Electrical energy: where it comes from and how it arrives
A lamp turns on instantly, but the energy it uses began its journey in a turbine, a reservoir or an oil field, and travelled hundreds of kilometres at voltages thousands of times higher than those in a home. Following that path —from the source to the socket— is understanding what everything studied later in Electrical Technology is for.
01Energy, power and efficiency
Energy is the capacity to do work or produce heat. It appears in several forms —chemical, thermal, mechanical, electrical, radiant, nuclear— and changes from one to another, but it is neither created nor destroyed: it is transformed. Electricity is almost never the origin of energy: it is the carrier that takes it from a primary source (a fuel, reservoir water, wind, the Sun) to an end use (light, motion, heat, information).
| Quantity | Symbol | Unit (SI) | Other common units | What it expresses |
|---|---|---|---|---|
| Energy | E | joule (J) | kWh (1 kWh = 3.6 MJ), kcal (1 kWh ≈ 860 kcal), toe (≈ 11.6 MWh) | What is stored, spent or transformed. |
| Power | P | watt (W = J/s) | kW, MW, HP (≈ 746 W) | How fast energy is transformed. |
| Efficiency | η | dimensionless (given in %) | — | What fraction of the input is put to use. |
Efficiency is always below 1 because in every transformation part of the energy ends up as dispersed heat that can no longer be used: friction, Joule heating, exhaust gases. It was not destroyed; it was degraded to a useless form. When several transformations are chained, the efficiencies are multiplied, not added:
The kW measures power (how much energy per second); the kWh measures energy. A 2 kW heater on for half an hour uses 1 kWh. Writing “kWh per hour” or “kW consumed in the month” is a conceptual error. The electricity bill charges energy in kWh and, for large customers, also the contracted power in kW.
An electric motor delivers 3 kW at its shaft with an efficiency of 0.88 and runs 8 hours a day for 22 days a month. What power does it draw from the grid, how much does it lose and how many kWh does it use?
Of the 600 kWh, 528 kWh (3 kW × 176 h) become work and 72 kWh are lost as heat in the motor. Raising the efficiency from 0.88 to 0.92 saves more than 25 kWh per month on that one machine.
02Thermal energy
Heat is energy in transit between two bodies at different temperatures; temperature measures the thermal state of a body, not the amount of energy it holds. Heat flows by conduction (through a solid), convection (carried by a fluid) and radiation (waves, no medium needed). Engineering uses the kelvin (K); a difference of 1 K equals one of 1 °C.
To heat a body without a change of state, the heat required is proportional to the mass and to the temperature rise:
The specific heat c of water is 4.18 kJ/(kg·K), one of the highest there is; that is why water is the working and cooling fluid par excellence. When the state changes, heat is absorbed without any rise in temperature: vaporizing 1 kg of water at 100 °C takes about 2.26 MJ (latent heat), which is the energy the steam gives back when it condenses.
Turning electricity into heat is easy and almost perfect: a resistor does it with an efficiency close to 1 (Joule heating, Electric current). Turning heat into work is hard: no heat engine can convert all the heat it receives into work, and the theoretical maximum depends only on the temperatures of the hot and cold sources (in kelvin):
A steam plant with a boiler at 540 °C (813 K) and a condenser at 30 °C (303 K) cannot exceed:
In practice they reach about 40 %. The rest is delivered to the condenser, which is cooled with river or lake water, or with cooling towers.
Fuels are compared by their calorific value: the thermal energy they release per unit of mass or volume when burned. The values below are orders of magnitude; each commercial fuel states its own in its datasheet.
| Fuel | Approximate calorific value |
|---|---|
| Natural gas | ≈ 10 kWh/m³ (about 9,000 kcal/m³) |
| Diesel | ≈ 10 kWh/L |
| Gasoline | ≈ 9 kWh/L |
| LPG (propane and butane) | ≈ 12 kWh/kg |
| Coal | ≈ 7 to 8 kWh/kg |
| Dry firewood | ≈ 4 kWh/kg |
An 80 L water heater (80 kg of water) must be heated from 15 °C to 55 °C. Its element is 1.5 kW and it uses 95 % of the electrical energy (the rest is lost through the walls).
The same calculation explains the bill: heating water with electricity is expensive because it turns high-value energy (electrical) into heat, which could be obtained by burning gas directly.
A lukewarm swimming pool holds far more thermal energy than an ember, even at a lower temperature, because it has much more mass. That is why heat is calculated with m, c and ΔT, and not with temperature alone.
03Energy sources and their uses
A source is primary if it is taken from nature as it is (oil, gas, uranium, river water, wind, solar radiation) and secondary if it results from transforming another: gasoline and electricity are secondary. Sources are also classified as non-renewable (depleted on a human time scale) and renewable (naturally replenished).
Conventional sources: oil and gas
They are organic remains buried for millions of years in sedimentary basins, which heat and pressure turned into hydrocarbons and which became trapped in porous rock. They are extracted through wells. Oil travels by pipeline and is separated at a refinery into gasoline, diesel, fuel oil, LPG and other products. Natural gas, mostly methane, travels through gas pipelines or, liquefied at about −160 °C (LNG), on ships. Conventional gas sits in permeable reservoirs and flows by itself; unconventional gas is held in very low-permeability rock (shale, tight) and requires hydraulic fracturing, as in the Vaca Muerta formation in the Neuquén basin.
In Argentina, oil and gas supply the great majority of primary energy —on the order of three quarters or more—, with producing basins such as the Neuquina, Golfo San Jorge, Austral and Noroeste. In electricity generation, most comes from thermal plants burning natural gas; hydro comes next, with nuclear and non-hydro renewables making up the rest. These are orders of magnitude: the share changes every year and the exact figure is found in CAMMESA reports.
| Source | Type | How it is used | Advantage | Limitation |
|---|---|---|---|---|
| Oil | Non-renewable | Refined into fuels; engines and thermal plants. | High energy density, easy to transport. | Emits CO₂; finite reserves. |
| Natural gas | Non-renewable | Gas pipelines; combined cycle, industry, homes. | Cleanest combustion among fossil fuels. | Emits CO₂; needs pipelines or LNG. |
| Coal | Non-renewable | Steam plants. | Abundant and cheap. | The most polluting; minor role in the country. |
| Uranium | Non-renewable | Fission: the heat produces steam. Atucha I, Atucha II and Embalse (Córdoba). | A lot of energy per kg; no CO₂ in operation. | Radioactive waste; initial cost. |
| Hydro | Renewable | Reservoir, run-of-river or pumped storage (Río Grande, in Córdoba). | High efficiency and adjustable power. | Impact on the river; depends on available water. |
| Wind | Renewable | Wind turbines in farms. | No fuel; very good resource in Patagonia. | Variable: depends on the wind. |
| Solar | Renewable | Photovoltaic panels (electricity) and thermal collectors (hot water). | No moving parts; modular. | Daytime only; needs backup or storage. |
| Biomass and biogas | Renewable | Burning waste or digesting organic matter. | Uses farm and livestock waste. | Local availability. |
Alternative energies
Alternative sources are those that replace or complement the conventional ones: solar, wind, biomass, geothermal, small hydro and tidal. In the country, Laws 26.190 and 27.191 promote their use in electricity generation with participation targets. Their technical challenge is variability: solar and wind output depends on the weather while demand follows its own rhythm, so the system needs dispatchable plants (hydro, gas), storage (pumped hydro, batteries) and good interconnections.
Every source has a cost: land use, materials to build the equipment, effects on a river or on birds, the need for backup. Comparing sources requires looking at the whole cycle —construction, operation, decommissioning— and not only the emissions at the moment of generation.
Uses of energy
| Sector | Typical end use | Predominant energy |
|---|---|---|
| Residential | Cooking, heating, hot water, lighting, appliances. | Natural gas and electricity; LPG where there is no grid. |
| Commercial and services | Lighting, air conditioning, computer equipment. | Electricity and gas. |
| Industrial | Process heat, motive power (motors), furnaces. | Natural gas and electricity. |
| Transport | Moving people and cargo. | Diesel, gasoline and compressed natural gas (CNG); electricity is growing in rail and vehicles. |
| Agriculture | Machinery, irrigation, grain drying. | Diesel and electricity. |
The cheapest energy is the energy not used: improving the efficiency of lamps, motors and building insulation (energy efficiency) cuts spending without giving up the service. Appliance efficiency labels exist precisely to compare them.
04From source to electricity: generation
Almost all the world’s electricity is produced with the same principle: electromagnetic induction (Magnetism and electromagnetism). An alternator spins an electromagnet inside a winding and an alternating voltage is induced in it. What changes from plant to plant is the prime mover that spins it: a turbine driven by steam, gas, water or wind, or a combustion engine. The exceptions are photovoltaic panels, which turn light directly into direct current and have no moving parts.
| Plant | Primary energy | Prime mover | Approximate efficiency |
|---|---|---|---|
| Steam thermal | Fuel oil, gas, coal | Boiler and steam turbine | 35 to 40 % |
| Gas turbine | Natural gas, diesel | Gas turbine | 30 to 40 % |
| Combined cycle | Natural gas | Gas turbine + heat-recovery boiler + steam turbine | 50 to 60 % |
| Nuclear | Uranium | Reactor, steam generator and turbine | 30 to 35 % |
| Hydro | Potential energy of water | Pelton, Francis or Kaplan turbine | 80 to 90 % |
| Wind | Kinetic energy of the wind | Rotor and generator (Betz limit: 59 %) | 35 to 45 % |
| Solar photovoltaic | Solar radiation | None: photovoltaic effect | 15 to 22 % |
| Generator set | Diesel, gasoline or gas | Internal combustion engine | 30 to 40 % |
Generator sets, emergency generation and each turbine type are covered in Generation and generator sets.
Why the system runs at 50 Hz
The frequency of the voltage a synchronous alternator delivers is tied to its rotational speed and to its number of pole pairs p:
| Pole pairs p | Speed at 50 Hz | Typical of |
|---|---|---|
| 1 | 3,000 rpm | Steam and gas turbogenerators. |
| 2 | 1,500 rpm | Nuclear plant turbogenerators, generator sets. |
| 6 | 500 rpm | High-head hydrogenerators. |
| 30 | 100 rpm | Slow, low-head, large-diameter hydrogenerators. |
All machines connected to the system turn in synchronism, “in step.” Frequency is therefore the thermometer of the balance between generation and consumption: if demand exceeds generation, the system slows a little and the frequency drops; if there is excess generation, it rises. Holding it at 50 Hz is one of the permanent tasks of dispatch.
At grid scale, electrical energy is hardly stored at all: what is switched on in a home must be generated, at that very instant, in some plant of the system. That is why generation follows demand minute by minute, and why dispatch —the order in which plants come online— is a core task of the system.
A home uses 300 kWh per month. It is supplied from a gas-fired steam plant with these efficiencies: boiler 0.90; steam cycle 0.40; generator 0.98; step-up transformer 0.995; transmission and distribution together, 0.92. How much fuel energy is needed and how much gas is that?
With a calorific value of about 10 kWh/m³:
For every kWh that reaches the home, about 3 kWh of fuel are burned. With a combined-cycle plant, which uses 0.55 of the gas energy instead of 0.36 (0.90 · 0.40), the same home would need about 61 m³.
05Transmission and transformation: why high voltage
The big plants are where the resource is —hydro in Comahue, in the littoral and in the sierras; thermal near the gas pipelines— and the loads are in the cities, hundreds of kilometres away. The power has to be transmitted, and every conductor has resistance: part of the energy turns into heat along the way. For a three-phase line, the transmitted power and the Joule loss are:
Solving for the current: for the same power P, raising the voltage V lowers the current I in the same proportion, and the losses, which depend on I2, drop with the square of the voltage. Raising the voltage 4 times cuts losses 16 times. That is the reason for high-voltage transmission.
A three-phase line has 3 Ω of resistance per conductor and carries 60 MW at power factor 1 (deliberately simplified to see the effect of voltage). It is calculated with a line voltage of 132 kV:
Repeated at the other two voltages:
| Line voltage | Current per conductor | Line losses | Fraction of the 60 MW |
|---|---|---|---|
| 33 kV | 1,050 A | 9.9 MW | 16.5 % |
| 132 kV | 262 A | 0.62 MW | 1.03 % |
| 500 kV | 69.3 A | 0.043 MW | 0.072 % |
Going from 33 to 132 kV cuts losses 16 times, just as the square of the voltage ratio predicts. At 33 kV the 1,050 A current would need enormous conductors, and even so a sixth of what is transmitted would be lost.
What makes changing the voltage possible
In alternating current, the transformer raises or lowers the voltage with an efficiency above 99 % in large units and with no moving parts. That is the historical reason the world adopted alternating current: it allows generating at a voltage convenient for the alternator, raising it for transmission and lowering it for use. Transmission is also three-phase (Three-phase circuits): three conductors carry the power that single-phase would need six for, and the currents of the three phases balance each other.
The voltage cannot be raised without limit: the cost of towers, insulator strings, safety clearances and switchgear grows. The voltage is chosen according to the power to be transmitted and the distance.
At high voltage, electricity can jump through the air to an object or a person who gets too close, without touching the conductor. Never bring fishing rods, kites, antennas, ladders or cranes near an overhead line. If a cable falls to the ground, keep your distance (the ground nearby can be energized) and notify the utility. Safety clearances and work procedures are set by Law 19.587 and Decree 351/79, as well as by AEA regulations and each company’s rules; see Occupational health and safety.
06From the interconnected system to the socket: distribution grids
The SADI (Argentine Interconnection System) joins the country’s plants and loads into a single grid running at 50 Hz. Connecting everything has two advantages: any plant can supply any region, and mutual backup means a local failure does not leave everyone without service. CAMMESA (the company that administers the wholesale electricity market) coordinates plant dispatch and administers the wholesale market. The activity is governed by Law 24.065 and overseen by bodies such as the ENRE nationally and, in Córdoba, the ERSeP for provincial distribution (EPEC and the cooperatives).
| Level | Typical voltage | Role |
|---|---|---|
| Generation | on the order of 10 to 20 kV | Alternator voltage; depends on each machine. |
| Trunk transmission | 500 kV | Large blocks of power over long distances between regions (SADI). |
| Regional transmission | 220 kV and 132 kV | Carry energy to provinces and large cities; feed substations. |
| Medium-voltage distribution | 33 kV and 13.2 kV | Run through towns and neighbourhoods; feed industries and transformer stations. |
| Low-voltage distribution | 380 V between phases (220 V phase-neutral) | Deliver to the end user; three-phase system with neutral, at 50 Hz. |
Voltages are grouped, according to regulations, into low, medium, high and extra-high voltage; the exact limits of each category are set by the standards applicable in each case. The points where the level changes are the substations (ET) and, at the last step, the distribution transformer stations (CT), where 13.2 kV is stepped down to 380/220 V. A CT can be pole-mounted or a civil-works unit, in a vault or at ground level. The equipment of each one —transformer, disconnectors, circuit breakers, protection— is studied in Substations and distribution lines and Electrical protection.
Ways of building a grid
| Topology | What it is | Advantage | Limitation |
|---|---|---|---|
| Radial | Each feeder leaves the source and keeps branching, with no return. | Simple and cheap; easy protection. | A fault leaves everything downstream without service. |
| Ring | The feeder returns to the source; normally operated open at one point. | Each section can be fed by two paths. | More expensive; needs switching to reconfigure. |
| Mesh | Many nodes connected to each other by several paths. | Maximum continuity of service. | Costly; more complex protection. Typical of transmission. |
Demand changes hour by hour
Consumption is not constant: it has an evening peak and a valley in the early hours, and it varies with the season (heating in winter, cooling in summer). The installed capacity must cover the maximum demand plus a reserve. Since energy is not stored at large scale, some plants run all day and others only during the peak hours. Some pumped-storage hydro plants, such as Río Grande, store water uphill when there is surplus energy and release it when it is short.
Energy reaches the user through the service connection and the meter. From there, the internal installation of a building is governed by the AEA regulation, AEA 90364 (Electrical installations and Housing installations). The quality with which the service is delivered —voltage, frequency, outages— has limits set by the regulators.
Fuel, water, wind or Sun → prime mover → alternator (10 to 20 kV) → step-up transformer → 500 kV over the SADI → substation (132 kV) → 33 or 13.2 kV over the medium-voltage grid → transformer station → 380/220 V → meter → the home’s switchboard. There are losses at every step, which is why the efficiency of the whole path is the product of the efficiencies of each stage.
07In the laboratory
Materials: electric kettle (or heating jug) of known power, plug-in energy meter (wattmeter), thermometer, measuring cylinder or scale, stopwatch.
Steps: 1) Put in 1 litre of water (1 kg) and measure the initial temperature. 2) Plug the energy meter and the kettle in, and reset the kWh counter. 3) Heat to about 80 °C and disconnect. 4) Note the final temperature, the energy recorded and the time. 5) Calculate the useful heat Q = m·c·ΔT and the efficiency as Q divided by the measured energy. Repeat and average.
What is measured: temperatures, energy in kWh (or Wh), time, power. Acceptance criterion: the efficiency comes out between roughly 80 and 95 %, never above 100 %, and two consecutive tests differ by less than 5 %. If it comes out above 100 %, there is a measurement or unit error.
Practices 1 and 3 are done with mains voltage. Use the laboratory’s meter and safety leads, without opening any equipment or touching live metal parts, with the board protected by a residual-current device and a circuit breaker, and always with the teacher present. Hot water also burns: do not lift the kettle while it is connected.
Materials: worksheet, the nameplate data of the appliances, a plug-in wattmeter if available.
Steps: 1) List every appliance in the classroom (luminaires, computers, projector, fan, heater) with its nameplate power. 2) Estimate the daily hours of use of each. 3) Calculate the daily energy of each appliance (E = P·t) and the monthly total. 4) Rank from highest to lowest consumption. 5) Compare with the school’s bill and propose three saving measures (LED lamps, switching off, better insulation) with the estimated saving in kWh.
Acceptance criterion: the estimated sum falls within 20 % of the consumption on the bill; if not, the assumed hours of use and the power of the larger devices are reviewed.
Materials: 220/12 V transformer of at least 100 VA, a second identical transformer, 12 V / 50 W halogen lamp, 100 m of 1 mm² copper cable (50 m out and back) on a reel, multimeter, clamp meter.
Steps: A) Feed the lamp at 12 V directly through the 100 m of cable. Measure the lamp voltage, the current and the drop in the cable. B) Step up to 220 V with the first transformer used backwards, run the same 100 m of cable at 220 V and step back down to 12 V with the second transformer to feed the lamp. Repeat the measurements. C) Calculate in each case the power lost in the cable.
Expected values (approximate, ignoring transformer losses):
In case A the lamp receives less than 8 V and barely lights, and the cable loses about 12 W; in B the lamp receives close to 12 V, and the cable loses less than 0.1 W. Acceptance criterion: the losses measured in B are at least ten times lower than in A, and the student explains the result with P = V·I.
08Common mistakes
| Mistake | Consequence |
|---|---|
| Confusing kW with kWh | Power and energy get mixed up; consumption and cost calculations give meaningless results. |
| Adding the efficiencies of a chain instead of multiplying them | The total efficiency is overestimated: 0.90 and 0.40 give 0.36, not 1.30 or 0.65. |
| Using °C in the Carnot formula | Absurd results (even negative ones). Carnot temperatures are always in kelvin. |
| Believing that “consuming energy” means destroying it | You lose sight of where the unused part goes: it is heat, and that heat has to be removed. |
| Thinking that if the voltage rises the current rises | For the same power it is the opposite: I = P/(√3·V·cos φ). Raising the voltage lowers the current. |
| Forgetting the 3 for the three phases in the losses of a three-phase line | Total losses are underestimated threefold: they are 3·R·I2, not R·I2. |
| Mixing line voltage with phase voltage | With 380 V between phases there are 220 V between phase and neutral (380/√3); using one for the other multiplies or divides the results. |
| Calling every renewable source “clean” or every fossil source “dirty” without looking at the full cycle | Sources are compared with incomplete arguments; impact, efficiency, cost and availability all have to be considered. |
| Using a calorific value without checking the units (kcal/m³ or kWh/m³) | An error of 860 times: 1 kWh equals about 860 kcal. |
09Self-assessment
What is the difference between power and energy? A 2 kW heater runs for 3 hours: how many kWh does it use?
Power is energy per unit of time (kW); energy is the total consumed (kWh). It is calculated E = P·t:
A motor delivers 4 kW at the shaft with an efficiency of 0.80. What power does it draw from the grid and how much does it lose?
It draws 5 kW and loses 1 kW as heat.
How much energy is needed to heat 50 kg of water from 20 °C to 60 °C?
Apply Q = m·c·ΔT with c = 4.18 kJ/(kg·K) and ΔT = 40 K, and convert to kWh by dividing by 3,600:
With an efficiency below 1, the energy actually paid for is higher.
Why does a steam plant not exceed 40 % efficiency?
Because of the second law of thermodynamics: a heat engine cannot convert all the heat into work, and the maximum (Carnot efficiency) depends on the temperatures of the hot and cold sources. With 540 °C and 30 °C the limit is 63 %; real losses bring it down to 40 %. The rest is delivered to the condenser.
Why is power transmitted at high voltage? If it goes from 132 to 500 kV, how do the current and the losses change?
For the same power, a higher voltage means a lower current, and losses depend on the square of the current. The current is divided by 3.79 and the losses by about 14:
A 4-pole alternator (p = 2) is connected to a 50 Hz grid. At what speed does it turn?
It turns at synchronous speed, 1,500 rpm:
What is the SADI, who coordinates dispatch, and why is the growth of solar and wind a challenge?
The SADI is the interconnected system that joins the country’s plants and loads at 50 Hz; CAMMESA coordinates plant dispatch and administers the wholesale market. Solar and wind are variable (they depend on the Sun and the wind), so the system needs dispatchable plants, storage and interconnections to keep generation and demand in balance.