APES chapter 16

How can electric utility companies help people reduce their energy waste?

A growing number of major corporations are saving money by improving energy efficiency. For example, the CEO of Dow Chemical Company, which operates 165 manufacturing plants in 37 countries, estimates that between 1996 and 2009, energy efficiency improvements cost Dow about $1.1 billion, but resulted in savings of about $9.4 billion.

List four ways to improve energy efficiency in new buildings and eight ways to improve energy efficiency in existing buildings.

Old: -We can apply the solar energy and chemical cycling principles of sustainability by using energy more efficiently and using more renewable energy resources, reusing and recycling materials, reducing pollution and waste, and in the process, creating jobs and saving money. -White or light-colored roofs are also used to help reduce cooling costs by reflecting incoming solar radiation, especially in hotter climates -Super-insulation is very important in energy-efficient design. A house can be so heavily insulated and airtight that heat from direct sunlight, appliances, and human bodies can warm it with little or no need for a backup heating system, even in extremely cold climates Existing: -Insulate the building and plug leaks. Adding insulation and plugging air leaks are two of the quickest and least expensive ways to improve energy efficiency and save money. -Use energy-efficient windows. Energy-efficient windows can pay for themselves within several years, cut expensive heat losses from a house or other building by two-thirds, lower cooling costs in the summer, and reduce heating system emissions. -Stop other heating and cooling losses. Leaky heating and cooling ducts in attics and unheated basements allow 20-30% of a home's heating and cooling energy to escape, and those in basements can draw unwanted moisture into homes. Careful sealing of duct joints can reduce this loss and save money. Also, use of white or light-colored roofs and living roofs can cut electricity use for air conditioning and reduce emissions. -Heat houses and other buildings more efficiently. In order, the most energy-efficient ways to heat indoor space or to conserve indoor heat are: superinsulation (which would include plugging leaks); a geothermal heat pump that transfers heat stored from underground into a home; passive solar heating; a high-efficiency, conventional heat pump (in warm climates only); and a high-efficiency natural gas furnace -Heat water more efficiently. One approach is to use a roof-mounted solar hot water heater. Another option is a tankless instant water heater fired by natural gas or LPG. -Use energy-efficient appliances. -Stop using the standby mode. Consumers can reduce their energy use and their monthly power bills by plugging their standby electronic devices into a smart power strip that cuts off power to a device when it detects that the device has been turned off. -Use energy-efficient computers. According to the EPA, if all computers sold in the United States met its Energy Star requirements, consumers would save $1.8 billion a year -Use energy-efficient lighting. by shifting to energy-efficient fluorescent lights in homes, office buildings, and factories and using LEDs for all traffic lights, we could save enough electricity to close more than 700 of the world's 2,800 coal-burning power plants. -Use automatic motion sensors to turn lights on and off as needed. In 2011, the Earth Policy Institute estimated that by shifting from incandescent bulbs to LEDs and installing automatic motion sensors and dimmers, we could cut the energy used for lighting in buildings by more than 90%.

What is the key concept for this section?

We can use water flowing over dams, tidal flows, and ocean waves to generate electricity, but environmental concerns and limited availability of suitable sites may limit the use of these energy resources.

What is energy efficiency?

a measure of how much useful work we can get from each unit of energy we use.

What are the major advantages of reducing energy waste?

it is the quickest, cleanest, and usually the cheapest way to provide more energy, reduce pollution and environmental degradation, and slow projected climate change.

What percentage of the energy used in the United States is unnecessarily wasted?

roughly 84% of all commercial energy used in the United States does not produce useful work About 41% of this energy unavoidably ends up as low-quality waste heat in the environment because of the degradation of energy quality imposed by the second law of thermodynamics The other 43% is wasted unnecessarily, mostly due to the inefficiency of light bulbs, industrial motors, motor vehicles, and power plants

What are the two key concepts for this section?

-Improvements in energy efficiency could save at least a third of the energy used in the world and up to 43% of the energy used in the United States. -We have a variety of technologies for sharply increasing the energy efficiency of industrial operations, motor vehicles, appliances, and buildings.

What are fuel cells and what are their advantages?

-a device that uses hydrogen gas as a fuel to produce electricity. -Fuel cells are at least twice as efficient as internal combustion engines, have no moving parts, and require little maintenance. -fuel cells would drastically reduce our emissions of and other pollutants that cause air pollution and climate change

What are the major advantages and disadvantages of using wind to produce electricity?

Advantages: -Medium Net energy yield -little or no direct emissions of CO2 or other air pollutants -easy to install, move around, and expand as needed -competitive costs for newer cells Disadvantages: -need access to sun -some designs have low net energy yield -need electricity storage system or backup -high costs for older systems but dropping rapidly -solar-cell power plants could disrupt desert ecosystems

What is the key concept for this section?

Geothermal energy has great potential for supplying many areas with heat and electricity, and has a generally low environmental impact, but the sites where it can be produced economically are limited.

What are four ways to save energy and money in transportation?

- One way to include more of these hidden costs in the market price is through gasoline taxes, which are widely used in Europe but are politically unpopular in the United States. -give consumers significant tax breaks or other economic incentives to encourage them to buy more fuel-efficient vehicles. -Other ways to save energy and money in transportation include building or expanding mass transit systems within cities, constructing high-speed rail lines between cities (as is done in Japan, much of Europe, and China), and carrying more freight by rail instead of in heavy trucks. -Another approach is to encourage bicycle use by building bike lanes along highways and on city streets.

Describe the potential for using renewable energy from the wind to produce most of the electricity used in the United States.

-Between the earth's equator and its polar regions, the sun's rays strike the earth at different angles. This results in more solar energy at the equator and less at the poles. Together with the earth's rotation, these differences create flows of air called wind -Because wind power is an indirect form of solar energy, relying more on it is a way to apply the solar energy principle of sustainability We can capture this energy with groups of wind turbines called wind farms that convert it into electrical energy that can be fed into electrical grids. -wind power is widely distributed and inexhaustible. The cost of producing electricity from wind is much lower than that of producing it by using the nuclear fuel cycle and roughly equal to the cost of producing electricity by burning coal. -greatly reduces our emissions of climate-changing carbon dioxide into the atmosphere. they can tap into the stronger and more reliable winds found at higher altitudes on land and at sea. could generate 4 times the electricity currently used in the lower 48 states. -Texas is the nation's leading producer of electricity from wind power, followed by California. -wind power has more advantages and fewer serious disadvantages than all other energy resources except for energy efficiency

What are three ways to cool houses naturally?

-Block the high summer sun with shade trees, broad overhanging eaves, window awnings, or shades -In warm climates, use a light-colored roof to reflect as much as 90% of the sun's heat (compared to only 10-15% for a dark-colored roof), or use a green roof -Use geothermal heat pumps for cooling (and for heating in winter).

List three general conclusions that energy experts have come to in considering possible energy futures

-First, during this century, there will likely be a gradual shift from large, centralized macropower systems to smaller, decentralized micropower systems such as wind turbines, household solar-cell panels, rooftop solar water heaters, and small natural gas turbines. -The second general conclusion of experts about the future of energy use is that a combination of improved energy efficiency and carefully regulated use of natural gas will be the best way to make the transition to using mostly renewable energy resources during this century -The third general conclusion is that because fossil fuels are still abundant and artificially cheap, we will continue to use them in large quantities

Explain three strategies that governments can use to encourage or discourage the use of an energy resource.

-First, they can keep the prices of selected energy resources artificially low to encourage use of those resources. They do this by providing research and development subsidies, tax breaks, and loan guarantees to encourage the development of those resources, and by enacting regulations that favor them. -A second major strategy that governments can use is to allow the prices of selected energy resources to rise to their natural market levels by removing government subsidies, thereby discouraging the use of these resources. They can do this by eliminating existing tax breaks and other subsidies that favor the use of the targeted resource. -Third, governments can emphasize consumer education. Even if governments offer generous financial incentives for energy efficiency and renewable energy use, people will not make such investments if they are uninformed—or misinformed—about the availability, advantages, disadvantages, and hidden environmental costs of various energy resources.

List eight questions that policymakers should ask about each source of energy.

-How much of the energy resource is likely to be available in the near future (the next 25 years) and in the long term (the next 50 years)? -What is the estimated net energy yield for the resource? -What are the estimated costs for developing, phasing in, and using the resource? -What kinds of government research and development subsidies and tax breaks will be needed to help develop the resource? -How will dependence on the resource affect national and global economic and military security? -How vulnerable is the resource to terrorism? -How will extracting, transporting, and using the resource likely affect the environment, the earth's climate, and human health? -Does use of the resource produce hazardous, toxic, or radioactive substances that we must safely store for very long periods of time?

List five widely used energy-inefficient technologies.

-Huge data centers, filled with racks of electronic servers that process information flowing on the Internet, use only about 10% of the electrical energy they pull off of the grid. The other 90% ends up as low-quality heat that flows into the environment. -The internal combustion engine that propels most motor vehicles wastes about 80% of the high-quality energy in its fuel. In other words, only about 20% of the money that people spend on gasoline gets put to good use. -A nuclear power plant, which produces electricity, wastes about 75% of the high-quality energy in its nuclear fuel and probably closer to 82% when we include the additional energy used in the nuclear fuel cycle -A coal-fired power plant wastes about 65% of the energy that is released by burning coal to produce electricity, and probably 75-80% if we include the energy used to dig up the coal and transport it to the plant, and to transport and store the ash by-product.

Define hydropower and summarize the potential for expanding it.

-Hydropower is any technology that uses the kinetic energy of flowing and falling water to produce electricity. It is an indirect form of solar energy because it depends on heat from the sun evaporating water, which is deposited as rain or snow at higher elevations where it can flow to lower elevations in rivers as part of the earth's solar-powered water cycle -Hydropower, the leading renewable energy source, produces about 16% of the world's electricity in 150 countries. -According to the United Nations, only about 13% of the world's potential for hydropower has been developed.

List five advantages of relying more on a variety of renewable energy sources and list three factors that are holding back such a transition.

-In 2012, the National Renewable Energy Laboratory projected that, with a crash program, the United States could get 50% of its electricity from renewable energy sources by 2050. -Studies show that with increased and consistent government backing in the form of research and development funds and subsidies and tax breaks, renewable energy could provide 20% of the world's electricity by 2025 and 50% by 2050. -There is growing evidence that a gradual shift from an economy powered primarily by climate-disrupting nonrenewable fossil fuels to one powered by a mix of climate-stabilizing renewable energy sources is underway and will gain momentum during this century. -result in more decentralized and energy-efficient national economies that are less vulnerable to energy supply cutoffs and natural disasters. -Also, it would greatly reduce air and water pollution, slow projected climate disruption, create large numbers of jobs, and save consumers money. -First, since 1950, government tax breaks, subsidies, and funding for research and development of renewable energy resources have been much lower than those for fossil fuels (especially oil) and nuclear power, although subsidies and tax breaks for renewables have increased in recent years. -Second, although subsidies and tax breaks for fossil fuels and nuclear power have essentially been guaranteed for many decades, those for renewable energy in the United States have to be renewed by Congress every few years. The resulting financial uncertainty makes it risky for companies to invest in renewable energy. -Third, the prices we pay for nonrenewable fossil fuels and nuclear power do not include the harmful environmental and human health costs of producing and using them. This helps to shield them from free-market competition with renewable sources of energy.

What is a smart grid and why is it important?

-In the United States, many energy experts place top priority on converting and expanding the outdated regional electrical grid system into what they call a regional and national smart grid. This would be an energy-efficient, digitally controlled, ultra-high-voltage (UHV) system with superefficient transmission lines. -A smart grid could adapt to a disaster that causes a power loss in one area by quickly bringing in electricity from other parts of the country. It would also make it easier for power companies and consumers to buy electricity produced from wind, solar, and other renewable forms of energy in different parts of the country. -over the next 20 years, would save the U.S. economy up to $2 trillion -A smart grid system would use smart meters to monitor the amount of electricity used and the patterns of use for each customer. -This information would help consumers to reduce their power consumption and power bills.

What are the two key concepts for this section?

-Solid biomass is a renewable resource for much of the world's population, but burning it faster than it is replenished produces a net gain in atmospheric greenhouse gases. -We can use liquid biofuels derived from biomass to lessen our dependence on oil-based fuels, but creating biofuel plantations can degrade soil and biodiversity, increase greenhouse gas emissions, and lead to higher food prices.

What are the hidden costs of using gasoline?

-The hidden costs not included in the price include government subsidies and tax breaks for oil companies, car manufacturers, and road builders; costs of pollution control and cleanup; time wasted idling in traffic jams; and higher medical bills and health insurance premiums resulting from illnesses caused by air and water pollution. -the hidden costs of gasoline for U.S. consumers were about $3.18 per liter ($12 per gallon). Thus, when gas costs $4 a gallon, U.S. consumers are really paying about $16 a gallon.

Explain the importance of developing better batteries and list some advances in this area.

-The major obstacle standing in the way of wider use of plug-in hybrid-electric and all-electric vehicles is the lack of an affordable, small, lightweight, and easily rechargeable car battery that can store enough energy to power a vehicle over long distances. -One promising type of battery is the lithium-ion battery, commonly used in laptop computers and cell phones -Much of the current research on batteries involves nanotechnology -researchers at the Massachusetts Institute of Technology (MIT) developed a new type of lithium-ion battery using such technology. -MIT researcher Michael Strano and his colleagues developed a thermal wave battery that produces electricity when a pulse of heat travels through carbon nanotubes. -MIT researcher Angela Belcher is working on another new type of battery. She has genetically engineered a virus that can coat itself with electricity-conducting materials to form a miniscule nanowire. -Scientists have also developed ultracapacitors, which are small mechanical batteries that quickly store and release large amounts of electrical energy, thus providing the power needed for quick acceleration.

Summarize the development of more energy-efficient vehicles.

-There is growing interest in developing modern, superefficient, ultralight, and ultrastrong cars that could get up to 130 kpl (300 mpg) using existing technology -One of these vehicles is the energy-efficient, gasoline-electric hybrid car (Figure 16-6, left). -These cars have a small, traditional gasoline-powered engine and a battery-powered electric motor used to provide the energy needed for acceleration and hill climbing. The most efficient current models of these cars get a combined city/highway mileage of up to 21 kpl (50 mpg) and emit about 65% less per kilometer driven than do comparable conventional cars. -the small gasoline motor kicks in, recharges the battery, and extends the driving range to 600 kilometers (370 miles) or more. The battery can be plugged into a conventional outlet and fully charged in 6 to 8 hours. -Another option is an all-electric vehicle that runs on a battery only. The problem for the average consumer is that the prices on hybrid, plug-in hybrid, and all-electric cars are high because of the high cost of their batteries. -Fully electric cars are promoted as not contributing to climate change because, while operating, they don't emit .

List six ways in which you can save energy where you live.

-Use microwave rather than stove or oven -Repair water leaks -Run only full loads in dishwasher -Clean refrigerator coils regularly -Hang clothes on racks for drying -Regularly clean or replace furnace filters

What is the potential for using tides and waves to produce electricity?

-We can also produce electricity from flowing water by tapping into the energy from ocean tides and waves. In some coastal bays and estuaries, water levels can rise or fall by 6 meters (20 feet) or more between daily high and low tides. Dams can be built across the mouths of such bays and estuaries to capture the energy in these flows for hydropower -Several turbines, resembling underwater wind turbines, have also been installed to tap the tidal flow of the East River near New York City. scientists and engineers have also been trying to produce electricity by tapping wave energy along seacoasts where there are almost continuous waves. -Scientists estimate that learning how to tap into the world's wave power at an affordable cost could provide more than twice the amount of electricity that the world uses. -Production of electricity from tidal and wave systems is limited because of a lack of suitable sites, citizen opposition at some sites, high costs, and equipment damage from saltwater corrosion and storms.

List four ways to save energy and money in industry.

-We can use cogeneration. -could use more energy-efficient electric motors. -Typical inefficient motors use one-fourth of the electricity produced in the United States and 65% of the electricity used in U.S. industry. -They can be replaced with more energy-efficient variable-speed motors, which run at the minimum rate needed for each job. -Recycling materials such as steel and other metals is a third way for industry to save energy and money. -A fourth way is to use more energy-efficient lighting -to redesign the software and cooling systems in government and private electronic data processing systems

What are this chapter's three big ideas?

-We should evaluate energy resources on the basis of their potential supplies, their net energy yields, and the environmental and health impacts of using them. -By using a mix of renewable energy sources—especially solar, wind, flowing water, sustainable biofuels, and geothermal energy—we could drastically reduce pollution, greenhouse gas emissions, and biodiversity losses. -Making the transition to a more sustainable energy future will require sharply increasing energy efficiency, using a mix of environmentally friendly renewable energy resources, and including the harmful environmental and health costs of energy resources in their market prices.

What are some technologies applied by green architecture?

-fuel-cell technology -Along with natural lighting, direct solar heating, insulated windows, and energy-efficient appliances and lighting, it makes use of solar hot water heaters, electricity from solar cells, windows that darken automatically to deflect heat from the sun, thin sheets of aerogel insulation, and recycling of wastewater.

Give three reasons why we waste so much energy.

-in the United States and other countries, energy resources such as fossil fuels and nuclear power are artificially cheap, primarily because of the government subsidies and tax breaks they receive and because their market prices do not include the harmful environmental and health costs of their production and use. -there are too few government tax breaks, rebates, low-interest and long-term loans, and other economic incentives for consumers and businesses that invest in improving energy efficiency. -A third reason is that some governments have not put a high priority on educating the public about the environmental and economic advantages of cutting energy waste.

List five major strategies suggested by such experts for making the transition to a more sustainable energy future.

-increase fuel efficiency standards for vehicles, buildings, and appliances -reward utilities for reducing demand for electricity -greatly increase use of renewable energy -greatly increase renewable energy research and development -phase out coal subsidies and tax breaks

Summarize the global potential for wind power.

-most of the world's rapidly growing number of wind farms have been built on land in parts of Europe, China, and the United States. -wind power has been the world's second fastest-growing source of energy after solar cells, with more than 80 countries now harnessing energy from the wind. -Wind power produced about 3.5% of the world's electricity in 2011 and by 2050, could produce about 31% of the world's electricity -Denmark, the world's most energy-efficient country, got 30% of its electricity from wind in 2012 and is aiming for 50% by 2020. Around the world, more than 400,000 people are employed in the production, installation, and maintenance of wind turbines

What is a solar cell (photovoltaic or PV cell) and what are the major advantages and disadvantages of using such devices to produce electricity?

-photovoltaic (PV) cells-Device that converts radiant (solar) energy directly into electrical energy. Also called a solar cell. Advantages: -Medium Net energy yield -little or no direct emissions of CO2 or other air pollutants -easy to install, move around, and expand as needed -competitive cost for newer cells Disadvantages: -need access to sun -some designs have low net energy yields -need electricity storage system or backup -high costs for older systems but dropping rapidly -solar-cell power plants could disrupt desert ecosystems

Explain how we would be applying the three principles of sustainability by improving energy efficiency and shifting to renewable energy resources.

-relying much more on direct and indirect forms of solar energy for our electricity, heating and cooling, and other needs; -recycling and reusing more materials and thus reducing wasteful and excessive consumption of energy and matter; and -mimicking nature's reliance on biodiversity by using a diverse mix of locally and regionally available renewable energy resources.

Explain how algae and bacteria can be used to produce fuels nearly identical to gasoline and diesel fuel.

-the algae remove from the atmosphere and convert it to oil, proteins, and other useful products. -They also require much less land, water, and other resources than biofuel plantations do and would not affect food prices by competing for cropland. -In addition, the algae could be grown in wastewater from sewage treatment plants where they would help to clean up the wastewater while producing biofuel. -Another possibility would be to transfer produced by coal-burning power plants into nearby algae ponds or bioreactors for use in making biofuel. -A different approach is to make gasoline or diesel fuel from rapidly multiplying bacteria by using techniques developed in the new field of synthetic biology. -Producing gasoline and diesel fuels from algae and bacteria could be done almost anywhere. -The resulting fuels could be distributed by the world's current gasoline and diesel fuel distribution system.

What are the advantages of using taller wind turbines?

-they can extract more energy from the wind more efficiently than smaller turbines nearer the ground can. -wind farms with these giant turbines can generate more energy with fewer turbines and a lower land and ecological footprint than those with the smaller turbines, and this will help to lower the price of wind energy.

What are the major advantages and disadvantages of using hydrogen as a fuel to use in producing electricity and powering vehicles?

Advantages: -can be produced from plentiful water at some sights -no CO2 emissions if produced with use of renewables -good substitute for oil -high efficiency in fuel cells Disadvantages: -negative net energy yield -CO2 emissions if produced from carbon-containing compounds -high costs create needs for subsidies -Needs H2 storage and distribution system

What are the major advantages and disadvantages of using hydropower?

Advantages: -high net energy yield -large untapped potential -low-cost electricity -low emissions of CO2 and other air pollutants in temperate areas Disadvantages: -large land disturbance and displacement of people -High CH4 emissions from rapid biomass decay in shallow tropical reservoirs -disrupts downstream aquatic ecosystems

What are the major advantages and disadvantages of using geothermal energy as a source of heat and to produce electricity?

Advantages: -medium net energy yield and high efficiency at accessible sights -lower CO2 emissions than fossil fuels -low cost at favorable sights Disadvantages: -high cost except at concentrated and accessible sources -scarcity at suitable sights -noise and some CO2 emissions

What are the major advantages and disadvantages of using biodiesel and ethanol to power motor vehicles?

Advantages: -reduced CO2 emissions for some crops -medium net energy yield for biodiesel from oil palms -medium net energy yield for ethanol from sugarcane Disadvantages: -fuel crops can compete with food crops for land and raise food prices -fuel crops can be invasive species -low net energy yield for corn ethanol and for biodiesel from soybeans -higher CO2 emissions from corn ethanol

What is biomass and what are the major advantages and disadvantages of using wood to provide heat and electricity?

Biomass consists of plant materials (such as wood and agricultural waste) that we can burn directly as a solid fuel or convert into gaseous or liquid biofuels. Biomass is another indirect form of solar energy because it consists of combustible organic (carbon-containing) compounds in plant matter produced mainly by photosynthesis. Advantages: -Widely available in some areas -Moderate costs -medium net energy yield -No net CO2 increase if harvested, burned, and replanted sustainably -plantations can help restore degraded lands Disadvantages: -contributes to deforestation -clear-cutting can cause soil erosion, water pollution, and loss of wildlife habitat -can open ecosystems to invasive species -increases CO2 emissions if harvested and burned unsustainably

What is geothermal energy and what are three sources of such energy?

Geothermal energy is heat stored in soil, underground rocks, and fluids in the earth's mantle. We can tap into this stored energy to heat and cool buildings and to produce electricity.

What is the key concept for this section?

Hydrogen is a clean energy source as long as it is not produced with the use of fossil fuels, but it has a negative net energy yield.

Explain why we can think of energy efficiency as an energy resource.

Improving energy efficiency means using less energy to provide the same amount of work in the forms of light, heat, transportation, and other benefits. This amounts to a largely untapped source of energy that is abundant, clean, cheap, and readily available.

What is the key concept for this section?

Passive and active solar heating systems can heat water and buildings effectively, and the costs of using direct sunlight to produce high-temperature heat and electricity are coming down.

What is cogeneration?

Production of two useful forms of energy, such as high-temperature heat or steam and electricity, from the same fuel source.

What are solar thermal systems, how are they used, and what are the major advantages and disadvantages of using them?

Solar thermal systems-System that uses any of various methods to collect and concentrate solar energy in order to boil water and produce steam for generating electricity. Advantages: -high potential for growth -no direct emissions of CO2 and other pollutants -lower costs with natural gas turbine backup -source of new jobs Disadvantages: -low net energy yield and high costs -needs backup or storage system on cloudy days -requires high water use -can disrupt desert ecosystems

Explain benefits of living roofs.

Such a roof can reduce the costs of cooling and heating a building by absorbing heat from the summer sun and helping to insulate the structure and retain heat in the winter. A living roof also absorbs precipitation, which would normally become part of a city's storm water runoff, adding to the pollution of its waterways.

What is the key concept for this section?

We can make the transition to a more sustainable energy future by greatly improving energy efficiency, using a mix of renewable energy resources, and including the environmental and health costs of energy resources in their market prices.

What is the key concept for this section?

When we include the environmental costs of using energy resources in their market prices, wind power is the least expensive and least polluting way to produce electricity.

Distinguish between a passive solar heating system and an active solar heating system and discuss the major advantages and disadvantages of using such systems for heating buildings.

passive solar heating system-System that, without the use of mechanical devices, captures sunlight directly within a structure and converts it into low-temperature heat for space heating or for heating water for domestic use. Compare active solar heating system. active solar heating system-System that uses solar collectors to capture energy from the sun and store it as heat for space heating and water heating. Liquid or air pumped through the collectors transfers the captured heat to a storage system such as an insulated water tank or rock bed. Pumps or fans then distribute the stored heat or hot water throughout a dwelling as needed. Compare passive solar heating system. Advantages: -medium net energy yield -very low emissions of CO2 and other air pollutants -very low land disturbance -moderate cost (Passive) Disadvantages: -need access of sun 60% of time during daylight -sun can be blocked by trees and other structures -high installation and maintenance costs for active systems -need backup system for cloudy days