with oil-free, magnetic bearing centrifugal compressor electric heat pumps and might be valuable to consider in many operations. Indeed, changing out demand-side heating equipment to that using lower temperature heating is nothing new — the change from traditional to condensing boiler technologies noted previously drove a similar change. And, as with that evolution, these demand-side actions can position users to benefit from the next great efficiency increase and emissions reduction opportunity — whatever it happens to be.
Operators seeking to reduce their carbon footprint and lower operating costs are looking strongly at their heating strategies, an activity which uses a large portion of the energy expended in many operations. The advent of oil-free, magnetic bearing centrifugal compressors in electric heat pumps may be a game changer in this regard, and Danfoss stands ready to work with innovative heat pump designers, installers, and operators, providing this state-of-the-art compressor, related heat pump components optimized for the new technology, as well as a wealth of technical assistance. As these compressors continue to generate impressive results in commercial demonstrations, the time is ripe for early adapters to consider the possibilities and get a fast edge over competition, reducing carbon footprint, fighting climate change, and lowering energy costs at the same time — now and for the long-term as well.
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