Components For Tomorrow's Heat Pumps
The center's latest research has involved dealing with minimum air conditioner and heat pump efficiencies - specifically finding ways to increase efficiency without increasing size too much, and without using R-22.
Most changes involve component innovations. He described some of the center's findings at the International Energy Agency (IEA) Heat Pump Conference in Las Vegas. The session was co-chaired by professor Thomas Kopp, an energy engineer from Switzerland, and Glenn Hourahan, P.E., of the Air Conditioning Contractors of America (ACCA).
Bullard's overall message: Component changes will solve some of the efficiency concerns, but they won't be enough. Integrated systems are more likely to help meet efficiency demands in air conditioning and refrigeration systems.
"Continuing efficiency improvements in air conditioning and refrigeration systems will require continued improvements in component technologies," Bullard said. "Such improvements are necessary, but they are not sufficient."
Concern with the efficiency of small (under 10 kW) systems is not exclusive to North America, he pointed out. Japan also has new efficiency standards, and other parts of Asia require higher efficiency systems because "the electricity distribution infrastructure cannot handle many inefficient systems," Bullard stated.
He walked his audience through the process of designing a heat pump system, with components designed to increase system efficiency. "Only five basic components are required - theoretically - for a system to operate the ideal vapor-compression cycle," he said: "two heat exchangers, compressor, expansion device, and refrigerant." These basic components have room for efficiency improvement.
"Additional improvements in system efficiency are being realized through the addition of other components that modify the thermodynamic cycle," Bullard said. These include the receiver, separator, internal heat exchanger, and ex-pander or multistage compressor.
Improving On The BasicsBullard started with the standard vapor compression cycle, with the system operating at a single design condition. The goal is to get as close to the ideal cycle efficiency as possible.
Condenser and evaporator: "The ideal heat evaporator or condenser would have infinite face area and infinitesimal thickness in order to minimize the power required to move an infinite amount of air or other fluid over the heat exchanger," Bullard said. "Today's most efficient heat exchangers, therefore, employ heat transfer enhancement techniques on both the refrigerant and air sides to minimize heat-transfer resistance subject to a face area constraint."
He described brazed-plate heat exchangers as "one of the most remarkable advances in this technology." Surface patterns on the plates "create a chaotic laminar flow in many parallel (and communicating) channels between the braze joints, resulting in large-scale vorticity that produces single-phase heat transfer coefficients that can be significantly larger than those in two-phase flow on the opposite side of the many parallel plates.
"When heat is transferred between two-phase refrigerant and air, the heat transfer resistance on the air side is often 90 percent or more," he explained. "Therefore, to minimize material cost and air-side pressure drop, recent technology development has focused on enhancing air-side performance."
Compressor: The task of the compressor designer is to maximize efficiency at constant entropy, by minimizing friction losses and internal heat transfers.
"Compressor efficiencies in-creased rather rapidly during the last two decades, primarily by eliminating excessive heating of the suction gas," Bullard pointed out. "This was accomplished by increasing motor efficiency, using rotary compressors with high-side oil sump, and rejecting more heat through the compressor shell instead of relying on cold suction gas to cool the motor."
Expansion device: Variable expansion devices (i.e., thermostatic and electronic expansion valves) operate with the goal of maximizing evaporator capacity at all operating conditions by maintaining the outlet as near as possible to the saturated vapor state, Bullard said. "Fixed short-tube orifices and capillary tubes are a less costly alternative, but such passive devices sized for a particular design condition generally fall short of that goal at off-design conditions."
Refrigerant: "The refrigerant charge is a key component of any system," he stated. "Together, the amount of charge and the expansion device opening enable the system to meet the target evaporator and condenser exit states in critically charged systems, at the design condition."
A system's refrigerant transport properties (such as conductivity and viscosity) can sometimes reduce overall system efficiency, he added. Sometimes these changes are addressed by tweaking a component design (i.e., changing tube diameter). However, "additional components are often needed to offset shortfalls in ideal cycle efficiency."
Off-design conditions: "After selecting components to get as close as possible to the ideal vapor compression cycle at a particular design condition, the next step towards improved efficiency is to modulate refrigerant and airflow rates to maintain near-ideal system performance at off-design conditions," Bullard said. Efficient, variable-speed drives help achieve this.
"Modulating compressor displacement rate is the key," he said, "because it allows the system to run continuously, utilizing the available heat transfer surface 100 percent of the time."
Variable-speed fans essentially modulate "the magnitude of the heat sink and the air-side heat transfer resistance. Their speed at any operating condition can therefore be selected to maximize system efficiency, or some other constrained tradeoff among other objectives such as low noise or dehumidification."
Adding ComponentsAdditional components in-stalled in vapor compression systems serve two main purposes, Bullard said: "to make the basic cycle and its components work better, or to bring the shape of the thermodynamic cycle closer to the Carnot ideal."
Receivers: "The combination of a simple, fixed expansion de-vice and a low-pressure receiver can be viewed as a low-cost, completely passive alternative to a TXV or EXV," he said. "Both protect the compressor and control the evaporator exit state." They are commonly used on window units and reversible heat pumps.
"High-side receivers are often used in conjunction with expansion valves in systems where high energy efficiency is required," he continued.
"In such cases, the expansion valve controls the evaporator outlet while the receiver maintains a saturated liquid condition at the condenser outlet over the full range of steady-state operating conditions."
Internal heat exchangers: Internal liquid-to-suction heat ex-changers are added to some systems in order to increase cycle efficiency or to protect the compressor from slugging, Bullard said. They are used on all domestic refrigerators.
However, the thermodynamic properties of refrigerants such as R-410A, he continued, "produce no net increase in COP when the cycle is changed by adding an internal heat exchanger, and in some cases (e.g., R-22) simple thermodynamics dictates a net loss of 5 percent or more, depending on the operating conditions."
Expanders: Subcooling, achieved either naturally, mechanically, or via an internal heat exchanger upstream of a throttling device, can help increase system efficiency by recovering some of its operational energy work by expanding through "a turbine or other positive-displacement device," Bullard said.
However, "The economic feasibility of this additional component is limited by several factors, mostly scale related."
Both internal heat exchangers seek to recover the same energy, he said; they would not be used together. "Overall, however, expanders offer the theoretical possibility of completely eliminating expansion losses, while internal heat exchange has unavoidable losses due to poor matching of temperature glides and the lowering of suction gas density."
Multistage compressors, intercoolers, and separators: "For many years, these additional components have been included in large, custom-built industrial refrigeration systems to increase system efficiency, but have not been employed in small, mass-produced systems because of the cost-reduction focus that dominates a commodity industry," said Bullard.
"However, recent research is revisiting the issue as modern manufacturing technologies make these components a potentially economical way to respond to demands for more efficient systems."
Secondary loops: "The refrigerants of the future will be more costly, flammable, or hazardous than those of the past," Bullard stated. "Therefore research and technology development activities are exploring ways to minimize refrigerant charge, even in refrigerant-to-air heat exchangers." These reductions can be achieved by transferring heat to a secondary fluid, such as a single-phase brine or "a two-phase bubble-pumping operation in an ultra-compact evaporator and condenser."
System IntegrationA reversible heat pump system already provides both heating and cooling, Bullard pointed out. "A slightly more complex system may also provide domestic water heating while sharing many of the same components with the heat pump used for space conditioning."
In large buildings, he continued, the heating-cooling system is already integrated with the ventilation system, and this trend may spread to smaller buildings.
"Already there are many demonstration projects, primarily in Europe, where conduction and infiltration loads have been reduced to a point where space heating loads can be met by heating the ventilation air alone."
Similar opportunities are available through improved building insulation, reduced infiltration, ventilators that recover sensible and latent heat, and glass that minimizes solar gain.
"The air conditioning system required to meet such small loads may require components that are radically different from those built today," Bullard said. "For example, in some climates it may be sufficient to meet a substantial portion sensible cooling load as a byproduct of the domestic water heating process, extracting heat from either the ventilation air or from radiant panels operating above the dewpoint of the indoor air.
"Latent loads, having been minimized by tight construction and an enthalpy-recovery ventilator, might be met separately with liquid desiccant in the building's air purifier."
It's possible that the heat pump of tomorrow might be constructed in a building's very walls, he speculated.
"Considering that a building's thermal loads are transmitted through such wall panels, it may be possible for the inner and outer surfaces to serve as the evaporator and condenser for an embedded heat pump system, relying only on natural convection and radiation because of the relatively low heat fluxes involved. It is even conceivable that such heat pumps could be integrated with photovoltaic cells on the outdoor surface, capturing much of the radiant energy not converted to power during winter."
All these scenarios have tremendous implications for the design of components for a vapor compression heat pump," he concluded.
"The lesson to be drawn from such brainstorming is not that any such scenario may in fact materialize; rather, it illustrates how component designs are dictated by larger system performance criteria.
"And the larger system may be required to deliver shelter, silence, and electricity as well as thermal comfort, hot water, and cold food."
Publication date: 06/27/2005