CHAPTER 2 What is energy? What is energy analysis? Energy exists in many forms, including: • kinetic energy • potential energy • chemical energy • nuclear energy • electromagnetic radiation • electricity heat Law of thermodynamics / law of conservation of energy; • First law = states that energy can neither be created nor destroyed, but can only be converted from one form to another (or from several forms combined to one or more other forms) • Second law = The entropy of any isolated system always increases. It is needed for proper analysis of energy conversions, especially when they involve heat transfer. • Third law = the entropy of a system approaches a constant value as the temperature approaches absolute zero Energy in energy systems • Energy production, conversion, and use can never be considered in isolation • mining of coal or uranium, the extraction of oil or natural gas, or the cultivation of biomass for energy purposes • natural form, so coal, natural gas, and crude • oil, as they are extracted from the Earth’s crust are primary energy carriers. • Crude biomass, like harvested wood, is also a primary energy carrier • Major energy conversion processes include power plants that convert fossil fuels to electricity and refineries that convert crude oil to a range of products, like petrol, naphtha, kerosene, and heavy fuel oil • Energy that is the product of an energy conversion process is called secondary energy. Heat, electricity, and gasoline are important secondary energy carriers • final energy - A lot of activities in the energy system, like transport, storage, and distribution, are needed to bring energy to the end user when it is needed. • end-use conversion - Once the user has acquired the energy, it may still not be in the right form, and further conversion may be needed, for example converting fuel to heat in a boiler • The resulting energy is often called useful energy • When the energy is finally in its ultimate form, it can provide a certain service (or function) for the user • Such services include heating or lighting a room, transporting a person in a car, or making steel or clothes • Energy Supply system - The part of the energy system that converts primary energy to final energy • Energy Demand system converts final energy to end-use services is called the Units of energy • The standard unit of energy = joule (J) • One joule is one kg m2 s–2, so it is the energy needed to accelerate a mass of 1 kg from 0 to 1 metre per second • The amount of one joule is too small to work with in energy analysis • In energy analysis we generally come across processes with a minimum amount of energy input or output of 106 J per year • annual primary energy use of countries typically lies in the order of exajoules • individuals it lies in the order of gigajoules • Companies have a much wider range, which of course depends on the size and type of company • energy use generally lies in the order of terajoules or petajoules per year Power Power = energy per unit of time Measured = Watt (J s–1) • This can be the power output (e.g. a power plant that produces 600 MW of electricity) or the power input (e.g. a compact fluorescent lamp that consumes 8 W of electricity) • For equipment with a constant power input or power output, the relation between energy and power is straightforward: EP=.t where: E = energy input (or output) P = power input (or output) t = time in use • kW is a unit of power and kWh is an energy unit Load factors and load duration curves • Useful concepts in energy analysis are therefore the load factor and the load duration curve • provide links between the (instantaneous) power uptake and the (annual) energy use • load factor - is the annual output divided by the nominal capacity • load factor – or capacity factor – is defined as the annual output (or input) of a system divided by the output (or input) that would have been achieved if the system had run at the nominal capacity for the full year (8,760 hours in a non-leap year) • input limitations can have a reducing effect on the load factor, for instance in the case of wind energy systems and solar energy systems • The load factor provides a measure of the total use of the equipment, but does not show how it varies over time • energy or the production of energy varies over time • Time series are often recorded to assist in good understanding of important characteristics of • energy use and energy supply/production. • For instance, time series of office building energy use often reveal substantial energy use at night and over the weekend • time - horizontal axis • energy use that is exceeded during the given number of hours is depicted on the vertical axis What are energy carriers? • coal, oil and oil products, and natural gas • food, plastics, and metals, contain energy • The standard convention in energy analysis • is that a substance is considered an energy carrier if the substance is predominantly used as a source of energy • Wood is increasingly considered an energy carrier, as the use of biomass for energy production increases The energy content of energy carriers • energy content is how much heat it can generate • mass m, temperature T at altitude h • In energy analysis, we generally use an environmental reference system • For kinetic energy and gravitational energy, the ground level is the reference • For thermal energy the environmental temperature is the reference • For chemical energy the reference state for each chemical element contained in the energy carrier • is the most stable naturally occurring compound of that element • and for hydrogen the reference is liquid water • for fuels, the energy of combustion is determined using a reference temperature of 25 °C and standard atmospheric pressure (1 atm = 101.325 kPa). • For water and steam on the other hand, the reference is the liquid at the triple point (0.01 °C) • changes of energy are measured under constant volume, whereas changes in enthalpy are measured under constant pressure. • Many processes take place at constant pressure • in which case the change in enthalpy gives the amount of heat generated or required Higher and Lower heating value of fuels • energy content of a fuel is determined by measuring the heat generated when a fuel is combusted and the combustion products are brought back to a reference situation • Thermodynamically this is the (chemical) enthalpy difference between the fuel/oxygen input and the combustion products, both at reference conditions of temperature and pressure • One of the combustion products is water, and at reference conditions water can be in either a liquid or a gaseous form H=U+p.V where: H = enthalpy U = internal energy p = pressure V = volume • For solids and liquids, the difference between H and U is very small, but for gases the difference cannot generally be neglected • methane (CH4), which is the major component of natural gas. The combustion reaction is: • After combustion and cooling down to the reference temperature (25 °C) the chemical enthalpy has decreased with 802 kJ per mol CH4 combusted, meaning that 802 kJ of heat is released • The water vapour in the combustion gas can con dense, giving up more heat: • The difference in enthalpy change (and therefore also the amount of heat obtained) is substantial: the process of condensation that brings water from the gaseous to the liquid form releases a substantial additional amount of heat (in our case 890 – 802 = 88 kJ per mol CH4) • energy content of fuels - one based on the higher heating value (HHV) and one based on the lower heating value (LHV) • higher heating value (HHV) - the condensation heat of the water that is the product of the combustion process is included in the HHV. • For methane the example shows that the HHV of methane equals 890 kJ/mol • lower heating value (LHV) - the water remains in the gaseous form and the heat of condensation is not included • The LHV of methane is therefore 802 kJ/mol. This distinction is relevant for fuels like coal, oil products, natural gas, and biomass, which contain hydrogen in one form or another • water is formed during combustion of the fuel • Alternative terms are net calorific value (NCV) for LHV, and gross calorific value (GCV) for HHV difference between the higher heating value and the lower heating value: 1. is equal to the condensation heat (evaporation heat) of the water that is the result of combustion (the original water content of the fuel is not taken into account) 2. The higher heating value is larger than the lower heating value • The relation between the two is given by the following formula: • The higher heating value gives the best indication of the utilisation possibilities of fuels • It indicates the total amount of heat that can be generated through combustion of the fuel • The rationale for using the lower heating value is that in many cases it is not feasible to utilise the condensation heat of the gaseous form of water that is present in the combustion gases of a fuel. • Example: in modern natural-gas-fired boilers for low-temperature heat production, the exhaust gases are cooled to such a degree that a substantial part of the water in the fuel gases condenses and the associated heat can be utilised, for example for heating buildings. Such boilers are often denoted as condensing boilers The efficiency of this condensing boiler can be calculated as useful heat output divided by energy content of fuel input. On an HHV and LHV basis the results are: which shows that LHV based efficiencies can be higher than 100 per cent due to the fact that LHVs do not account for the heat of condensation. Energy use or energy consumption? • The terms energy use and energy consumption are used interchangeably • the term energy consumption is less suitable: energy may be converted to other forms, but it never really gets lost • energy is actually consumed; in general, the energy left after it has been used no longer has any economic value and can be considered as consumed Final Energy Use • Energy use can be measured at various levels, first of all for individual entities • entities are called energy users • purchased energy - What can be measured most easily is the amount of energy that enters such an entity; we call this • purchased energy is not equal to the total amount that such an entity uses • In addition to the purchased energy, an entity may extract energy itself in one form or another • Example: a household with a solar domestic hot water system, or a firm that runs a hydropower plant • An entity may also sell energy of which it has an excess: e.g. a farmer with a wind turbine may sell excess electricity to the power grid • an entity may add energy carriers to its stocks or extract energy carriers from its stocks we define final energy use: Final energy use = Purchased energy - Sold energy + Own extraction + Extraction from Stock • *stock changes are negative when energy is added to the stock - By giving the unit joule an extra subscript, one is aware of the different quality of the different types of energy - By 1 Je, we indicate one joule of electricity - The index ‘fuel’ is used for fuels like natural gas, coal, or - oil The index ‘p’ is used to indicate primary energy For heat, the index ‘th’ (thermal) is used, so 1 Jthis 1 joule of heat By giving the unit an extra index, we avoid mixing up different types of energy • An important energy conversion process is electricity generation, e.g. in the form of combined generation of heat and power (CHP) • Consequently, in these cases final energy use needs to take into account the conversions that take place in these own conversion processes Final energy use = Purchased energy - Sold energy + Own extraction +Stock extraction - Input to conversion processes + Output version processes Calculating primary energy use from final energy use • The conversion of energy can cause substantial energy losses • If an energy user uses both coal and electricity, it therefore does not make sense to simply count together the coal use and the electricity use as if they were equal • We are more interested in the primary energy use of such an energy user • But that means that the primary energy use needs to be calculated by tracing back through the energy supply chain • We assume that a final energy user uses both fuels and electricity The primary energy use can then be estimated in the following simple way: Energy balances and energy statistics • A distinction is generally made between energy balances and energy statistics. • Energy statistics contain the basic information, generally in the original, physical units (tonnes for coal, m3 for natural gas, toe for oil) for the various energy carriers, whereas energy balances give all quantities in the same unit (e.g. J or toe) • An energy balance is a matrix in which each row represents a sector, and each column rep resents an energy carrier • Each matrix element presents the amount of the energy carrier that is used (or produced) in that sector • important row in the energy balance is the one presenting the total primary energy supply (TPES), which is generally considered the indicator for a country’s total energy use. It is calculated as follows. TPES = (indigenous) production + imports exports -international bunkers + stock changes • The five components that make up the TPES are listed above the TPES row in the energy balance • nb* - stock changes are positive when energy is extracted from the stocks • Indigenous production - is the extraction of primary energy by the country, for example mining of coal • The category ‘international bunkers’ covers the amount of energy delivered to outgoing ships and aeroplanes that leave the country • This category is excluded from the national consumption figures in the same way as exports, as this consumption cannot be attributed to a specific country • Energy use by inland shipping and aircraft is included in the TPES of a country • The various energy conversion sectors (like electricity plants and refineries) are listed below the TPES row • Energy conversion sectors have energy as both an input (negative numbers) and an output (positive numbers) • The treatment of combined generation of heat and power • CHP plants produce both heat and electricity After all energy conversion sectors have been taken into account, the total final consumption (TFC) is left over; TFC = TPES - inputs to conversion processes + outputs of conversion processes • The total final consumption is also the sum of the final energy consumption in all the end-use sectors listed in the rows below the TFC row in the IEA energy balance • Non-energetic energy use - A specific category of energy demand is the so-called non-energetic use of energy carriers – i.e. the use of energy carriers in products • An important example is the use of naphtha and other refinery products as a feedstock for the production of chemicals like ethylene in the petrochemical industry • Other non-energetic uses are waxes, lubricants, bitumen and graphite electrodes • Primary nuclear energy - In IEA energy statistics and in many other cases it is assumed that the heat generated by nuclear reactions in nuclear power stations is the primary energy to be registered • Primary energy of renewables other than bioenergy - In IEA energy statistics the electricity from hydropower, wind, and solar photovoltaics (PV) is considered to be the primary energy. • primary energy from non-combustible sources like nuclear, wind, and solar energy is equivalent to the LHV of combustible fuels that would have been required in conventional thermal power plants to substitute the generated electricity or other form of secondary energy • An effciency of 35 per cent is used for electricity from non-combustible sources and an effciency of 85 per cent for heat from non-combustible sources. • The difference between IEA and GEA illustrates that with increasing amounts of renewables the amounts of primary energy calculated are becoming more and more dependent on conventions used.
