Stirling solare - Tesina per tecnico industriale

Stirling solare

Tesi di maturità: a. s. 2011/2012

Candidato: Abdelhafid Atifi

Premessa

In questo lavoro di tesi andremo alla scoperta, in modo sintetico, delle principali energie rinnovabili, soffermandoci su quella con le maggiori potenzialità: l’energia solare. In seguito analizzeremo il motore Stirling, inventato nel 1816. Questo motore, dopo un iniziale successo, fu quasi totalmente abbandonato a causa dello sviluppo, prima del motore a vapore e successivamente dei motori endotermici. Ma scopriremo come questa “vecchia” tecnologia sia tornata alla ribalta. Infine, con l’aiuto di un modellino dimostrativo, spiegheremo come funziona un motore Stirling alimentato dalla luce solare.

Lo scopo di questa tesi è capire se la combinazione motore Stirling-energia solare possa rappresentare una risorsa energetica in grado di risolvere i problemi energetici e soprattutto ambientali che al giorno d’oggi siamo costretti ad affrontare.

Indice

• Renewable energy 1
  • Introduction 1
  • Wind energy 2
  • Geothermal energy 3
  • Hydroelectricity 4
  • Solar energy 4
• L’energia solare 5
  • Cenni storici 5
  • “L’energia del sole” 5
  • Pannelli fotovoltaici 6
  • Collettori solari 9
  • Solare a concentrazione 9
  • Vantaggi 10
  • Svantaggi 10
• Una “nuova”, “vecchia” tecnologia: il motore Stirling 11
  • Introduzione 11
  • Cenni storici ed applicazioni 12
  • Principio di funzionamento 14
• Quando il motore Stirling incontra l’energia solare 15
  • Stirling solare 15
  • Come è fatto? 16
  • Come funziona? 17
• Conclusioni 18
• Bibliografia 19

Renewable energy

Introduction

A renewable energy source is one that is replaced over a reasonable period of time by natural processes; examples include wind, solar energy, hydroelectricity and geothermal energy. About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from hydroelectricity. New renewables (wind, solar and geothermal) accounted for another 3% and are growing very rapidly.

While many renewable energy projects are large-scale, renewable technologies are also suited to rural and remote areas, where energy is often crucial in human development. As of 2011, small solar photovoltaic systems provide electricity to a few million households, and micro-hydro configured into mini-grids serves even more. Over 44 million households use biogas made in household-scale digesters for lighting and/or cooking. United Nations' Secretary-General Ban Ki-moon has said that renewable energy has the ability to lift the poorest nations to new levels of prosperity.

The most widely used forms of renewable energy are: wind power, geothermal energy, hydroelectricity and solar energy. These sources of energy provide an alternate ‘cleaner’ source of energy, helping to negate the effects of certain forms of pollution.

1

Wind energy

Since ancient times man has harnessed the power of the wind, the earliest known windmills were vertical axis type, developed by the Persians in 500-900 A.D.

Wind power is the conversion of wind energy into a useful form of energy, such as using:

• Wind turbines to make electricity;
• Windmills for mechanical power;
• Wind-pumps for water pumping or drainage.

A wind farm is a group of wind turbines in the same location used for production of electricity. Wind farms can be located onshore or offshore.

In an onshore farm a group of eolian turbines, placed in the same area, are interconnected by a middle-voltage linking net. This net gathers the energy created by each turbine, convoying it to a collection station. Here a transformer converts the middle-voltage electric energy in high-tension electric current, introducing it in the distribution system. A large wind farm may have dozens of eolian generators and more than one hundred of single turbines, being placed on an area of several km²; however, being the space of each generator very little, all the places between the turbines could be used for agriculture or livestock holdings. United States of America currently own the most quantity of wind farms, followed by Germany, Spain and Denmark. Italy is on the fourth place.

Onshore Wind-farm

The most recent wind farms are usually placed on the sea or on the offshore, great lakes, far from the coasts, where it is possible to exploit the strong winds not delayed by obstacles. Unluckily, the realization and maintenance costs of these offshore wind farms are more elevated than the onshore ones, because of the transportation costs, the great building problems, the difficulty to anchor their towers on the bottom and, in the end, the corrosive action of the sea water on their structures.

Lillgrund Wind farm, Sweden 2

Anyway, these marine plants have great productivity advantages. On the sea surface, as matter of fact, winds blow without any obstacle, with a higher speed. The offshore placement of great wind farms also solve the acoustic and aesthetical problems, the tower being placed beyond the line of the horizon, at least 3 km from the coast. The offshore plants represent, according to the most part of the specialists of this sector, the true future of the aeolian energy, for what concerns both the environmental problem and the production potential.

In 2007, the offshore plants produced about the 3,5% of the European aeolian energy, owned for the most part from Denmark and United Kingdom, followed by Holland and Sweden; in 2008 the largest aeolian offshore production happened in the UK, followed by Denmark. At present, the largest offshore wind farm is placed off the Lincolnshire coasts (Great Britain), with an installation power of 194 MW.

Geothermal energy

Geothermal energy is generated in the Earth’s almost 6,000 km beneath the Earth’s surface. The double-layered core is made up of very hot (melted rock) surrounding magma a solid iron center. Very high temperatures are continuously produced inside the Earth by the slow decay of radioactive particles. This process is natural in all rocks.

Geothermal energy was used by ancient people for heating and bathing. Even today, hot springs are used worldwide for bathing, and many people believe hot mineral waters have natural healing powers. The exploitation of geothermal energy to produce electricity is a new industry. A group of Italians first used it in 1904. The Italians used the natural steam erupting from the Earth to power a turbine generator.

3

There is more than one type of geothermal energy, but only one kind is widely used to make electricity. It is called hydrothermal energy. Hydrothermal resources have two common ingredients: water (hydro) and heat (thermal). Depending on the temperature of the hydrothermal resource, the heat energy can either be used for making electricity or for heating.

Low Temperature Resources: Heating

Hydrothermal resources at low temperatures (50 to 300 degrees Fahrenheit) are used for heating homes and buildings, growing crops, and drying lumber, fruits, and vegetables. In this application are used Ground source heat pumps that are special heating and cooling systems that use heat from the ground to control temperature inside a structure. In winter, when homes and offices are chilled by the low air temperatures above ground, a geothermal heat pump will use the earth below a structure as a heat source.

Scheme of a plant at low temperature

High Temperature Resources: Electricity

Hydrothermal resources at high temperatures (300 to 700 degrees Fahrenheit) can be used to make electricity.

These high-temperature resources may come from either dry steam wells or hot water wells. We can use these resources by drilling wells into the Earth and piping the steam or hot water to the surface. Geothermal wells are two to four km deep.

In a dry steam power plant, the steam is piped directly from the geothermal reservoir to a turbine generator to make electricity. In a hot water plant, some of the hot water is turned into steam. The steam powers a turbine generator just like a dry steam plant. When the steam cools, it condenses to water and is injected back into the ground to be used over and over again. 4

Geothermal power station in Iceland

Hydroelectricity