Vapor Compression Technology Applied in Heat Pumps to Reduce CO2 Emissions


The inefficiency of fossil fuels, along with the negative environmental impact coming from their burning and resulting emissions, is driving companies to find alternative heating and cooling solutions. While renewable sources – such as wind and solar power – are decreasing this impact, other fossil fuel-burning sources need to be replaced with electric-driven alternatives to fully realize their emissions reduction potential. New vapor compression technology can help reduce heating and cooling operations while providing these additional CO2 emissions reductions.

Air-to-water heat pumps provide much more efficient heating compared to fossil fuel-sourced solutions. They are approximately 3.5 times more efficient than boilers, before accounting for the transmission and distribution losses. Even with the inefficiencies of the power generation needed to fuel electric heat pumps, there is still an opportunity for this air-to-water heat pump to realize about 35% operating cost reduction, as well as 60% emissions reduction.

Recent heat pump vapor compression innovations are much more efficient – particularly at part-load operation – whereas boilers and furnaces do not see significant additional efficiencies at part-load conditions. The goal of eliminating CO2 emissions requires a reduction in energy consumption and the decarbonization of the energy consumed, while finding alternatives to inefficient fossil fuel-fired heating sources. By nature, heat pumps – and especially these more recent innovations – are intended to optimize energy for both cooling and heating at those varying operating conditions.

 

U.S. Clean Energy Commitments

As U.S. federal climate policies and strategies continue to evolve, efforts from individual states are already underway to implement a significant increase of renewable energy sources. According to the World Resources Institute, in 2019, multiple states committed to a 100% renewable energy source. Deadlines for reaching these goals ranged from 2035 to 2050.

However, the reality must match the commitment. To this end, 76% of all 2020 new US planned power generation capacity is either wind or solar. Additionally, the IEA in November 2020 provided an update that, globally, renewables constituted 90% of this year’s new installed power generation. As the nation transitions to renewables, there will always be a need for some level of backup, since these natural resources are at the mercy of the environment. These backups – historically known as peaking plants – are necessary when the demand side is high and there are renewable source deficiencies. These constitute the remaining 24% of new 2020 planned generation capacity. Significant growth in the percentage of renewables in the electric power generation portfolio is driving the need for heat pumps that are replacing fossil fuel-based sources and driving an additional increase in the decarbonization potential.

Utilities Are Changing to Support Renewables

In response to the shift to renewable energy sources, utility programs are being pushed to balance demand based on those same supply variations. Again, since renewable power sources may not be available during peak load times, utilities must turn to clean, alternative sources that can balance demand.

Heat pumps are critical to programs that can react when renewable generation sources are not available. As renewable energy volume grows, utilities are motivated to implement heat pumps that can adjust to those supply variations. Since the wind and sun do not always cooperate – leading to a disconnect between supply and demand – addressing that disconnect requires energy storage and heat pump technologies that can modulate energy usage to available supply.

Energy storage takes many forms. There is battery storage, pumped hydro storage and thermal storage, among others. For HVAC, thermal storage is today anywhere from 1-10% of the applied cost of battery storage, which translates to significant savings over an extended period. Therefore, thermal storage becomes integral to addressing the disconnect between integrating more renewables to supply the demand side. 

Grid-interactive efficient buildings require efficiency, load, shift, shed and modulation to quickly act on the demand side to deficiencies on the supply side. These building are also shifting to the concept of separate, sensible and latent load cooling – which can increase efficiency and reduce resulting energy use.

 

District Energy Systems

First developed in the US in the Edison days, district energy systems are a highly efficient way to heat and cool a group of buildings from a central plant. In Europe, district energy systems have evolved over time to operate at lower temperatures, which – in turn – allows other heat recovery and heat pump sources. Grid-interactive heat pumps and thermal storage provide an optimal flexibility district energy system. While district energy systems offer flexibility to meet efficiency goals, they are challenging to implement where no required infrastructure currently exists. However, the long-term value of district energy systems lies in the ability to create a symbiosis that enables heat recovery from vapor compression cooling of multiple heat pump sources, replacing former chillers in cooling-only applications.

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Compression Technology Supports the Trends

There are two main compression technologies for HVAC systems: dynamic compression and positive displacement compression. Dynamic compression adds kinetic energy to the refrigerant, while positive displacement harnesses potential energy by “squeezing” it and reducing the area.

There are also a variety of bearing types. Historically, oil-based bearings are found in most traditional compression technologies. Today, there are several oil-free options, including magnetic, ceramic and gas bearings. Among newer compression technologies, magnetic bearings provide flexibility in terms of capacity, reliability and efficiency.

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Because oil-free compressors minimize physical size through high-/variable-speed operation, they are ideal for retrofit applications where a smaller footprint may be required. Additionally, since these same compressors have no mechanical contact, oil-free machines are quieter and easier to maintain.

 

Next Phase of Refrigerants

These evolving dynamic, variable speed, oil-free vapor compression technologies utilizing new HFO refrigerants can potentially further reduce refrigerant CO2 emissions, as reduced pressure and density can lead to lower global warming potential (GWP) with minimized flammability. The coming years will likely bring lower-pressure and lower-density refrigerants, and supporting technologies optimized for their use.

It is critical to optimize these dynamic centrifugal compressors to their target application operating temperatures. There are three basic optimizations on the market today:

  • Standard lift optimization for lower lift or lower differential operating temperatures, which support water-cooled, evaporative-cooled and hybrid chillers
  • Medium lift optimization for air-cooled chillers, water-cooled chillers in more challenging environments and water-to-water heat pumps with lower required heating temperatures and/or higher cooling/heat-source temperatures
  • High lift optimization – the most recent innovation for oil-free compressors – is on the mechanical design of the compressor, along with the aerodynamic compression process. It supports the most challenging applications, including very high ambient air-cooled chillers, water-to-water heat pumps with higher heating and/or lower cooling/heat source temperatures, air-to-water heat pumps in milder climates, medium-temp process cooling and thermal storage applications.

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All vapor compressors have two sets of forces in the compression process: the radial (side-to-side) motion of the shaft driving the compression process and the axial (back-and-forth) motion. While older compressor designs had both stages of compression on one end of the compressor, new high-lift optimization technology moves the second stage of compression to the opposite end, which balances those axial forces. The two compression stages pulling back against each other enables a higher differential temperature capability, despite the fact there is still no mechanical contact.

 

New Technology Supports Multiple Applications

The most recent version of this vapor compression technology can now provide those same technology benefits formerly relegated mainly to comfort cooling chillers applied in milder climates, now to the hottest environments, provide cooling at lower temperatures – including thermal storage – or provide heating with heat pumps. What were once unattainable applications for magnetic-bearing-based compressors is no longer the case, thanks to this evolving technology that enables operation in more challenging applications.

The success of European district energy systems demonstrates the best way to optimize energy efficiencies is through a combination of technologies and heat sources. There may not be one compressor type that fits into the optimal system. Depending on operating temperatures, heating, return and water temperature may require a combination of a medium lift and high lift compression design in a series counter flow arrangement to, in-turn, enable the most efficient solution that meets the application requirements. 

Oil-free design allows engineers to combine a variety of technology to optimize based on real-world conditions. For example, where cooling is required for industrial manufacturing, this technology can provide cooling and then recover the heat through a symbiosis heat pump, which boosts the temperature to supply the district heating system. When the industrial process is not operating, a parallel geothermal ground-source loop can also supply heat to the district heating system via that same heat pump. This dual cycle system can enable full-time supply as the baseload of a district heating system, maximizing operating hours and helping buildings recoup the first cost investment in less than three years.

 

Finding the Right Solution

An evolving portfolio of magnetic bearing, variable speed dynamic vapor compression technology can support an expanding array of applications with a variety of compressor combinations to maximize operational efficiency while lowering CO2 emissions.

It is important to understand the needs of the specific application to design the optimal system solution that maximizes energy efficiency and minimizes CO2 emissions, while at the same time lowering operating costs. Manufacturers like Danfoss offer components, including compressors and other oil-free system components, that are designed, optimized and rigorously tested to meet the demands for a wide range of applications, now including heat pumps and others even more challenging – the portfolio is expanding with industry trends in-turn driven by the need to reduce CO2 emissions.

 

About the Author

Drew Turner, global marketing manager for Danfoss, has 23 years’ experience in the HVAC industry. He holds a bachelor’s degree in industrial engineering from Oklahoma State University and a master’s degree in business administration from the University of Colorado.

Danfoss engineers advanced technologies that enable us to build a better, smarter and more efficient tomorrow.  In the world’s growing cities, we ensure the supply of fresh food and optimal comfort in our homes and offices, while meeting the need for energy-efficient infrastructure, connected systems and integrated renewable energy. Our solutions are used in areas such as refrigeration, air conditioning, heating, motor control and mobile machinery. Our innovative engineering dates back to 1933 and today Danfoss holds market-leading positions, employing 27,000 and serving customers in more than 100 countries. We are privately held by the founding family. Read more about us at www.danfoss.com.

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