In 2008, The New York Times reported 56% of the energy generated in the United States was wasted. In electricity generation, 66% was lost as heat out the smoke stacks of remote power plants and another ten percent lost during transmission. Of transportation energy, 71% was lost from heavy, idling vehicles and cars carrying only a driver. Meaning, in a time when fossil fuel resources are declining around the world, over half the 100 Quadrillion BTU’s generated in the US consumed fuel without doing any work.
The first step to eliminating such enormous waste is of course compact, car-free, 3D eco-cities composed of mid-rise, mixed use buildings that incorporate urban farming and local agriculture. Cities themselves save energy by shortening transportation and energy transmission distances, trading horizontal movement snarled in traffic for moving people vertically in elevators. In New York City, for example, 75% of families do not own automobiles. Eco-cities take this a step further, seeking balance between the density necessary to walk or cycle to home, work, school, cultural attractions and entertainment and open green spaces for aesthetic beauty and urban farming to lure people away from sprawling suburbs by making cities highly desirable places to live.
There is no shortage of speculation about how these eco-cities will get their energy, from solar panels and wind turbines to hydrogen fuel cells and biofuels. Unfortunately, there are much greater fundamental energy needs than electricity and transportation fuel within cities in northern latitudes. Such as heating in cold winter months, cooking fuel to reconstitute stored grains and pulses, waste disposal and wastewater treatment to avoid odors and disease, and most important of all, fertilizer to grow food after inorganic fertilizers made from fossil fuels are no longer available. Needs solar panels and wind turbines are of no use for. To maximize the smaller amounts of energy available in the post-carbon future, eco-cities will need to carry out as many of these functions simultaneously. Working together cooperatively allows us to turn the laws of thermodynamics from an enemy into an ally. The first step towards such an urban infrastructure of the future is trading large, remote power plants for local combined heat and power plants that are small enough and quiet enough to be located close to residences to utilize waste heat for district heating.
Combined heat and power can be explained using the example of an automobile engine. Fuel is burned in the engine cylinders and somewhere around 30% of the energy in the fuel is turned into work moving the pistons, while the remaining 70% is lost as heat out the exhaust pipe or carried away by the car’s cooling system. The coolant carries this waste heat to the car’s radiator where it is dissipated into the atmosphere and lost. A combined heat and power system takes this waste heat from stationary electric generators and steam boilers and pipes it to where it can be used for heating and hot water in near-by homes and buildings, simultaneously generating electricity and heat. New York City, for example, heats over 100,000 buildings with seven local combined heat and power plants. Combined heat and power can potentially increase a generator’s efficiency from 30% to 90% or even 100%.
What can fuel these combined heat and power plants without fossil carbon? Energy can be generated at the same time waste is treated and recycled in waste-to-energy processes like anaerobic digestion and thermal gasification. These two processes complement one another and work together with the seasonal availability of organic wastes. Biogas is generated from wet, high-water content waste such as sewage sludge and fresh garden and kitchen scraps and gasification works best with low-water content wastes such as straw, wood and pellets made from annually-renewable dry waste.
Biogas is an excellent, clean-burning gas made up mostly of methane, the same flammable component in fossil natural gas. Like natural gas, biogas can be used for cooking, fueling generators and boilers and even vehicle fuel. Where natural gas contains 1,000 BTU per cubic foot, biogas has around 600 BTU. A difference that is scarcely noticeable when cooking side-by-side with them. Only the biogas process, called anaerobic digestion, has a number of important advantages over even fossil fuels. It is a highly effective wastewater treatment process that can kill 99.99% of harmful pathogens and parasites and the by-product of the process is a high-quality, nitrogen-rich fertilizer and soil amendment that can replace inorganic fertilizers made with fossil fuels. Nitrogen is the limiting factor of human life on earth, as most plants we all need to live need it to grow. Where nitrogen in ordinary compost evaporates, the anaerobic digestion process not only retains nitrogen, but converts it into ammonia that is more readily absorbed by plants.
Gasification is a thermal process similar to burning wood or pellets in a wood stove, only the hydrogen gas that normally escapes out the chimney is captured and utilized to fuel engines or boilers. Gasification emissions do not impact air quality like direct burning does, as particulates must be filtered to separate the syngas, allowing gasification to be used in the densest urban environments. Syngas is a poorer quality energy source than biogas, only around 230 BTU per cubic foot, and the by-product is largely unusable ash, however, far more gas can be produced per pound of dry waste and some organic matter, such as the lignin in wood, cannot be anaerobically digested. Gasification generates heat that can be captured to feed district heating systems during both the making of and the burning of syngas in engines and boilers, making it an ideal combined heat and power fuel.
Integrated waste-to-energy combined heat and power systems can meet essential needs for food, clean water, public health and sufficient electrification for elevators and public transportation in a well-planned eco-city and provide a modest amount of energy for local light industry. When considering how much energy would be available, it would be misleading to look at total city energy use as planners do with today’s centralized utilities. Instead, it would be more helpful to examine how much energy you and your family could produce yourselves with a small, household anaerobic digester and gasifier and home combined heat and power system. In summer months when fresh organic waste was plentiful, only the biogas would be needed to provide cooking fuel and some electricity. Kitchen and garden waste together with bathroom waste from a family of 4-5 people could generate 70 cubic feet of biogas per day. A volume sufficient to cook 3 meals per day for about 10 people or run a 1 kW electric generator at full load for 3 hours or power a 55-inch LCD flatscreen television for 15 hours.
There are many variables that can affect home heating requirements, such as insulation value and climate, however, if we use for example a well-insulated, energy-efficient home in an eco-city located where heating was needed for 10 hours a day for 200 days of the year. This might require 150,000 BTU’s of heat a day, which could be derived from 30 lbs. of wood or 17 lbs. of pellets. The home’s annual heating needs could be met with 3-tons---about 1-1/4 cords---of wood or 1-1/2 tons of pellets. As heating and hot water needs were being met by the gasifier, sufficient syngas would be produced to run a 1 kW generator for another 10 hours in addition to the electricity provided by the biogas. Enough additional electricity to run another five 55-inch LCD flatscreen TVs for five hours and dry a load of clothes in a 5,000 watt clothes dryer.
Such integrated waste-to-energy combined heat and power systems could of course further increase efficiency by working cooperatively throughout an entire building or neighborhood-scale to maximize fuel and share surplus heat and electricity where it is needed, although, such larger systems generally employ steam turbines instead of internal combustion engines, which are far quieter and require less maintenance. The most important consideration is to keep these systems small enough to supply energy on-demand and to be economically shut down at night or in warm summer months when not needed.
Simply transitioning from remote coal and natural gas power plants to local combined heat and power could cut the 60 Quads used for heating and electricity by two-thirds and switching from suburban sprawl to eco-cities served by public transportation could cut the 40 Quads used in transportation by three-quarters. Reducing today’s 100 Quads to around 30. Decentralized waste-to-energy makes this figure highly realistic, but more importantly, this energy would be indefinitely sustainable irregardless of population growth. It is interesting to note the use of both biogas and gasification predates the Industrial Age; the most recent component in the infrastructure of the distant future is the internal combustion engine, perfected in the 1880’s. Meaning humanity could already be enjoying a richer, fuller quality of life in beautiful, completely carbon-free and ecologically sustainable eco-cities and could have been doing so for at least the past 100 years.
Warren Weisman is an internationally recognized expert on anaerobic digestion and biogas and an expert on the use of historical machines and processes in the post-carbon future. He has over 20 years experience as a builder and has worked as a mechanic and power plant operator.