The Energy System

Compiled by:
Maurice Skelton (seecon international gmbh)
Adapted from:
IEA (2012)

Executive Summary

Energy is all around us; we make use of it, harness it, change it – it is an aspect of everyday life. This factsheet covers three distinct domain of the energy system – the different energy forms, sources and carriers, the global energy system, and re-use options in the Resource Recovery and Safe Reuse (RRR) sector. After introducing the physical properties of energy, it lists the energy forms used and transformed by humans, and distinguishes between renewable and non-renewable energy sources as well as energy carriers. This difference is important for a good understanding of the energy system, to get a better grip of energy recovery and reuse options. A chapter on the current global energy consumption shows the global energy system, and highlights the need for change in this unsustainable system – it is still based mainly on fossil fuels. Finally, the factsheets describes re-use options in the SSWM sector.

Physical Properties of Energy

Energy is the property of matter and radiation, which manifests itself as a capacity to perform work (such as causing motion or the interaction of molecules). Power can be derived from the utilisation of physical or chemical energy sources (OXFORD DICTIONARY 2014).

It follows that energy cannot be destroyed, but only can be transformed between different forms of energy via work. Energy is always bound to mass.

The different energy forms are: gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, and mass energy. Energy can be transformed from one form into the other at various clearly mathematically stated efficiencies via work and heat.

The second law of thermodynamics states, that heat energy as such cannot be converted into work with no additional input, it is a non-reversible transformation. Heat will – up to a point – always be lost in the process of transformation. It is however possible to re-use waste heat where humans use other sources of energy to produce heat – e.g. for heating.

Energy Forms Used & Transformed by Humans Commonly

From the various energy forms, humans extract and use mainly the following forms:

Kinetic energy / Energy contained in movement

  • The flow of water: e.g. hydropower plants and tidal energy
  • The flow of air: windmills

Heat energy

  • Heat as a by-product of work (waste heat): factories, machines
  • Heat from natural hot springs: geothermal

Electric Energy

  • All in all electricity has to be produced from other forms of energy (transformation of energy).

Chemical energy:

  • Oil, coal and gas
  • Organic material from plants: sugars, fats, lipids, amino acids
  • Hydrogen

Radiant energy:

  • Solar radiation from the sun: solar-thermal power plants, for electricity production, hydrogen generation, heat collection

Nuclear Energy

  • Nuclear radiation as harnessed in nuclear power plants

Renewable Energy Sources

Flow of Water

The movement of water can be harnessed by letting the flow turn a turbine, which then produces electricity and heat. Examples of such power plants are small scale and large scale hydropower plants. Tidal movements of water can also be harnessed for electricity generation.

Flow of Wind

The movement of air can turn a turbine inside a windmill, the wind acts as a carrier of energy. The turbine converts the kinetic energy into electrical energy and heat.

Geothermal Sources

The earth’s crust contains vast amount of thermal energy. Geothermal power plants can harness this energy for heat collection and electricity generation.


Solar radiation is the main source for natural energy transformation. It can be harnessed by plants, algae and various microorganisms and be transformed into chemical energy. Animals and many microorganisms can only live because they eat the chemical energy produced by plants. The plants natural powerhouse is the chlorophyll. Biofuel can be made from certain crops, harnessing essentially the power of the sun.

The sun also powers the global water cycle which in turn can be harnessed indirectly by small- and large-scale hydropower plants.

Humans can nowadays transform solar radiation indirectly by machines, e.g. solar panels or solar-thermal power plants, into electricity, heat, and hydrogen.

Non-Renewable Energy Sources

Oil, Coal and Gas

Oil, coal and gas are classified non-renewable as their creation took place over several million years. Even though these so-called fossil fuels have got their drawbacks – in the transformation of it carbon dioxide (CO2) is released into the atmosphere – it still comprises the most important energy source world-wide, accounting for more than 80% of global energy consumption.

Nuclear Energy

Nuclear power stations harness nuclear radiation and transform it into electricity, a process called nuclear fission. Most of the energy (approximately two thirds) is lost as heat during this process (EURELECTRIC 2003).

Energy Distribution & Energy Carriers

Energy is always linked to mass, and vice versa: An object has energy from its sheer existence. Einstein was the first to find this in in the famous formula E = mc2, energy equals mass times the speed of light squared.

Now we have established that energy can be transformed and is conserved in various sources. One of the main challenges in energy management is the distribution of energy to the places where it’s needed. Wind may not be abundant in a city, and hydropower plants may well be in areas not densely populated. Heat generally is let out into the air or the water. Sun may shine in places where it cannot be directly used and has to be conserved in an energy carrier fit for transport. Modern energy sources are hardly local (BURN 2012). Several methods have been developed which can conserve and / or transport energy easily.


Electricity is one single product: an energy carrier that is generally distributed through a grid from generators to final users. Today there is no great way to store large amounts of electricity. Almost all has to be produced within moments of when it’s consumed (BURN 2012). Electricity however can power many devices, and the transformation into mechanical energy for engines is highly efficient with losses smaller than 25%. Combustion engines (such as cars use) only transform between 30% and 35% into mechanical energy, the rest is lost as waste heat (ANSWERS 2014).

Petrol, Diesel, Kerosene and Bio-Fuels

These liquid fuels are flexible energy storage carriers which can transform their chemical energy into mechanical energy, e.g. in a combustion engine, and heat. Petrol, Diesel and kerosene are derived from crude oil. The advantages are its storage ability, which makes it useful for transportation. Distribution takes place through ships, lorries, and pipes.

Many fossil fuels are also used to produce electricity, as is the case in co-generation, the combined production of heat and power.

Natural Gas, Bio-Gas and Liquefied Petroleum Gas (LPG)

These gaseous fuels are chemical energy storage carriers; they can be burnt to produce heat and mechanical energy. Gas can be stored in adequate containers and is distributed through pipes through a local grid or in smaller pressure containers to the end consumer.

Solid Fuels

The feedstock from biomass sources (forests, agricultural and livestock residues, short-rotation plantations, dedicated herbaceous energy crops, the organic component of municipal solid waste (MSW), and other organic waste streams) can be processed into solid fuels, such as chips, pellets, briquettes, logs.

(Waste) Heat

Heat and waste heat can be used for heating factories and housing using heat pumps and co-generation, the combined production of heat and power. Community or district heating can replace the on-site burning of fossil fuels, re-using waste heat. Transporting heat a long way is not economically viable, community heating is therefore restricted locally. It is distributed through adequate pipes forming a grid.


Hydrogen is a flexible energy carrier with potential applications across all end-use sectors. It can be produced from various conventional and renewable energy sources, including natural gas, coal, biomass, and non-renewable and renewable electricity. It is one of only a few near-zero-emission energy carriers (along with electricity and biofuels) and should be carefully considered as part of a global decarbonisation strategy. Hydrogen can be stored and can be used for future demand. Hydrogen as an energy carrier is still a niche player.

Global Energy Consumption [Adapted from IEA 2012b]

The current energy system is dominated by large, centralised generation based mainly on fossil fuels – oil, coal and gas – in all sectors. The global economy runs on energy: virtually all goods and services require an input of energy. As consumer demand for more goods and services grows, energy demand also increases. Continuing to supply energy by today’s means is unsustainable: surging demand will translate into higher energy prices and aggravated energy security concerns, and experts predict the resulting greenhouse-gas (GHG) emissions (including CO2 emissions) would increase average global temperatures by 6°C in the long term.


Figure 1: The Global Energy System. Complex interactions between primary energy sources and energy carriers to meet societal needs for energy services as used by the transport, buildings, industry and primary industry sectors. Re-use streams (in red) are added to the original graph. Source: Adapted from SIMS et al. (2007)

This would have disastrous impacts on the Earth and its inhabitants. The clear correlations between economic growth, energy demand, CO2 emissions and energy prices must be seen not as an insurmountable obstacle but rather as the starting point for a clean energy future. Strategic policy actions have the potential to break – and eventually reverse – past trends.

Global energy demand has nearly doubled since 1980 (Figure 2). If current trends continue unabated, it will rise another 85% by 2050. While efficiency measures have achieved some reduction in global energy intensity, the rate of improvement has slowed in recent years, which is worrisome. The virtually unbroken trend of increasing energy demand over the last 30 years has driven up energy-related CO2 emissions. As energy-related CO2 emissions make up two-thirds of total global GHG emissions, this trend must be reversed in order to address concerns over climate change and long-term energy security.

Figure 2: Total primary energy supply and CO2 emissions. Since 2003, energy demand has stabilised in OECD regions but grown rapidly in non-OECD countries, reflecting higher rates of economic development and population growth. If current trends persist, global CO2 emissions will double by 2050, resulting in a projected average temperature increase of 6°C in the long term. Source: IEA (2012a)

Heating and Cooling

Heating (and cooling) account for as much as 46% of global final energy demand, yet little progress towards decarbonisation has been made. While energy is an overarching theme of the climate change debate, in practice most of the attention focuses on electricity and transport. Few low-carbon policies explicitly address the provision of heating (or cooling).

The main uses of thermal (heating and cooling) energy span all sectors: buildings, where indoor spaces are warmed or cooled to comfort levels and water is heated for various uses; industry, where heat is used to drive industrial processes or machinery; and power, where thermal plants (fossil fuel, nuclear) transform heat into electricity.

Energy consumption to generate heat varies with the level of economic development. The highest percentages of total final energy in the form of heat are seen in Africa (71%) and Asia (60%), largely due to widespread, inefficient use of biomass for cooking and heating (Figure 3). Developing countries have a high percentage of heat as an energy source: easily accessible, low-cost energy sources are combusted inefficiently, providing minimum comfort in relatively small spaces. In developed countries, higher living standards have brought heating distribution systems to larger living areas, which allows efficient use of more valuable energy sources (e.g. gas, electricity).


Figure 3: The share of energy used for heating purposes in the emerging economies of Asia, Latin America and Africa is relatively high. Non-energy use refers to fuels used for chemical feedstocks and nonenergy products. Examples of non-energy products include lubricants, paraffin waxes, coal tars and oils as timber preservatives. Source: IEA (2012a)

The high percentage of combustible renewables in developing countries (Figure 4) reflects the use of traditional forms of biomass (e.g. wood, waste, cattle dung). While these might seem beneficial when viewed solely in light of their global warming potential, their use decreases indoor air quality and has associated health impacts. Deforestation is also a major environmental concern in many regions of the world. Traditional biomass sources are often dispersed, and much time and effort is spent, almost entirely by women, to collect firewood or adequate wastes. All of this constitutes a significant barrier to further development due to the loss in productive capability. On the whole, the continued use of traditional biomass is unsustainable in the long term.


Figure 4: Fossil fuels dominate the energy mix for providing heating services. Source: IEA (2012a)

Energy Use, Re-Use and Transformation in the SSWM sector

Energy efficiency is critical: Energy that is not consumed does not have to be produced, refined, transported or imported; and, of course, it produces no emissions. Reducing global energy consumption reduces vulnerability to all the things that might go wrong across the value chain and also contributes to achieving climate change goals.

As a general principle, re-use of energy should try harness the waste produced in the step where the waste still contains much energy, sitting on a high energy level. Energy loss can thus be reduced. An example for this would be the re-use of urine and faecal sludge, a highly concentrated chemical energy carrier, rich in nutrients. Diluting this with vast amounts of unpolluted fresh water for transport, and then re-concentrating the nutrients is a high loss of energy at first, and needs additional input of energy for the wastewater treatment.

Figure 5 is a representation of biomass resources from industry, agriculture, forestry, waste and energy supply. Through energy transformation, the energy contained in the biomass can be re-used in the various sectors, thus reducing overall energy consumption.


Figure 5: Biomass supplies originate from a wide range of sources and, after conversion in many designs of plants from domestic to industrial scales, are converted to useful forms of bioenergy. Source: SIMS et al. (2007)

Extending the Energy Flow in the SSWM sector

Harnessing Energy from Mass Flow

Drinking water as well as wastewater flows through the distribution and collection network. Many of these networks aren’t completely flat, but have got slopes in part of them. Drinking water power plants with pressure reduction turbines can produce electricity (EAWAG 2011).

Harnessing Energy from Waste Heat

Large quantities of heat are currently wasted in power stations and high-temperature industries, problems that will only increase as emerging economies continue to industrialise. This waste heat can be reused in other industrial processes, adjacent industries or nearby urban areas to provide both heating and cooling. Thermal power plants in both OECD and non-OECD countries emit large amounts of energy in the form of heat to the environment. This heat has the potential to be captured and reused economically with greater use of co-generation, or fed to energy networks to provide heat to buildings or industrial processes (Figure 6).


Figure 6: Thermal power plants in both OECD and non-OECD countries emit large amounts of energy in the form of heat to the environment. This heat has the potential to be captured and reused economically with greater use of co-generation, or fed to energy networks to provide heat to buildings or industrial processes. Source: IEA (2012a)

Higher-than-ambient-temperature wastewater, groundwater or lake water is also used for heating purposes, via heat pumps (EAWAG 2011). This recovery can happen up to about 10% of initial energy input from a technological perspective (KROISS 2014).

District heating and cooling networks are being installed more and more commonly, and are fundamental for decarbonisation. In combination with daily and seasonal storage, district networks open up opportunities beyond co-generation for other low-carbon technologies (such as heat pumps or solar heating and cooling), to participate in energy networks that interact with the electricity and transport sectors. District energy networks are an important component of smart energy networks, and allow many of the newer technologies to expand their potential.

Harnessing Energy from Biomass and Organic Waste

Biomass continues to be the world’s major source of food, stock fodder and fibre as well as a renewable resource of hydrocarbons for use as a source of heat, electricity, liquid fuels and chemicals. Woody biomass and straw can be used as materials, which can be recycled for energy at the end of their life. Biomass sources include forest, agricultural and livestock residues, short-rotation forest plantations, dedicated herbaceous energy crops, the organic component of municipal solid waste (MSW), and other organic waste streams. These are used as feedstocks to produce energy carriers in the form of solid fuels (chips, pellets, briquettes, logs), liquid fuels (methanol, ethanol, butanol, biodiesel), gaseous fuels (synthesis gas, biogas, hydrogen), electricity and heat.

A wide range of conversion technologies is under continuous development to produce bioenergy carriers for both small- and large-scale applications. Organic residues and wastes are often cost-effective feedstocks for bioenergy conversion plants, resulting in niche markets for forest, food processing and other industries. Industrial use of biomass in OECD countries was mainly in the form of black liquor in pulp mills, biogas in food processing plants, and bark, sawdust, rice husks etc. in process heat boilers.

Modern biomass combustion to produce heat is a mature technology and, in many cases, is competitive with fossil fuels. Biomass is also used in co-generation, which is more efficient than electricity or heat alone. Where the heat can be usefully employed, overall conversion efficiencies of 70% to 90% are possible.

Common feedstocks in biomass-fired co-generation plants are forestry and agricultural wastes and the biogenic component of municipal residues and wastes.


Figure 7: Overview on Sanitation Systems, with highlighted systems with re-use options for heat, biogas or electricity generation. Adapted from SPUHLER (2010).

An alternative to providing heat directly through combustion of biomass resources is to produce a biomass-derived gas. Biogas digesters can have a capacity of a few kilowatts (household size) to several megawatts in commercial agricultural biogas plants. Biogas can be burned for heat-only purposes or in co-generation plants to produce electricity. Small-scale and large-scale biogas-electricity transformation options exist. After refinement, it can also be fed into gas networks and substituted for natural gas.

Energy Recovery and Re-Use Options

References Library

ANSWERS (2014): Why is electricity the most versatile form of energy?. URL [Accessed: 27.06.2014].

BURN (2012): Getting Energy to You: How Sources Differ. URL [Accessed: 27.06.2014].

EAWAG (2011): Factsheet: Water and Energy. URL [Accessed: 27.06.2014].

EURELECTRIC (2011): Efficiency in Electricity Generation. URL [Accessed: 27.06.2014].

FEYNMAN, R.P.; LEIGHTON, R.B.; SANDS, M. (1964): The Feynman Lectures on Physics. Volume I. URL [Accessed: 26.06.2014].

IEA (2012): Energy Technology Perspectives 2012. Pathways to a Clean Energy System. Paris: International Energy Agency IEA. URL [Accessed: 27.06.2014]. PDF

KROISS (2014): Water and Energy. Presentation held at the IWA Workshop in Tokyo. Vienna: Technische Universität Wien. URL [Accessed: 27.06.2015]. PDF

OXFORD DICTIONARY (2014): Definition of Energy in English. Oxford: Oxford University Press. URL [Accessed: 27.06.2014].

SIMS, R.E.H. ; SCHOCK, R.N.; ADEGBULULGBE, A.; FENHANN, J.; KONSTANTINAVICIUTE, I.; MOOMAW, W.; NIMIR, H.B.; SCHLAMADINGER, B.; TORRES-MARTÍNEZ, J.; TURNER, C.; UCHIYAMA, Y.; VUORI, S.J.V.; WAMUKONYA, N.; ZHANG, X. (2007): Energy supply. In Climate Change 2007: Mitigation. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change . Cambridge U.K and New York: Cambridge University Press. URL [Accessed: 27.06.2014]. PDF

SPUHLER, D. (2010): Sanitation Systems. In: CONRADIN, K. (Editor); KROPAC, M. (Editor); SPUHLER, D. (Editor) (2010): SSWM Toolbox. Sustainable Sanitation and Water Management Toolbox. Basel. URL [Accessed: 27.06.2014].

Further Readings Library

Reference icon

(2014): A value proposition: Resource recovery from faecal sludge—Can it bethe driver for improved sanitation?. In: Resources, Conservation and Recycling 88, 32-38. URL [Accessed: 16.07.2014]. PDF

This scientific article describes various options for resource recovery from faecal sludge, describing five end-uses in more detail: dry sludge as fuel for combustion; biogas from anaerobic digestion of sludge; protein derived from sludge processing to be used as animal feed; dried sludge for use as a component in building materials; and treated sludge as a soil conditioner or organic fertilizer.

Reference icon

GOLD, M.; MURRAY, A.; NIWAGABA, C.; NIANG, S.; STRANDE, L. (2014): Faecal Sludge – From Waste to Solid Biofuel?. In: SANDEC NEWS 14. Dübendorf: Eawag/Sandec. URL [Accessed: 16.07.2014]. PDF

This article gives a brief summary on faecal sludge as a solid biofuel. Research in urban areas of Senegal, Ghana and Uganda proved that there are widely untapped markets for faecal sludge end-products as financial drivers to sustain reliable and safe faecal sludge management. As a fuel, it especially shows promise as an industrial energy resource and a means to generate revenue.

Reference icon

DIENER, S.; REISER, J.C.; MURRAY, A.; MBÉGUÉRÉ, M.; STRANDE, L Recovery of Industrial Waste Heat for Faecal Sludge Drying. In: SANDEC NEWS 13. Dübendorf: Eawag/Sandec. URL [Accessed: 16.07.2014]. PDF

This article is a brief summary on the enhanced use of waste heat from industries for the faecal sludge drying process.

Reference icon

DIENER, S.; REISER, J.C.; MBÉGUÉRÉ, M.; STRANDE, L. (2012): Waste heat recovery from cement production for faecal sludge dryin. Dübendorf: Eawag: Swiss Federal Institute of Aquatic Science and Technology. URL [Accessed: 16.07.2014]. PDF

This project report informs on waste heat recovery for faecal sludge drying, giving the example of a cement production factory. The conclusion is, that the combination of the two waste sources, faecal sludge and waste heat, can create a financially attractive alternative to fossil fuel in industries.