More air, less juice:Hot technology for A/C

The worldwide energy footprint of air conditioning is about to get a lot bigger. That’s the conclusion of researchers who have modeled the impact of 100 million air-conditioned vehicles on the roads of China by the year 2015. Add to that statistic the reality that U.S. energy consumption for A/C has doubled over the last 20 years and the size of the energy demand associated with A/C becomes apparent.

Indeed, rising levels of affluence in developing economies make A/C energy demand a hot topic. In India, for example, the number of room A/C units now grows annually by 20%. The A/C demand of Mumbai alone is estimated to be one-quarter that of the entire U.S. And there are similar trends in the Middle East and in tropical South American mega-cities.

No wonder, then, that The Dept. of Energy has turned its attention to promoting more efficient A/C designs. It is doing so both by setting minimum energy efficiency standards for commercial A/C units, and by funding “rocket science” A/C ideas through its Advanced Research Projects Agency - Energy (ARPA-E).

Last year, the DOE set minimum efficiency standards reflecting regional requirements for heating, cooling and water-heating equipment. The requirements — slightly different for the South, Southwest, and Northern regions of the U.S. — go into effect in 2013 for non-weatherized furnaces and in 2015 for weatherized air conditioners, furnaces, and heat pumps. For the two warmer southern regions, minimum cooling efficiency (as defined by the standard Seasonal Energy Efficiency Ratio) is 14 SEER for central air conditioners. In contrast, cooling efficiency minimums for installations in the north remain 13 SEER.

Industry watchers say these standards haven’t yet been implemented because they’re potentially onerous. They force manufacturers to create different products for different regions of the U.S. that also meet efficiency codes based on the design. Of course, they might be easier to meet with some of the innovative A/C ideas coming out of ARPA - E, but these super-efficient designs aren’t likely to see commercial reality for years.

The system to beat:Air-source vapor-compression A/C

Traditional A/Cs use either refrigeration or evaporation (as in boiler/cooling-tower setups) to generate cold air. Vapor-compression refrigeration A/C is most common. To briefly review this scheme, a blower sends air over an evaporator coil filled with cold circulating refrigerant to transfer some of the air’s thermal energy to the refrigerant — which in turn vaporizes. Next, the vaporized refrigerant flows to a compressor, which further boosts refrigerant pressure and temperature. The hot refrigerant then passes through condenser coils which get cooled by the air from a fan. The vapor cools and recondenses, then flows back to the heat exchanger via a valve that lets the liquid refrigerant expand — causing its temperature to drop further in a manner resembling that of a pressurized canister being relieved. Finally, a blower pushes the cooled air into the conditioned areas, often through duct systems.

Such air-source vapor-compression A/Cs have a coefficient of performance (COP) that is at best 60% — COP to COP_Carnot of maximum theoretical efficiency. This figure is what A/C researchers and engineers aim to transcend.

Technologies now and on the horizon

Some technologies around since the early 1900s are quite efficient theoretically, but still too expensive to manufacture, use, and maintain. Nevertheless, there are several methods in commercial use that improve on the basic vapor-compression scheme.

1. Chilled beams: Also called induction diffusers, these incorporate pipes of chilled water running through finned, beam-shaped heat exchangers mounted on ceilings, walls, and floors.An evaporator or heat exchanger chills the water and may include a pump, expansion vessel, and buffer tank as needed. This interfaces with another secondary closed circuit or directly circulates through the beam pipes to absorb heat from the conditioned volume’s air via radiant heat transfer or radiant-plus-convection heat transfer.

Chilled-beam operation is typically 20% more efficient than air-source vapor-compression A/C. The beams use the fact that water can carry more energy than air. They also boost efficiency by segregating cooling (and heating) functions from ventilation, so vent fans need produce less airflow and pressure and thus consume less energy. Finally, the design allows for the thermal retreatment of indoor air for better efficiency — drawing external air only when needed for ventilation.

Though they are a mature technology, chilled-beam setups are uncommon because they cost more to build. However, the replacement market has grown because of sluggish commercial construction in the U.S., making chilled-beam technology increasingly viable. Here, the beams are retrofitted rather than being part a building’s core structure.

2. Variable-refrigerant flow (VRF): This design starts with traditional vapor-compression A/C technology but implements it with multi-stage refrigerant compression that can be adjusted to meet the distinct demands of multiple conditioned zones. The classic application example is in a hotel, as a means of accommodating the preferences of individual guests.

In a nutshell, VRF uses one specialized condensing unit that services multiple evaporators. The evaporators are at each independently controlled zone and include an electronic expansion valve. Liquid refrigerant flows through their coils to undergo evaporation and cool the conditioned volume or condense and heat the zone. However, refrigerant flow to each evaporator is continually adjusted to heat or cool air at commanded rates, locations, and times. Larger VRFs can even transfer heat between areas, simultaneously heating some zones while cooling others.

VRF adaptive control, based on occupancy and demand, minimizes energy use — sometimes making VRFs 25% more efficient than competing technologies such as water-source heat-pump systems (which rely on evaporative cooling from towers.) This efficiency makes VRF technology suitable for some facilities seeking Leadership in Energy and Environmental Design (LEED) certification.

Because VRFs don’t rely on central ventilation for cooling distribution, they also eliminate duct losses — for 10 to 20% energy savings over VAV systems. Finally, thanks to variable drives on both the refrigerant compressors and on distribution, part-load efficiency is up to 15% higher than that of traditional heat pumps or electric chillers. This is noteworthy because A/Cs most commonly operate at partial load capacity. Widely used in large commercial buildings, VRF technology is new to the U.S. but has been employed in Asia and Europe for years.

3. Solar air-conditioning with adsorption: Solar cooling uses the same approach as vapor-compression A/C but omits the motor-powered compressor. The most common iteration utilizes a twin-chamber adsorption (surface-only absorption) chiller. Pipes filled with chilled water run through the conditioned space and collect room heat. Once heat is collected, the water is piped through a low-pressure evaporator open to a chamber cooled and packed with silica sorbent. Injected water draws heat from the room-cooling circuit and is vaporized. This vapor adheres to the silica’s surface via Van der Waals forces for exothermic energy loss. Then the room-cooling circuit runs back through the conditioned volume to chill it once again.

Meanwhile (thanks to a generated pressure differential) another circuit of water flows through the chiller’s off-duty chamber to regenerate its silica by drying. This circuit is connected to and heated by tube collectors embedded in rooftop panels that collect solar heat. Water vaporized off of the silica collects in a cooled condenser for reuse.

Once the first chamber’s silica is saturated, ports to the evaporator and condenser chambers are switched and the regenerated chamber takes over adsorption cooling. The chilling function can be bypassed in winter to provide direct heating form solar energy.

Adsorption chillers use relatively little energy. So solar air conditioning is up to 75% efficient, hitting higher efficiencies with radiant rather than forced-air distribution. Problems with this technique include finding a location for the panels on retrofits, as most commercial buildings put chillers, VRFs, and other mechanical systems on the roof.

Even so, some facilities have incorporated solar air-condition technology with excellent results. For example, the Festo Corp. headquarters in Germany has Europe’s largest solar-powered HVAC system. An array of more than 1,200 m² of evacuated tube collectors (in the panels) helps produce 500 MW-hr of primary energy per year. Combined with an adsorption cooling system, the design cools and warms the building via water pipes in concrete floors.

4. Geothermal heat pumps: These rely on pipes running through 20 to 300-foot-deep shafts to access the stable 50 to 60°F temperature caused by solar warming of earth’s crust. For A/C, refrigerant runs through a heat exchanger interfacing a chilled beam variant or other forced-air system to collect heat from the volume being cooled. Next, the geothermal circuit leads the refrigerant into the buried portion of the design though pipes sunk either horizontally or vertically. Pumps at regular intervals maintain refrigerant pressure and flow. The cool ground serves as a heat sink, absorbing by induction the refrigerant’s collected heat.

Efficiencies can exceed 85% mainly through elimination of electrical or gas-powered cooling apparatus. According to the Geothermal Energy Association, installations have climbed in the U.S. to exceed one million. The EPA is promoting the trend with the Enhanced Geothermal Systems (EGS) initiative — building engineered reservoirs to produce energy from geothermal sources where lack of water or permeability wouldn’t permit it otherwise. Geothermal designs work in a range of source temperatures, though warmer sources boost efficiency.

5. Smart membrane cooling: One water-based air conditioner developed by Dais Analytic Corp. omits both refrigerant and compressor. Called NanoAir, it’s built around a membrane of high-charge-density electrolyte material that separates dehumidification and cooling. A vacuum is drawn on one side of the membrane, while air travels past the membrane on the other. Passing through are water molecules only nanometers in diameter because they have a high dielectric constant. To cool air, the system transits water through the membrane to make gas, pulling off the water’s vaporization heat to produce cold water. Moisture from the dehumidification and water from the water cooling both go to a vapor pump for expulsion. This process can be much more efficient than a conventional refrigeration cycle — even to 50% more efficient. Such efficiencies arise partly because conventional air-conditioner cooling coils must be set to a temperature lower than the ambient air’s dew point. The NanoAir system can be set to cool at the target temperature.

6. Elastocaloric-based cooling: In 2010, researchers at the University of Maryland began work on thermally elastic metal alloys for use in refrigeration and air conditioning. Based on a Martensitic phase transformation of latent heat, the solid coolant refrigerant aims to replace the fluids used in conventional A/C compressors. Martensitic transformation is basically a small homogeneous shift in a material’s lattice of atoms in response to mechanical stress. When undergoing this process, NiTi and other shape-memory alloys generate heat. Upon release, the alloys exhibit endothermic behavior.

Investigations into a prototype employing one NiTi two-state alloy shows that the material alternately absorbs or creates heat as a compressor-based system, but uses less energy. In a recent update on the project, researcher Ichiro Takeuchi and his team confirmed experimentally that this effect works and can be actively used for cooling and refrigeration. “The prototypes we built include a 30-W hand-cranked water cooler and a 1-kW thermoelastic air-conditioner. The project continues, and we are moving ahead with this technology in different directions,” says Takeuchi.

Note that elastocaloric-based cooling is not the only technology aiming to transcend the heat transfer of traditional vapor-compression A/C evaporators and condensers with solid refrigerants. These allow more effective direct-contact heat transfer, as refrigerant-side resistance to heat transfer is eliminated for a 10 to 20% improvement in performance coefficient. Another such technology includes magnetocaloric designs that promise COP/COP_Carnot to 70% or more.

7. Pre-cooling schemes: University of California, Davis and A/C manufacturer Trane of Ingersoll Rand recently built a rooftop air conditioner said to be 40% more energy-efficient than conventional units.The Voyager DC is a hybrid rooftop air conditioner that uses evaporative cooling to reduce peak electrical demand. In short, water evaporation cools outside air for the condenser on an otherwise conventional air conditioner. Then water chilled by evaporation cools hot outside air used for building ventilation — for hours of free cooling and less full-speed operation. In addition, the Trane Voyager DC incorporates variable speed fans and staged compressors to maintain high efficiency.

Duct work plays a role in A/C efficiency

The cool air an A/C generates must be delivered to its intended space, typically through some kind of air duct. Duct design can impact cooling efficiency. Modern duct systems incorporate a means of varying the amount of air delivered to specific subspaces.

The typical scheme in such variable air volume (VAV) systems is to install terminal units at each outlet off the main duct. These terminal units contain automated butterfly dampers to modulate delivered air quantity. Digital controls coupled with sensors monitoring pressure and temperature modulate supply air temperature. Such controls are basically sophisticated thermostats, customizable with setpoints, time schedules, trend logs, logic, and alarms. Some even interface with building automation systems. VAV installations so equipped are up to 30% more efficient that older air-distribution schemes; their ducts and central air handling units can be smaller, demanding less energy for fans. Fan control via variable-speed drives further boosts efficiency.