[ DOE Report ] Increasing Efficiency, Building Systems and Tech
Excerpt from the Quadrennial Technology Review: An Assessment of Energy Technologies and Research. Full report published by the U.S. Department of Energy (DOE), September 2015.
More than 76% of all U.S. electricity use and more than 40% of all U.S. energy use and associated greenhouse gas (GHG) emissions are used to provide comfortable, well-lit, residential and commercial buildings—and to provide space conditioning and lighting for industrial buildings. Successfully meeting priority technology goals for performance and cost will make it possible to significantly reduce this energy use by 2030 in spite of forecasted growth in population and business activity.
Figure 5.1 shows U.S. building energy use in 2014. Space conditioning, water heating, and lighting represent well over half of the total, including energy used in outdoor lighting and cooling most data centers.
The building sector’s share of electricity use has grown dramatically in the past five decades from 25% of U.S. annual electricity consumption in the 1950s to 40% in the early 1970s to more than 76% by 2012. Absent significant increases in building efficiency, total U.S. electricity demand would have grown much more rapidly than it did during this period.
Research and Innovations Aimed At Reducing Energy Use. Buildings last for decades (consider that more than half of all commercial buildings in operation today were built before 1970),5 so (when assessing energy efficiency solutions) it’s important to consider technologies that can be used to retro t existing buildings as well as new buildings. Many of the technologies assumed in Figure 5.2 and Figure 5.3 can be used in both new and existing structures (e.g., light-emitting diodes [LEDs]). Retrofits present unique challenges, and technologies focused on retrofits merit attention because of the large, existing stock and its generally lower efficiency. These include low-cost solutions such as thin, easily-installed insulation, leak detectors, devices to detect equipment and systems problems (e.g., air conditioners low on refrigerants), and better ways to collect and disseminate best practices.
Energy use in buildings depends on a combination of good architecture and energy systems design and on effective operations and maintenance once the building is occupied. Buildings should be treated as sophisticated, integrated, interrelated systems. It should also be understood that different climates probably require different designs and equipment, and that the performance and value of any component technology depends on the system in which it is embedded. Attractive lighting depends on the performance of the devices that convert electricity to visible light, as well as on window design, window and window covering controls, occupancy detectors, and other lighting controls. As the light fixture efficiency is greatly increased, lighting controls will have a reduced net impact on energy use. In addition, the thermal energy released into the room by lighting would decrease, which then affects building heating and cooling loads.
Since buildings consume a large fraction of the output of electric utilities, they can greatly impact utility operations. Specifically, buildings’ ability to shift energy demand away from peak periods, such as on hot summer afternoons, can greatly reduce both cost and GHG emissions by allowing utilities to reduce the need for their least efficient and most polluting power plants. Coordinating building energy systems, on-site generation, and energy storage with other buildings and the utility can lower overall costs, decrease GHG emissions, and increase system-wide reliability.
Next Generation Research and Priorities
Providing a comfortable and healthy interior environment is one of the core functions of building energy systems and accounts for about a third of total building energy use. New technologies for heating, cooling, and ventilation not only can achieve large gains in efficiency, but they can improve the way building systems meet occupant needs and preferences by providing greater control, reducing unwanted temperature variations, and improving indoor air quality. Opportunities for improvements fall into the following basic categories:
- Good building design, including passive systems and landscaping
- Improved building envelope, including roofs, walls, and windows
- Improved equipment for heating and cooling air and removing humidity
- Thermal energy storage that can be a part of the building structure or separate equipment
- Improved sensors, control systems, and control algorithms for optimizing system performance
Both building designs and the selection of equipment depend on the climate where the building operates.
The Building Envelope. The walls, foundation, roof, and windows of a building couple the exterior environment with the interior environment in complex ways (see Table 5.2).6 e insulating properties of the building envelope and construction quality together control the way heat and moisture flows into or out of the building. e color of the building envelope and other optical properties govern how solar energy is reflected and how thermal energy (heat) is radiated from the building. Windows bring sunlight and the sun’s energy into the building. About 50% of the heating load in residential buildings and 60% in commercial buildings results from flows through walls, foundations, and the roof (see Table 5.2).7 Virtually the entire commercial cooling load comes from energy entering through the windows (i.e., solar heat gain). The bulk of residential cooling results from window heat gains although infiltration also has a significant role. Future cooling may be a larger share of total demand since U.S. regions with high population growth are largely in warmer climates.
Windows and Skylights. The quality of a window is measured by its insulating value and its transparency to the sun’s visible and infrared light recognizing that an ideal system would allow these parameters to be controlled independently. An ideal window would provide attractive lighting levels without glare, high levels of thermal insulation, and allow infrared light to enter when it is useful for heating but block it when it would add to cooling loads (see Figure 5.4).10 It would also block ultraviolet light that can damage skin and materials.
Windows should also be effective parts of building climate control and lighting systems. Without active control of optical properties, static window requirements will depend on the climate, orientation, and interior space use. If cooling loads dominate, windows that block the invisible (i.e., infrared) part of the solar spectrum are desirable.
Significant progress has been made in window technology over the past three decades. Thanks in large part to DOE’s research investment, sealed windows (multiple panes sealed in a factory) now comprise about 95% of windows sold for residential installation and 89% of windows sold for nonresidential installation. Low-emissivity ENERGY STAR windows make up more than 80% of the market and are twice as insulating as the single-glazing windows that were the default option for generations.
Innovations include glass coatings that reduce absorption and re-emission of infrared light, thermal conductivity improvements (e.g., multiple panes of glass, filling gaps between glass panes using argon, krypton, or xenon, and improved frame design), and the use of low-iron glass to improve visible clarity. Commercial products are now available that provide seven times the insulation provided by single-glazing windows without compromising optical properties. A typical single-glazed window has an R value of one, but R-11 glazing materials and combined frame/glazing units with R-8.1 are commercially available. The “solar heat gain coefficient” is a measure of the fraction of total sunlight energy that can pass through the window while the “visual transmittance” measures the fraction of visible sunlight that gets through. A typical single-glazed window has a solar heat gain coefficient and visual transmittance of about 0.7. Commercially available windows can come close to this with a transmittance of 0.71 and a solar heat gain coefficient that can be selected in the range 0.29–0.62.15 Window frames transmit unwanted heat directly through rigid materials. While progress has been made both in insulating framing materials and in frame design to reduce conduction, challenges still remain. Durable edge seals remain a challenge, and stress under large temperature differences remains problematic.
The biggest challenge is providing superior performance at an affordable cost. There are also practical considerations. Windows with three or four layers of glass are too heavy and costly for most conventional installations. Using a vacuum between the panes eliminates conduction and convection completely, but it requires very small spacers or other mechanisms to keep the glass panes from touching. The cost of highly insulating windows using filler gas would be reduced if the price of producing the gas can be cut (they are now made by liquefying air) or if substitutes are found.
In summary, all current approaches face cost and visual quality challenges.
Building Walls, Roofs and Foundations. The walls, roofs, and foundations of buildings also control the flow of heat, moisture, and air. Their color and other optical properties a effect the way heat is absorbed and how the building radiates heat back into the atmosphere, but they must do so in ways that meet aesthetic standards and serve functions such as building stability and re-resistance. Ideal materials are thin, light, and easy-to-install, and provide opportunities to adjust their resistance to flows of heat and moisture.
Thin materials offering high levels of insulation are valuable for all building applications but are particularly important for retrofits since space for additional insulation is o en limited. Promising approaches include vacuum insulation and lightweight silica aerogel. Flexible insulation materials with thermal resistance of nearly R-10 per inch are available from several suppliers. Because of high costs, use of these insulating materials has been limited to industrial applications such as pipelines, although building applications have been explored. More federal research here is justified only if there is evidence that there are significant opportunities to find novel materials that over high levels of insulation in thin products that can cost-effectively meet fire, safety, and other building code requirements that the private sector is not pursuing on its own. New materials must also be practical for construction—ideally it should be possible to cut, bend, or nail them.
More work is needed in tools and methods to measure and continuously monitor heat and moisture flows through building shells. This includes analytical tools capable of converting sensor data into actionable information about the source of failures in insulation and vapor barriers.
Building shells also affect the way buildings absorb and radiate heat. Ideally, the optical properties of building materials would be adjustable to changes in the weather and other external conditions such as sunlight. Current technologies don’t allow dynamic control, and designs often use a solution that optimizes annual performance even if it isn’t ideal in extreme conditions. In situations where air conditioning is a significant load, roofing should reflect sunlight instead of absorbing it and be able to efficiently radiate heat from the building. New roofing materials are available that help reduce cooling loads in buildings, lengthen the life expectancy of roofing materials, and cut the “heat island” effect in which buildings and other artificial surfaces heated by the sun actually increase the ambient temperature of cities.
It has proven difficult to find materials that can both reflect the sun’s energy and radiate heat during the daytime (when radiative cooling would be most important). Radiating infrared is particularly di cult in areas with significant humidity since water vapor in the air blocks most infrared transmission. is problem has recently been overcome in a laboratory-scale sample. A material created from seven layers of hafnium oxide and silicon dioxide reflects 97% of the sun’s shortwave energy while radiating infrared heat at such a high rate that the material was 5°C below ambient temperatures, even in strong sunlight. It achieves this by having very high emissions in the narrow range of infrared where the atmosphere is transparent to infrared (between eight and thirteen micrometers).
Ventilation and Air Quality. Many people spend most of their time indoors, and the quality of indoor air has a significant impact on their health and comfort. Inadequate ventilation can make a room stuffy and uncomfortable. Exposure to indoor pollutants such as mold, radon, secondhand smoke, pressed wood products (that may contain formaldehyde), and other materials can lead to health effects, including asthma and lung cancer. Moisture buildups can also lead to structural damage to the building.
These problems can be addressed most effectively by minimizing and managing pollutant sources in the building. Problems that remain after steps have been taken to reduce pollutants can be addressed by improved building design and operations, as well as by systems bringing in filtered, outside air and exhausting contaminated interior air. Fresh air may infiltrate the building unintentionally through leaks or through controlled ventilation. Standards typically require different minimum-ventilation rates for different space-use types and occupant densities. Some facilities, such as hospitals and labs, require significantly more fresh air than others. However, increased ventilation increases energy consumption when unconditioned, outside air must be heated or cooled as it replaces conditioned, indoor air that is being exhausted. In 2010, unwanted residential air leaks were responsible for more than two quads of space-heating energy loss and one-half quads of space-cooling energy loss, and more than one quad of commercial heating energy loss. Building codes specify maximum allowed leakage, but detecting leaks can be difficult and expensive, and compliance rates are often poor. New technologies, such as the Acoustic Building Infiltration Measuring System, may improve accuracy and reduce costs.
There are many ways to reduce the energy lost in ventilation systems, which include the following:
- Reduce leaks in building shells and ducts: While minimizing uncontrolled infiltration is a critical part of building design and construction, locating and fixing leaks in existing buildings presents a greater challenge, especially in commercial buildings where pressurization tests cannot be easily used to measure and locate leaks. DOE research led to the development of material that can be sprayed into existing ducts to seal leaks from the inside.
- Use natural ventilation where possible: In some climates and at certain times of the year, natural ventilation can be used to introduce fresh air using natural circulation or fans. Good building design, carefully chosen orientation, windows that open, and ridge vents are some of the many strategies that can be used. Economizers are devices that bring in fresh air when appropriate and can reduce cooling loads by 30% when operated by a well-designed control system. Economizer designs that minimize or eliminate failures can be important for efficiency, but a significant fraction of installed economizers
may not be operative because of poor maintenance. The next generation of sensors and controls can automate detection and maintenance notification to help address this issue, and economizer designs can be improved to minimize maintenance.
- Advanced sensor and control systems provide ventilation only where and when it’s needed: Most installed systems implement fixed air-exchange rates as specified by code, but ventilation needs depend upon occupancy, building purpose and internal activities, and other factors (e.g., a hospital). Significant efficiencies could be gained if ventilation systems provided only the fresh air needed to maintain required levels of carbon dioxide (CO2) and other compounds. Such systems are known as demand- controlled ventilation. Modern systems can use sensors to detect concentrations of CO2 and other contaminants, and this information can be used to make appropriate adjustments to ventilation rates. However, keeping them in calibration has proven di cult. Good control systems may be able to reduce ventilation-related energy use in residences by as much as 40%.
- Use efficient, variable speed motors: Most ventilation systems adjust flow rates only by turning motors o and on or by using dampers. Significant energy savings can be achieved using efficient, variable air volume systems with variable-speed fans along with properly designed and sealed ducts. There are also major opportunities for improving the efficiency and lowering the cost of variable speed motors and motor controls. Innovations that improve the performance and lower the cost of wide bandgap semiconductors are an important part of this work.
- Use heat and moisture exchange devices: Even greater energy savings can be achieved by using heat exchangers that allow incoming cool air to be heated by warm building air being exhausted (or the reverse if the building is cooled). Advanced systems can also exchange moisture (i.e., enthalpy exchangers). These systems are discussed in the section on heat pumps.
- It has been particularly difficult to get advanced systems into smaller buildings. More than half of buildings larger than 10,000 square feet use economizers and variable air volume systems, but less than 10% of buildings smaller than 10,000 square feet use them. Technologies that are inexpensive and easy to use in smaller buildings would be particularly useful.
Space Conditioning Equipment. Although well-designed building envelopes can dramatically reduce heating and cooling loads, there will always be a need for mechanical systems to condition air. Fresh outdoor air will need to be brought into the building and conditioned to replace exhaust air and the heat and moisture generated by occupants and building equipment will need to be removed.
Space conditioning involves two distinct operations: 1) increasing or decreasing air temperature (i.e., adding or removing “sensible heat”), and 2) humidifying or dehumidifying air (i.e., adding or removing “latent heat”).
Because warmer air can also contain more moisture (water vapor), heating usually needs to be coupled with humidification and cooling with dehumidification. Traditional air-heating equipment includes furnaces and heat pumps (see Figure 5.5).38 About half of the floor space is heated with systems that burn fuels and produce CO2 that cannot practically be captured or sequestered with conventional technology. In large commercial buildings, space heating typically uses boilers to heat water, piping the hot water to spaces (i.e., offices and other rooms), and then blowing air over compact hot water coils or running the coils through the floor or wall and radiating heat into the space.
These systems require a separate, dedicated outdoor air system to bring in fresh air. The combination of water pipes/pumps and small air ducts/efficient fans not only requires less energy than large air ducts, it also needs less space between floors.
Air conditioning involves both cooling the air and removing moisture. The traditional approach does both using vapor-compression heat pumps. Smaller systems, including most residential systems, move conditioned air while most large commercial buildings use central chillers to cool water and transfer heat from water to air closer to the occupied spaces. Dehumidification is the process of taking water out of air, and it accounts for nearly 3% of all U.S. energy use. It is typically achieved by inefficiently cooling moist air until the water vapor condenses out and then re-heating the air to a comfortable temperature, which is an inefficient process. Efficiency improvements in heating, ventilation, and air conditioning (HVAC) systems will involve efforts to improve the efficiency of heating or cooling air and technology that can efficiently remove moisture from air.
Heat pump systems are o en used for heating in regions where natural gas is not available. Next-generation cold weather heat pumps can be cost effective in a wide range of climates. Current heat pumps lose 60% of their capacity and operate at half the efficiency when operating at -13oF. Work is underway to develop a heat pump capable of achieving a Coefficient of Performance (COP) of 3.0 for residential applications at that temperature (compared with a COP of 3.6 for an ENERGY STAR® heat pump operating with no more than a 25% reduction in capacity). Work is also underway to improve the performance of cold-weather gas furnaces. Heat pumps have the advantage of providing both heating and cooling with a single unit offering an opportunity to lower initial costs.
Vapor-compression heat pumps and air conditioners rely on refrigerants (working fluids) such as hydro fluorocarbons that have a significantly higher global warming potential (GWP) than CO2 when they are released to the atmosphere. The search for substitutes has proven difficult since alternatives present challenges in toxicity, flammability, lower efficiency, and/or increased equipment cost. It is an area of active, ongoing research by the National Institute of Standards and Technology (NIST) and others. See Table 5.3 for more information.
There is a number of promising heat-pump technologies that have the potential to increase system efficiency and eliminate refrigerants with high GWP. Some use vapor- compression with CO2, ionic liquids, water, and various combinations as working fluids. Heat pumps can also be built that do not require vapor compression (see Table 5.3). ere are also opportunities to improve thermally driven technologies using adsorption and absorption devices and duplex-Stirling heat pumps.
While a key interest in developing these new approaches is to reduce GHG emissions, some can exceed the efficiency of current vapor-compression units.
Moisture Removal. Well-designed building shells and foundations can greatly reduce moisture infiltration, but residual moisture transfer coupled with moisture generated by people and building operations will continue to make moisture removal a priority in building energy systems. A number of new approaches do not require heat pumps and could lead to major gains in efficiency. Membrane technologies allow water vapor to pass but block the passage of dry air or can be used to separate moisture from air using only the difference in vapor pressure, passing thermal energy from outgoing to incoming air. Alternatively, these systems may develop a vacuum on one side of the membrane and then compress and exhaust the water vapor removed. These systems can be combined with evaporative cooling stages to provide both dehumidification and chilling.
Heat Exchangers. Heating and cooling systems depend on devices called “heat exchangers” that transfer heat from the surfaces of the equipment, usually metal surfaces, to air. Efficient heat exchangers are typically large and expensive. It may be possible to greatly improve heat exchange efficiency through improved designs such as microchannel devices or the rotating heat exchanger. New manufacturing methods, including additive manufacturing, may allow production of heat exchange designs not possible with traditional approaches, which could increase the efficiency of commercial air conditioners by as much as 20%.
Thermal Storage. The performance of building heating and cooling systems and the electric grid system serving the building can be enhanced by systems that store thermal energy, particularly cooling capacity. Thermal storage can be provided with a number of different technologies and a number of commercial products are available. Approaches include the following:
- Designing buildings to store and remove thermal energy in the mass of the building itself (i.e., floors, support columns, etc.)
- Using ice and other phase change materials
Since chillers are more efficient when outdoor air is coolest, systems that pre-cool buildings in the early morning can result in energy savings. Chillers can also store cooling capacity by pre-cooling chilled water or ice during night hours and then shutting o the vapor compression systems during peak cooling demand periods in the afternoon. This can yield small site energy savings through chiller efficiency improvements during the cooler nighttime hours, but the largest site benefit of thermal energy storage lies in reducing the site peak demand and peak energy usage. Shifting energy demand away from peak periods could improve electric utility operations by requiring fewer generation plants to be brought on line and reducing the need to build new plants and distribution systems. Thermal storage could also be a dispatchable asset, mitigating problems associated with the intermittent output of wind and solar energy systems. Such systems must be operated as part of an integrated building control system (this is discussed in a subsequent section of this report).
Integrated System Analysis. Taken together, the technologies described above can achieve major improvements in efficiency. (An analysis of current data) shows that a new residence, built using the best available technology today, could reduce its cooling energy needs by 61% while systems operating at the thermodynamic limit would see an 82% reduction.
This analysis assumes that improvements in windows and the opaque envelope were applied first, since they are passive approaches, and the remaining cooling demand was then met with more efficient equipment. As a result, (data shows) envelope improvements contributing more to the overall primary energy use intensity reductions in both cases.
The savings potential of residential heating is even greater since the occupants and household appliances and other devices generate enough heat to meet a large fraction of the home’s heating needs given high quality insulation, windows, and controlled ventilation.
The results for commercial buildings differ in part because lighting plays a large role in energy use. Improved lighting efficiency decreases the heat energy released into the building by the lighting systems and thus reduces the demand for cooling. In the heating season, increasing lighting efficiency actually increases the demand for heating energy. This can be offset by improved insulation and heating equipment (Figure 5.9). These summary Figures cover all building types and U.S. climate regions; actual building loads will depend heavily on climate region, size, and other design features.
Research Opportunities. Primary areas for improving the efficiency and quality of building thermal comfort are the following:
- Materials that facilitate deep retro ts of existing buildings (e.g., thin insulating materials)
- Improved low-GWP heat-pumping systems
- Improved tools for diagnosing heat ows over the lifetime of a building
- Clear metrics for the performance of building shells in heat management and air flows
A detailed discussion of research opportunities for windows and wall materials can be found in a DOE report on windows and buildings envelope RDD&D,53 and a detailed discussion of advanced non-vapor compression heat pumps can be found in a report on that topic.54 In brief, areas where fundamental research problems remain unresolved include the following:
- Glazing materials with tunable optical properties (transmissivity and emissivity adjustable by wavelength) including materials that could be applied to existing windows
- Materials that are thin and provide tunable insulating and vapor permeability and materials that could be used in next-generation enthalpy exchange devices
- Technologies that could lower the cost of producing noble gases and identifying transparent, low- conductivity gases that could substitute for noble gases
- Strategies for using vacuum as a window insulation
- Innovative heat exchanger designs for heat pumps and other uses (variety of scales) that reduce the volume and weight of heat exchangers
- New ways to enhance ventilation and health that are cost-e ective, energy-e cient, and practical to implement
- Improved ways to control moisture transfer into and out of buildings
- Components for non-GHG heat pumps including magnetocaloric, thermoelastic, thermoelectric, electrochemical, and electrocaloric systems
In a number of cases, the technology for achieving needed system performance is known but products are too expensive. In most cases, costs will decline as production volumes increase. Emphasis should also be placed on lowering manufacturing costs. In some cases, Finding inexpensive materials is also important. Areas with opportunities include electrochromic windows, variable speed motors, vacuum insulation/advanced insulation (e.g., aerogel), sensors, and controls.
Continuing research brings the goal of creating a “net-zero energy façade or envelope” within reach. A window could reduce a building’s need for external energy sources more than a highly-insulated opaque wall. While the specifics vary with location and orientation, the opportunities to do this include: 1) reduce thermal losses by a factor of two to three below current code requirements; 2) provide active control of solar gain and daylight over a wide range; 3) introduce sufficient daylight to adequately light the outer thirty-foot depth of floor space; and 4) use natural ventilation when it can o set HVAC use. These systems require careful integration with other building systems to be effective and to provide the required levels of thermal and visual comfort.