Energy efficiency in the built environment

Policymakers, commercial enterprises, and scientists have focused increased attention over the past few years on the issue of energy. Of primary concern is the increase of atmospheric carbon dioxide from fossil-fuel consumption, which leads to global warming. National security is an additional concern since we in the US rely on unfriendly areas of the world for a substantial portion of our fossil fuels.

Most of the recent effort has been directed toward new and better sources of energy that have a reduced impact on global warming. All the proposed sources have potential constraints and uncertainties. For example, increased use of coal for power generation will require the separation and sequestration of CO₂ by techniques that are expensive and presently unproven at large scales. Nuclear energy carries with it worries about safe operation, long-term waste storage, and possible nuclear proliferation. Renewable energy sources have their own uncertainties and constraints. There are suggestions that wind-powered turbines numerous enough to meet a large fraction of our energy needs can seriously deplete global surface wind velocities and cause temperature increases. Similarly, a large array of photovoltaic panels will decrease Earth’s average surface albedo since only a small fraction of the incident solar energy on a PV panel is converted to electricity—most is absorbed as heat. In addition, as we will see, today’s cost of energy generation from most renewable sources is too expensive for widespread deployment. Even in the most optimistic scenarios, most solutions require long lead times, on the order of decades, before they can make a substantial contribution to global energy supplies.

The dearth of near-term affordable solutions for clean energy supplies presents serious obstacles to limiting global warming. The problem is compounded by the worldwide increase in energy consumption. As the developed world continues its modest energy growth, it is being overtaken by the explosive energy consumption of China, India, and other developing countries.

A more rational approach to limiting global warming is a balanced solution that places equal weight on energy efficiency to limit or reverse energy consumption until long-term, environmentally acceptable energy-supply technologies can be deployed at meaningful levels. In the transportation sector, improvement of the fuel economy for vehicles is finally receiving serious consideration in the US. Although it will certainly help reduce our reliance on liquid fossil fuels, it is not sufficient, by itself, to halt overall global warming.

A good starting point in addressing energy efficiency is to examine where the energy is used. Figure 1 shows the proportions of primary energy used in the three main end-use consumption sectors: industry, transportation, and buildings—both residential and commercial. ¹ Primary energy refers to the fossil energy directly consumed as well as the fossil and other energy sources needed to generate the electricity consumed. Electricity is not included as a separate end-use sector because it represents an intermediate conversion of energy rather than an end use.

As figure 1 illustrates, buildings are the largest energy consumer in the US, which is a surprise to many people. The combined residential and commercial building sectors consume close to 40% of the total primary US energy. Over the past three decades, residential energy consumption has grown due to more and larger homes, although the fraction of total US energy for residences has remained at about 20–22%. Over the same time frame, the commercial portion of US energy consumption has risen from 13% to 18% of the total. The combined residential and commercial building sector also uses 70% of US electricity. To properly account for the primary energy charge for that electricity, one must take into account the inefficiencies of electrical generation, transmission, and distribution. So each kilowatt-hour of electrical energy consumed at a building requires roughly 3 kWh of primary-energy supply at the generation plant.

In some European countries, including the UK, buildings consume 50% of the total primary energy. In China the buildings’ percentage of total consumption is much smaller than in the West, but China is building an estimated 10 million residential units a year. ² The new units are larger, and Chinese homebuyers increasingly demand central heating and air conditioning. It is safe to assume that buildings will represent a good fraction of the overall energy growth in China and India.

Opportunities, costs

Given the enormous energy consumption by the building sector, a viable part of a CO₂ control strategy should include increased energy efficiency for that sector. Over the life of a typical building, the energy consumed in its operation is almost an order of magnitude larger than the energy needed to manufacture the building components—the so-called embodied energy. Thus more efficient building operation should be the focus of our attention.

Robert Socolow has proposed a possible set of strategies ³ that would use existing technologies to maintain the world’s atmospheric concentration of CO₂ at or below the limit of 500 ppm (the current concentration is about 375 ppm). His strategy involves seven equal wedges of action beyond business as usual for the next half century. One of the seven is an average 25% reduction of energy consumption by all buildings. Some studies suggest we can exceed a 25% reduction. Given the long lifetime of buildings and the large existing stock, an important component of such a program must be the retrofit of existing buildings. For example, the US has more than 100 million homes, and that number is growing by only 1–2 million each year.

Pursuing energy efficiency depends not only on having available technology but also on the economics of efficiency versus other options. The table on page 37 compares the cost of electricity generation from conventional and renewable technologies with the cost of saving electricity by amortizing the increased capital cost of a more efficient new building over its lifetime. ⁴ Note that the listed costs of generation do not include transmission and distribution costs. Also, the cost for fossil fuels does not include costs for carbon abatement. Today’s costs for many of the renewable technologies receiving the most attention are much higher than the cost of efficiency. A recent report reaches a similar conclusion. ⁵ According to that report, the cost of carbon reduction from many efficiency actions yields a positive life-cycle return—the present value of the long-term savings is worth more than the up-front expense. Furthermore, the efficiency investments are far less expensive than energy-supply options that include carbon abatement. Economies of scale and new technological innovations will probably reduce the cost of renewable sources of energy, but the same factors should also reduce the cost of efficiency improvements.

Goals for energy efficiency

Several organizations have established ambitious goals for the efficiency levels of future buildings. The American Institute of Architects has a goal of achieving, by the year 2010, a minimum 50% reduction from the current level of consumption of fossil fuels used to construct and operate new and renovated buildings. They also would promote further reductions of remaining fossil-fuel consumption by 10% or more in each of the following five years. ⁶ Other groups have called for substantial energy reductions, with some advocating that new construction within the next few decades approach zero net energy—the point at which on-site renewable energy production over the year compensates for all primary energy consumed the same period. The capital of the United Arab Emirates, Abu Dhabi, is developing a new urban zone, the Masdar Development, a 6-square-kilometer sustainable development that will include a new graduate university and commercial and residential buildings. The complex is designed to achieve zero net energy. From economic considerations, such a goal is best achieved by first substantially reducing energy consumption and then meeting the remaining demand with renewable supplies.

Achieving substantial levels of energy efficiency requires a combination of technologies. If there is something approaching a silver bullet, it is integrated design—architects, developers, engineers, and energy consultants working together from conceptual design to finished construction. Such an approach not only will ensure that the major building elements such as the building skin or envelope, heating and cooling systems, ventilation, and lighting are well matched, but it also will result in substantial economies. For example, to counteract the discomfort caused by cold windows usually requires a separate heating system directly adjacent to the windows, but a high-performance envelope system may eliminate that need. The high-performance envelope will also reduce the size of the heating and cooling equipment needed to meet the peak demand. To facilitate the integrated design process, simplified tools are needed to assess the energy implications of different conceptual possibilities. One such tool, developed by several graduate students and me, is the MIT Design Advisor, available at http://designadvisor.mit.edu .

Energy-efficient building designs involve technologies that will improve by evolutionary advances and, in some instances, revolutionary ones. Some designers are tempted to concentrate on an approach that emphasizes a building design that “looks green” or has “green bling.” But in most applications, the most effective components do not outwardly look that dissimilar from conventional designs. Another concern is the piling on of measures that offer little or no energy-performance improvements. The usual result is a trophy building that is not economically rational. Such an approach makes it difficult to assess the key elements of the performance gains.

Residential buildings

Space heating constitutes the largest energy use in residential buildings. Active or passive solar-energy systems can meet a majority of the heating needs in most climates. Such systems require windows or solar thermal collectors—which collect the Sun’s energy for heating purposes—oriented to receive a maximum of solar irradiation in the winter. Figure 2 shows two early solar homes designed by MIT and a contemporary passive residence in Sweden that is heated without a central heating system. Solar-heated homes require high-efficiency windows and heavily insulated walls to minimize heat loss by conduction, convection, and IR radiation. In addition, thermal mass is needed to store excess solar-energy gains during the daytime for use during the night. The storage can be in the form of temperature change in a solid, such as masonry, or in water. In some designs, the energy is stored in phase-changing materials embedded in wall panels.

Electricity power costs

To meet ventilation needs, an air-to-air heat exchanger can transfer heat from warm exhaust air to incoming cold ventilation air. Passive houses such as the Swedish residence in figure 2(c) can achieve comfortable indoor climates without the need for a central heating system. As part of the demonstration project CEPHEUS (Cost Efficient Passive Houses as European Standards), funded by the European Union, 14 passive buildings with 221 dwelling units have been built at different building sites in central and northern Europe. The units demonstrated that interior comfort conditions could be maintained while using only 15 to 20% of the space-heating demand of conventional new buildings. Compared with conventional new buildings in those regions, the total primary energy savings for electricity and heat were more than 50%. ⁷

From a thermodynamics perspective, burning fossil fuels for low-temperature applications in home heating is very inefficient. A potentially more efficient system includes high-efficiency heat pumps. A heat pump is basically an air conditioner cycle that draws energy from the ambient outside air (rather than from the interior as in the usual air-conditioning cycle) and rejects heat to the building interior at higher temperature. Since the process operates in a closed cycle, the first law of thermodynamics ensures that the heat delivered—the sum of the heat absorbed from the ambient air and the energy from electrical power consumption—is always greater than the electrical power consumption.

Current heat pumps have a coefficient of performance (COP)—the ratio of thermal energy delivered to electrical energy consumed—of 2.5–3. In contrast, the ideal, reversible heat pump, a Carnot cycle, has a COP given by 1/(1 − T _L/T _H). The ideal COP is about 14 for typical interior temperatures T _H and exterior temperature T _L. Major irreversibilities in existing heat pumps are caused by large temperature differences in the heat-transfer process in the evaporator and condenser stages of the heat pump. Evaporators and condensers are heat exchangers that typically contain refrigerant within tubes; heat is transferred between the refrigerant and air that is circulated over the tubes’ exterior. The heat-transfer process can be substantially enhanced by applying techniques under development for cooling computer integrated circuits. Using microgrooved heat-exchange surfaces can increase the heat-transfer rate by an order of magnitude. Nanotechnology offers the possibility of an order-of-magnitude increase in the thermal conductivity of the surface materials, and nanofluidics may enhance overall convective heat transfer. Heat pumps that draw heat from the outside air suffer decreased output as the air temperature drops, just when it is needed most. Ground-source systems that draw energy from large masses of earth or water underground can avoid that decrease and have a somewhat higher COP.

When using fossil fuels in urban locations, a more efficient alternative to a conventional heating system is a co-generation system, which combines heating and power systems and can serve a single building or a local urban region. In such a system, the hot exhaust from the power-producing cycle—possibly a fuel cell, gas turbine, or Stirling engine—is used to produce heat. The net primary energy used by a combined cycle is less than the primary energy needed to produce an identical amount of electricity and heat from two separate systems. In the case of identical electrical and heat production, if the power-producing system has an efficiency of 50% for electrical generation, a combined cycle requires only 2/3 as much primary energy as two separate systems. But balancing the co-gen’s electricity and thermal output with the building demands can be difficult, as can achieving the full benefit of the system in some seasons or in some climates, such as the southern US.

Considerable strides have been made in reducing the energy needed for large appliances and in substituting compact fluorescent lights for incandescent ones. Solid-state LEDs promise even higher efficiency. Although refrigerators have become larger over the past several decades, improvements in the insulation, seals, and refrigerant systems have resulted in substantial increases in overall efficiency so that a large refrigerator uses, on average, only about 60 W. Figure 3 shows the trends in refrigerator efficiency. Many of the gains have been driven by stricter appliance standards and financial incentives. But as the figure shows, the gains have been offset by the rise in energy consumption of numerous new home electronic devices in their standby mode. When turned off, TVs, power supplies, copy machines, and other devices continue to draw power. That consumption can be inexpensively remedied with modest changes in future appliances.

Commercial buildings

Recent studies have demonstrated the effectiveness of energy-efficient retrofits and new construction for commercial buildings. For example, an analysis of real-world commissioning results—the detection and correction of construction defects, malfunctioning equipment, and deferred maintenance—for 224 existing commercial buildings demonstrated the median energy cost savings of 15% and a median payback time of 0.7 years. ⁸ Only about 10% of US commercial buildings (which together account for 33% of the total US floor area) have centralized energy management and control systems, while occupancy sensors for lighting serve less than 10% of commercial building floor space. Selected controls and diagnostics have the potential ⁹ to reduce total commercial building primary energy by 14–38%.

For commercial buildings, lighting represents the largest primary energy consumption. Higher-efficiency lighting has seen continued progress through compact fluorescent bulbs, higher-efficiency commercial fluorescent tubes and ballasts, and solid-state LEDs. Effective use of daylight combined with dimmers and occupancy sensors that eliminate unneeded artificial light can reduce energy use for lighting by more than 50%. ¹⁰ The new headquarters for the New York Times has such a system along with automated blinds to control glare. The inset of figure 4 shows how the interior looks. The system is a prototype, not widely available yet because a detailed engineering design is required for each installation. Figure 5 shows the result of an advanced daylighting system being tested: an advanced reflective collector deployed in a conventional window to reduce glare near the window while casting the daylight deeper into the space.

Energy for cooling both commercial and residential buildings is becoming a larger portion of the energy demand as the US population in southern regions continues to increase. Developing countries in south Asia and large regions of China are witnessing a similar trend in energy use. Although considerable work, especially in Europe, has identified means to reduce building energy use in cold climates, similar research in warm and humid climates is more challenging.

Shading and spectrally selective glazing can reduce solar heating of building interiors. Highly reflecting roofs, so-called cool roofs, can also reduce heat transfer to interiors; that approach is especially useful for single-story buildings with flat roofs. And novel integration of heat pumps and air conditioners in the buildings can meet the remaining cooling loads while yielding an overall energy savings. Air conditioning a space typically consists of two tasks: dehumidification and cooling. Dehumidification requires cooling below the dew point; if it can be separated from air-space cooling, which can be performed at more moderate temperatures, it can lead to higher air-conditioner efficiency. For example, systems using liquid-cooled radiant panels along with intelligent controls and variable-speed compressors are projected to reduce overall energy requirements for air conditioning by a factor of 3 or more. ¹¹ Research to reduce irreversibilities in evaporators and condensers has the potential to produce important efficiency improvements in air conditioners and heat pumps. In addition, new dehumidification technologies, such as the use of desiccants, may be integrated into cooling-system improvements.

Natural ventilation can reduce the seasonal energy requirements for cooling commercial buildings by 50% or more in many US and European climates. Moreover, preliminary research has indicated an improvement in human comfort and satisfaction with that technology when compared to a sealed-up mechanically cooled space. Figure 4 shows an office building in Copenhagen that is cooled solely by natural ventilation. Natural ventilation requires a facade with windows whose opening can be controlled, and careful interior design to ensure adequate air flow throughout. It is challenging to design a naturally ventilated building that works well and can be controlled effectively under the wide range of real-world conditions. Successful design requires a detailed understanding of fluid dynamics and thermal conditions through a large interconnected space subject to a variety of thermal loads and wind conditions.

Some early experimental work gave a preview of the complications to be encountered. ¹² The simplest possible steady-state experiment was constructed with heated and cooled end walls surrounded by insulated side walls, ceiling, and floor. Clockwise circulation of air throughout the room was confidently expected, with warmer air flowing along the ceiling from the hot wall to the cold wall and cooler air returning along the floor. Close to the ceiling, such a flow was observed. But a reverse flow was observed away from the ceiling and floor (figure 6(a)) due to flow separation in the outer portions of the boundary layers. Figure 6(b) shows a similar reverse flow observed in a naturally ventilated multistory building with open floor plans and a central atrium. With a careful analysis of possible flows, the building’s window geometry and controls can be designed to provide comfortable conditions throughout the occupied space. By maximizing natural nighttime ventilation flows to cool concrete floors and other interior thermal masses, the interior air temperatures throughout the day can be maintained well below the peak outdoor ambient air temperatures observed during the afternoon.

The influence of indoor air quality and daylighting on health and productivity is just beginning to be understood. Preliminary research has indicated that productivity gains can be achieved with such green techniques. Students in classrooms with enhanced daylighting have achieved higher test scores than those in similar classes in conventional classrooms. ¹³ And employees in buildings with higher outdoor ventilation have lower rates of absenteeism. ¹⁴

Barriers

In many instances, energy-efficient actions with only a one-or two-year payback period have gone begging. There are a number of barriers to their adoption. The building industry is very conservative. It is widely fragmented with a huge number of suppliers, builders, designers, and developers, and it lacks vertical or horizontal integration. Many buildings are still constructed with minimized up-front costs, since in most cases the developer is divorced from the occupant who pays the operating costs. For the occupant, energy costs are an order of magnitude lower than the rent or mortgage costs and two orders less than the salary costs. Education on the potential and merits of energy efficiency is lacking, as are adequate good data from real projects to solidify the projected savings.

But there are signs of growing awareness. Some major retailers have begun ambitious pilot programs to save energy in their stores. In several new buildings, including the New York Times building, the Chesapeake Bay Foundation headquarters in Maryland, and the new federal government building in San Francisco, an integrated process of design and development has produced much greater energy efficiency while also providing a more healthy, comfortable, and productive workplace.

Energy efficiency, when properly applied, can substantially reduce energy use and CO₂ creation in the near term at modest life-cycle costs. Indeed, in many instances efficiency investments will more than pay for themselves over their lifetime. The key is an integrated design process that brings the relevant technologies together into an effective whole.

Interest and activity in building-sector efficiency have been increasing at the governmental level: The state of California and the cities of Austin, Texas, and Berlin, Germany, are good examples. ¹⁵ Private corporations are becoming more active as well. However, efforts to date fall far short of reasonable goals such as those proposed by the American Institute of Architects or the California Energy Commission.

An aggressive efficiency program in the building sector would forestall a substantial portion of the demand for new power stations and give more time to develop environmentally friendly energy-supply concepts. Not only are there a host of opportunities to apply today’s efficiency knowledge, but advanced technologies—drawing on basic research by scientists and engineers in solid-state lighting, thermodynamics, turbulent flows, and nanotechnology—could allow us to economically reduce building energy demand by far more than 50% from today’s level. To accelerate those developments, governmental and industrial R&D efforts need to be substantially expanded, which would have the added benefit of enhancing the educational opportunities for the next generation of leaders in the building-efficiency field.