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In any discussion of energy use—whether renewable or nonrenewable— energy conservation and increased energy efficiency rank among the most crucial factors to consider. We begin our discussion of renewable energy with a look at conservation and efficiency. We will then explore the range of renewable energy resources that are available.

Learning Objectives

After reading this module you should be able to

A truly sustainable approach to energy use must incorporate both energy conservation and energy efficiency. Conservation and efficiency efforts save energy that can then be used later, just as you might save money in a bank account to use later when the need arises. In this sense, conservation and efficiency are sustainable energy “sources.”

Energy conservation and energy efficiency are the least expensive and most environmentally sound options for maximizing our energy resources. In many cases, they are also the easiest approaches to implement because they require fairly simple changes to existing systems rather than a switch to a completely new technology. In this section we will examine ways to achieve both objectives.

Energy Conservation and Efficiency

Energy conservation means finding and implementing ways to use less energy. As we saw in Chapters 2 and 12, increasing energy efficiency means obtaining the same work from a smaller amount of energy. Energy conservation and energy efficiency are closely linked. One can conserve energy by not using an electrical appliance; doing so results in less energy consumption. But one can also conserve energy by using a more efficient appliance—one that does the same work but uses less energy.

Conservation

FIGURE 37.1 lists some of the ways that an individual might conserve energy, including lowering the household thermostat during cold months, consolidating errands in order to drive fewer miles, or turning off a computer when it is not being used. On a larger scale, a government might implement energy conservation measures that encourage or even require individuals to adopt strategies or habits that use less energy. One such top-down approach is to improve the availability of public transportation. Governments can also facilitate energy conservation by taxing electricity, oil, and natural gas, since higher taxes discourage use. Alternatively, governments might offer rebates or tax credits for retrofitting a home or business so it will operate on less energy. Some electric companies bill customers with a tiered rate system in which customers pay a low rate for the first increment of electricity they use and pay higher rates as use goes up. All of these practices encourage people to reduce the amount of energy they use.

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As we saw in Chapter 12, the demand for energy varies with time of day, season, and weather. When electricity-generating plants are unable to handle the demand during high-use periods, brownouts or blackouts may occur. To avoid this problem, electric companies must be able to provide enough energy to satisfy peak demand, the greatest quantity of energy used at any one time. Peak demand may be several times the overall average demand, which means that substantially more energy must be available than is needed under average conditions. To meet peak demand for electricity, electric companies often keep backup sources of energy available—primarily fossil fuel–fired generators.

Therefore, an important aspect of energy conservation is the reduction of peak demand, which would make it less likely that electric companies will have to build excess generating capacity that is used only sporadically. One way of reducing peak demand is to set up a variable price structure under which customers pay less to use electricity when demand is lowest (typically in the middle of the night and on weekends) and more when demand is highest. This approach helps even out the use of electricity, which both reduces the burden on the generating capacity of the utility and rewards the electric consumer at the same time.

The second law of thermodynamics tells us that whenever energy is converted from one form into another, some energy is lost as unusable heat. In a typical thermal fossil fuel or nuclear power plant, only about one-third of the energy consumed goes to its intended purpose; the rest is lost during energy conversions. We need to consider these losses in order to fully account for all energy conservation savings. So, the amount of energy we save is the sum of both the energy we did not use together with the energy that would have been lost in converting that energy into the form in which we would have used it. For example, if we can reduce our electricity use by 100 kWh, we may actually be conserving 300 kWh of an energy resource such as coal, since we save both the 100 kWh that we decide not to use and the 200 kWh that would have been lost during the conversion process to make the 100 kWh available to us.

Efficiency

Modern changes in electric lighting are a good example of how steadily increasing energy efficiency results in overall energy conservation. Compact fluorescent light bulbs use one-fourth as much energy to provide the same amount of light as incandescent bulbs. LED (light-emitting diode) light bulbs are even more efficient; they use one-sixth as much energy as incandescent bulbs. Over time, the widespread adoption of these efficient bulbs will result in substantially less energy used to provide lighting.

Another way in which consumers can increase energy efficiency is by switching to products that meet the efficiency standards of the Energy Star program set by the U.S. Environmental Protection Agency. For example, an Energy Star air conditioner may use 0.2 kilowatt-hours (kWh, or 200 watt-hours) less electricity per hour than a non–Energy Star unit. In terms of cost, a single consumer may save only 2 to 5 cents per hour by switching to an Energy Star unit. However, if 100,000 households in a city switched to Energy Star air conditioners, the city would reduce its energy use by 20 MW, or 4 percent of the output of a typical power plant. “Do the Math: Energy Star” on page 436 shows you how to calculate Energy Star savings.

Sustainable Design

Sustainable design can improve the efficiency of the buildings and communities in which we live and work. FIGURE 37.2 shows some key features of sustainable design applied to a single-family dwelling. Insulating foundation walls and basement floors, orienting a house properly in relation to the Sun, and planting shade trees in warm climates are all appropriate design features. As we saw in Chapter 10, good community planning also conserves energy. Building houses close to where residents work reduces reliance on fossil fuels used for transportation, which in turn reduces the amount of pollution and carbon dioxide released into the atmosphere.

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Buildings consume a great deal of energy for cooling, heating, and lighting. Many sustainable building strategies rely on passive solar design, a construction technique designed to take advantage of solar radiation without the use of active technology. FIGURE 37.3 illustrates key features of passive solar design. Passive solar design stabilizes indoor temperatures without the need for pumps or other mechanical devices. For example, in the Northern Hemisphere, constructing a house with windows along a south-facing wall allows the Sun’s rays to penetrate and warm the house, especially in winter when the Sun is more prominent in the southern sky. Double-paned windows insulate while still allowing incoming solar radiation to warm the house. Carefully placed windows also allow the maximum amount of light into a building and reduce the need for artificial lighting. Dark materials on the roof or exterior walls of a building absorb more solar energy than light-colored materials, further warming the structure. Conversely, using light-colored materials on a roof reflects heat away from the building, which keeps it cooler. In summer, when the Sun is high in the sky for much of the day, an overhanging roof helps block out sunlight during the hottest period, which makes the indoor temperature cooler and reduces the need for ventilation fans or air conditioning. Window shades can also reduce solar energy entering the house.

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To reduce demand for heating at night and for cooling during the day, builders can use construction materials that have high thermal mass. Thermal mass is a property of a building material that allows it to retain heat or cold. Materials with high thermal mass stay hot once they have been heated and cool once they have been cooled. Stone and concrete have high thermal mass, whereas wood and glass do not; think of how a cement sidewalk stays warm longer than a wooden boardwalk after a hot day. A south-facing room with stone walls and a stone floor will heat up on sunny winter days and retain that heat long after the Sun has set.

Although building a house into the side of a hill or roofing a building with soil and plants are less-common approaches, these measures also provide insulation and reduce the need for both heating and cooling. While “green roofs”—roofs with soil and growing plants—are somewhat unusual in the United States, many European cities, such as Berlin, have them on new or rebuilt structures. They are especially common on high-rise buildings in downtown areas that have little natural plant cover. These green roofs cool and shade the buildings and the surrounding environment. And the addition of plants to an urban environment also improves overall air quality.

The use of recycled building materials is another method of energy conservation. Recycling reduces the need for new construction materials, which reduces the amount of energy required to produce the components of the building. For example, many buildings now use recycled denim insulation in the walls and ceilings, and fly ash (a byproduct recovered from coal-fired power plants) in the foundation.

Many homes constructed today incorporate some or all of these sustainable design strategies, but it is possible to achieve energy efficiency even in very large buildings. The building that houses the California Academy of Sciences in Golden Gate Park in San Francisco is a showcase for several of these sustainable design techniques (FIGURE 37.4). This structure, which incorporates a combination of passive solar design, radiant heating, solar panels, and skylights, actually uses 30 percent less energy than the amount permitted under national building energy requirements. Natural light fills 90 percent of the office space and many of the public areas. Windows, blinds, and skylights open as needed to allow air to circulate, capturing the ocean breezes and ventilating the building. Recycled denim insulation in the walls and a soil-covered rooftop garden provide insulation that reduces heating and cooling costs. As an added benefit, the living green roof grows native plants and captures 13.6 million liters (3.6 million gallons) of rainwater per year, which is then used to recharge groundwater stores.

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In addition to these passive techniques, the designers of this building incorporated active technologies that further reduce its use of energy. An efficient radiant heating system carries warm water through tubes embedded in the concrete floor, using a fraction of the energy required by a standard forced-air heating system. To produce some of the electricity used in the building directly, the designers added 60,000 photovoltaic solar cells to the roof. These solar panels convert energy from the Sun into 213,000 kWh of electricity per year and reduce greenhouse gas emissions by about 200 metric tons per year.

The California Academy of Sciences took an innovative approach to meeting its energy needs through a combination of energy efficiency and use of renewable energy resources. However, many, if not all, of these approaches will have to become commonplace if we are going to use energy in a sustainable way.

As fossil fuels become less available and more expensive, what will take their place? Probably it will be a mix of energy efficiency strategies, energy conservation, and new energy sources. In the rest of this chapter we will explore our renewable energy options.

In Chapter 12 we learned that conventional energy resources, such as petroleum, natural gas, coal, and uranium ore, are nonrenewable. From a systems analysis perspective, fossil fuels constitute an energy reservoir we are depleting much faster than it can ever be replenished. Similarly, we have a finite amount of uranium ore available to use as fuel in nuclear reactors.

In contrast, some other sources of energy can be regenerated rapidly. Biomass energy resources are potentially renewable because those resources can be regenerated indefinitely as long as we do not consume them more quickly than they can be replenished. There are still other energy resources that cannot be depleted no matter how much we use them. Solar, wind, geothermal, hydroelectric, and tidal energy are essentially nondepletable in the span of human time; no matter how much we use there will always be more. The amount of a nondepletable resource available tomorrow does not depend on how much we use today. In this book we refer to potentially renewable and nondepletable energy resources together as renewable energy resources. FIGURE 37.5 illustrates the categories of energy resources.

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Many renewable energy resources have been used by humans for thousands of years. In fact, before humans began using fossil fuels, the only available energy sources were wood and plants, animal manure, and fish or animal oils. Today, in parts of the developing world where there is little access to fossil fuels, people still rely on local biomass energy sources such as manure and wood for cooking and heating—sometimes to such an extent that they overuse the resource. For example, according to the U.S. Energy Information Administration, biomass is currently the source of 86 percent of the energy consumed in sub-Saharan Africa (excluding South Africa) and much of it is not harvested sustainably.

As FIGURE 37.6 shows, renewable energy resources account for approximately 13 percent of the energy used worldwide, most of which is in the form of biomass. In the United States, which depends heavily on fossil fuels, renewable energy resources provide only about 7 percent of the energy used. That 7 percent, shown in detail in FIGURE 37.7, comes primarily from biomass and hydroelectricity.