Declan Baugh

I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that.

- Thomas Edison in conversation with Henry Ford, 1931.

Abstract

This report is aimed to examine the effects of investing in solar energy in a "Renewable Strategy For Ireland". Every year, we are burning 15,000 years of accumulated fossil fuels, and our limited supply of fossil fuels such as gas and oil is quickly diminishing, while there is also strong evidence to suggest that their by-products such as Co2 is harming our atmosphere and harmfully raising the temperature of the globe. We must discover methods of producing rewnewable energy, costly and efficiently, and implementing them worldwide, in such a way that we can realistically have a competitive alternative to burning fossil fuels.

Our planet receives vasts amounts of energy from the sun daily, however the means for efficiently converting this solar energy to electrical energy is not currently economically viable. More intelligent and innovative ways of producing and installing cheap sources of energy which can harness this radiation must be invested in.

Photovoltaic cells are devices that can convert solar radiation into electricity. We will study the current and future states of technology with photovoltaic cells, and will concentrate on how we can use these devices to harvest the energy we recieve from the sun.

The total primary energy requirements of Ireland and the percentage of energy which is used by each sector of the economy is examined in detail. We use this analysis to deduce what credible avenues are available to us, to combat the problem at hand.

A case study of DCU is explored. The potential solar energy which could be produced in DCU is carefully measured and calculated, and our results is analysed.

The total energy which could be saved is projected, as well as the cost necessary to install such an operation.

Current trends show that the price of photovoltaic cells are decreasing significantly annually, while the efficiency is increasing. Many sources believe withing 10 years, grid parity may be viable, where it becomes equal or cheaper to generate electricity locally, that it would to purchase it from the electrical grid. This is a benchmark in renewable energy technology which we hope to reach before all our fossil fuels diminish forever

Current State of Technology in photovoltaic cells

This chapter studies and demonstrates technologies in development which aim to tackle the problems of declining fossil fuel reserves, with particular focus on the field of photovoltaic cells. This rising necessity to develop renewable energy sources which are both inexpensive and efficient continues to inspire new approaches and innovations in low cost photovoltaic devices.

Solid state photovoltaic cells are devices which may be a solution to this problem. One of the greatest downfalls to these devices is the cost. The current cost of large-scale production, installation and maintenance of solid state solar cells is too expensive to act as a viable alternative to fossil fuel powered electricity stations. Solid state solar cells require a relatively thick layer of semiconductor in order to acquire enough solar flux to achieve sufficient optoelectronic energy conversion, and semiconductor fabrication is costly.

Introduction to Band Theory

In order to understand the function of photovoltaic cells, one must understand the basic concepts of quantum theory. In 1913 Niels Bohr depicted the structure of the atom as a positively charged nucleus, surrounded by negatively charged orbiting electrons. This model states that an electron may only travel in discrete quantized energy levels about the nucleus. We now know that this is a very primitive view of the atomic structure and this model is seen to break down heavily in crystalline solids. As matter solidifies and forms a crystal lattice, the distance between adjacent atoms reduce, hence there is an overlap of the wavefunctions of individual electrons. This creates an inconsistency with the Pauli Exclusion Principle, which states that no two identical fermions may occupy the same quantum state simultaneously. This results in the splitting of the discrete energy levels, which creates energy bands.

For example, in the specific case of silicon (Si), Si has an electron configuration of 1s2, 2s2, 2p6, 3s2, 3p2. The 3p orbital’s have a degeneracy of six, which means the silicon 3p orbital has two occupied stated, and four unoccupied states. However, as an ingot of silicon crystal is grown, the Si atoms crystallize to form a zincblende lattice structure. In doing so, the lattice constant reduces, which result in sp3 hybridization. The 3s and 3p energy levels split into energy bands, which merge to form one large energy band with four occupied and four unoccupied levels [Fig 1]. As the lattice constant reduces further, the band splits again, and the two electrons in the upper 3p band pass through to the lower valence band leaving the upper conduction band empty. Electrons may exist in either of these bands, but cannot exist with energies corresponding to values of the band gap between the bands [Fig 1].

Figure 1: A schematic diagram of silicon energy levels with respect to interatomic spacing of the lattice constant. (a) Represents the energy bands of a free silicon atom. As the interatomic spacing between atoms is reduced to (b), the energy levels split into bands. Further reduction caused the bands to merge at (c), and then separate until an upper conduction band, and lower valence band are created at interatomic spacing (d), which matches the interatomic spacing of stable crystalline silicone. Adapted from Figure 1 in Reference [1].

At zero Kelvin, electrons will occupy the lowest possible energy levels and acts as an insulator, i.e. the valance band is completely filled, and the conduction band is unoccupied. At temperatures above zero Kelvin, electrons in the maximum of the valence band may be thermally excited and cross the bandgap and rest at the minimum of the conduction band, immediately above the gamma point in the Brillouin zone. The material is now branded a semiconductor.

In order to excite an electron to the conduction band from the valence band, the electron must absorb a photon of equal or greater energy to that or the band gap of the material. In doing so, a loosely bound conduction electron is positioned in the nth energy level, and in the valence band a positively charged hole is created. The energy level within the conduction in which the electron is excited to, is determents by the energy of the applied photon.

Introduction to the Photovoltaic Effect

Naturally occurring semiconductor elements such as silicon and germanium have a valence number of 4. To produce a photovoltaic cell, the semiconductor must be contaminated (doped) with controlled amounts of chemical elements with valencies of 3 and 5. For the case of silicon, we use boron and phosphorous. Doping with these elements will introduce an excess of either positive charge carriers (doped with boron), or negative charge carriers (doped with phosphorus). We call these p-type and n-type respectively. When a p-type and n-type semiconductor is placed together, the boundary between these two layers is called a p-n junction. An internal electric field develops at this junction, and if an incident photon excites an electron, creating an electron-hole pair, then the induced electric field will force the charge carriers in opposite directions and create an electric current.

When an external circuit is attached to the semiconductor, this electrical current can be harvested.

Figure 2: Schematic diagram demonstrating the operation of a p-n junction photovoltaic cell. Image adapted from Reference [2]

First Generation Photovoltaic Cells

Currently, the majority of photovoltaic cells in operation are first generation cells, such as the solid state p-n junction photovoltaic cells we previously discussed.

P-n junctions may be composed of various different semiconducting materials, but the majority is composed of silicon. Silicon is an indirect bandgap semiconductor, and therefore, the transition from the valence band to the conduction band must be accompanied by a phonon [Fig 3]. For indirect bandgap semiconductors, phonon assisted transitions result in a percentage of the incident energy being lost through phonon vibrations.

Silicon is one of the most abundant materials in earth’s crust [3], so raw silicon is widely available, however, the cost of producing electronic grade, monocrystalline silicon is very high. Due to these factors, there is a reluctance to invest in this technology when it may take decades to get any return on their investment. New approaches and innovations in low cost photovoltaic devices are imperative if we wish to realistically view solar energy as a future alternative to burning fossil fuels for electricity generation.

Figure 3. Band diagram illustrating the transition of an electron between the conduction band and valence band of a direct bandgap (a) and an indirect bandgap (b) semiconductor. It is shown that in order for an electron to pass from the valence band to the conduction band of an indirect semiconductor, the transition must be accompanied by a phonon, which decreases optical efficiency. Adapted from Figure 5, p 186, Reference [4].

Second Generation Photovoltaic Cells

Second generation photovoltaic cells, widely known as thin-film photovoltaic cells (TFPV), are an alternative to the costly 1^st generation solid state cells. This category of solar cell is created by depositing thin layers of photovoltaic material on a substrate. Materials such as amorphous silicon, micro-crystalline silicon, cadmium telluride (CdTe), and copper indium selenide/sulfide (CIS), are used to create TFPV cells. Significantly less time, energy and cost is required to create amorphous silicon, as opposed to monocrystalline silicon. Due to this, TFPV cells are considerably cheaper to produce than 1^st generation cells. They are also more versatile, as they are extremely thin (ranging from a few nanometers to 10s of micrometers), flexible and lightweight. TFPV cells can be mounted on glass substrates and are generally more aesthetically pleasing than solid state cells. However, the efficiency of 2^nd generation cells are currently less than that of 1^st generation cells, so higher efficiency cells must be researched and developed before the use of 2^nd generation photovoltaic cells becomes widespread.

Third Generation Photovoltaic Cells

Third generation solar cells may also be called exitonic solar cells. An exiton is the term given to the system which consists of an electron and a hole, electrostatically bound together; hence, an exiton is a quasiparticle which has a major influence on the optical properties of the material which it resides. Excitons are seen to have huge optical absorption and emission intensities. Furthermore, excitonic solar cells can be manufactured at a far lower cost than traditional solid state cells, which makes them promising candidates to revolutionize the way electricity is produced today.

When a photon of light is incident on an electron in the valence band and the photon as energy greater than the bandgap energy, then the electron can be excited into the conduction band an electron hole pair will be created. Once an exciton is formed, there are three methods in which the excitation energy can be dissipated. Energy may be converted into heat via lattice vibrations, a free electron and free hole may be release through ionization, or the electron may relax back into the valance band accompanied by the emission of a photon of light with energy equal to that of the bandgap. If the electron is forced to stay within the vicinity of the hole, the two oppositely charged particles experience a coulombic attraction between one another and form an exciton. An exciton can therefore be looked as being electrostatically comparable to that of the hydrogen atom. However, the potential energy, binding the exciton together is considerably smaller than that of the hydrogen atom and hence will have a different absorption spectrum.

In order to harvest the benefits of excitons in relation to efficient electricity production we must concentrate on the use of exciton based photovoltaic cells for efficient conversion of solar energy into electricity. The fundamental difference between the conventional solid state p-n junction solar cells and exitonic solar cells is that light absorption in excitonic photovoltaic cells is a result of the creation of excitons, and not the creation of electron hole pairs directly. However, the binding energy of exitonic solar cells are considerably less than that of conventional 1^st generation solar cells, hence photons of less energy can induce an electrical current, resulting in more efficient photovoltaic cells.

At present, there are three types of excitonic solar cells; organic photovoltaic cells with planar interfaces, organic photovoltaic heterojunction cells, and dye-sensitized solar cells.

Organic Photovoltaic Cells with Planer Interfaces

An organic based solar cell is a solar cell which uses conductive organic polymers to confine excitons for light absorption and electric current creation.

In a conventional solar cell, light incident on a p-n junction creates electron-hole pairs. If an external electric field is applied to the system, then the electron and hole will drift in opposite direction, resulting in a DC current. However, excitonic solar cells function through a different process. There is a specific type of exiton called the Davydov exciton [5]. The Davydov exciton is found exclusively in organic substances whose unit cell consists of polymer chains bond together into closed rings, such as benzene or C60 [Fig. 4]

Figure 4: The molecular structure of (a) the Benezene Ring C6H6 , and (b) Buckminsterfullerene C60. Both are organic molecules whose structure consists of closed polymer chains of carbon atoms which are capable of confining localized excitons.

When a photon is absorbed by a polymer ring, or sphere in the case of C60, an electron hole pair is created. Electrostatic interaction between neighboring rings is small compared to interactions between the tightly molecule, hence the electron hole pair does not exit the molecule, instead they become electrostaticly bound together within the ring creating an exciton [6]

In order to achieve a transfer of charge, an external force must be applied to break the bond between the electron and hole, and then transfer the charge carriers from molecule to molecule creating a DC current. One technique to generate this electric field is to deposit a thin film of the organic material between two metals electrodes [Fig 5.a]. The metal contacts must have a relatively high and low work function such as Indium tin oxide (ITO) (work function of 4.4–4.5 eV) and aluminum (Al) (work function of 4.06-4.26 eV) [6][7]. The difference in work function creates an electric field across the organic material. This electric field breaks apart the exciton and the free electron and hole moves through the electric field to create a current across the electrodes. However, in reality, this is quite an inefficient process and has very low quantum efficiency. A sufficient number of electron-hole recombination occurs before the charge carriers actually reach the electrodes. Also the electric field between the two electrodes is rarely strong enough to ionize the excitons.

Heterojunction Organic Photovoltaic Cells

To increase efficiency, multilayer-heterojunction organic photovoltaic cells were developed [Fig 5. b]. These cells consist of two metal contacts with a relatively high and low work function just as before. However rather than a single film of organic material, two layers of organic material which have difference in ionization energy are used [8]. There is now an additional electric potential induced directly across the heterojunction. This localized potential has a greater probability of having sufficient energy to break apart the excitons. The material with a low ionizing energy acts as an electron donor, and high ionizing energy acts as an electron acceptor. If an exciton is created which is not directly on the, heterojunction interface, the exciton must diffuse across the interface in order to generate practical charge carriers [8]

Unfortunately in order to absorb enough light to create sufficient exciton production, the material must be approximately 100nm. The diffusion length of an exciton in organic material is roughly 10nm, so the majority of excitons won’t actually reach the heterojunction interface and so don’t influence the DC current [8]. This is the major problem with this model, so dispersed heterojunction photovoltaic cells were developed. In this photovoltaic cell, the donor and acceptor are grown together simultaneously, creating a blend of donor and accepter material as shown in [Fig 5. c]. Controlled growth of this blend can ensure that average distance between donor and accepter portion are within the range of the exciton diffusion distance. This will ensure that the majority of excitons produced in either material can reach the heterojunction, allowing the excitons to ionize, and electrons and holes to be pulled towards the electrodes.

Figure 5: (a) Schematic diagram of an organic photovoltaic cell structure. (b) Schematic diagram of an organic photovoltaic heterojunction cell structure. (c) Schematic diagram of a dispersed organic photovoltaic cell. Adapted from Figure 2, Reference [6]

Dye-Sensitized solar cells (DSSC)

The DSSC, otherwise known as the Grätzel cell, was invented by the Swiss professor Michael Grätzel in 1991. He describes the DSSC as [“a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits commercially realistic energy-conversion efficiency” [9].

DSSC are currently the most efficient excitonic solar cell available. Its operation and construction is much different from the previous organic cell model [6]. DSSC can be thought of as a hybrid cell, a combination of organic and inorganic components. As the raw materials for DSSC is rather cheap, and does not require the high level of metrology and manufacturing costs of solid state solar cells. DSSC offers the prospect of an exceptionally cheap and simple technology which could be utilized for the large scale production of solar cells, and as an actual competitor to the more conventional fossil fuel electricity plants [6][10]

The top half of a DSSC consists of a transparent anode of fluorine doped tin dioxide (SnO2). Approximately 10μm of titanium dioxide (TiO2) is deposits on the anode. TiO2 forms an assembly of ≈20nm diameter nanoparticles bound together, which creates a highly porous structure with an extremely large surface area [9]. During the photo absorption process, the TiO2 does not contribute to photon absorption; it only absorbs a negligible amount of photons in the UV region. However TiO2 functions act a photo catalyst for transport of electrons. The TiO2 is immersed in a mixture of ruthenium- polypyridine and a solvent. Ruthenium-polypyridine is a molecular sensitizer which is responsible for the majority absorption of photons. This acts as a dye and covalently bonds to the TiO2 [9].

The bottom half of a DSSC consists of a concentrated redox electrolyte solution of iodide coated onto a conductive film, such as platinum. The redox property of the electrolyte is essential for the operation of the DSSC. The electrolyte molecules must undergo a constant cycle of oxidation and reduction in order to transport holes across the cell. The top and bottom halves of the DSSC are joined together and sealed in a transparent glass case to avoid the iodide electrolyte from leaking out. The electrolyte then penetrates into the porosity of the TiO2 and makes contact with the dye nanoparticles, allowing the removal of positive charge from the dye subsequent to it ejecting an electron into the TiO2. The large surface area of the TiO2 allows photon absorption to take place, despite a minuscule 10μm of film thickness. When light is incident on the cell, the molecular sensitizers are capable of harvesting a large percentage of the solar energy flux (≈ 46%) and has extremely high efficiencies for photovoltaic conversion (≈ 80%) through the creation of excitons [10].

The dye molecules are a mere few nanometers in size, so a tightly bound exciton created will easily diffuse to the TiO2 /dye interface and dissociate. The released electron will travel through the TiO2, into the anode, through an external circuit, and into the platinum electrode [Fig. 6]. However a major difference between a DSSC and an organic photo voltaic cell, is that after an exciton is ionized, the possibility of electron – hole recombination occurring is negligible. Once an electron is ejected by the dye molecule, it becomes oxidized and an electron is immediately absorbed from the underlying iodide electrolyte. This concentrated redox electrolyte solution which permeates the TiO2 film acts as a medium to transfer of holes from the oxidized dye to the underlying electrode [10]. The positively charged iodide now reacts with neighbouring atoms to form triiodide. The triiodide molecule diffuses through the electrolyte solution until it becomes in contact with the platinum electrode. At this point the reduction of the triiodide molecule takes place and the cycle may begin again.

Figure 6: Illustration of the structure and operation of a dye-sensitized solar cell (DSSC). The DSSC consists of nanoparticles of an optical sensitizer covalently bonded to the surface of an extremely porous 10μm thick TiO2 film, coated in an iodide electrolyte and sandwiched between SnO2 and Pt electrodes. Excitation of the sensitizer is followed by electron injection into the conduction band of the TiO2, and creation of a triiodide molecule in the electrolyte. Adapted from Figure 1 in Reference [10].

One major disadvantage to DSSC is the fact that it uses a liquid electrolyte. This introduces instability to the model. The electrolyte may freeze, preventing the triiodide molecules from diffusing to the anode and preventing current from flowing. The electrolyte may expand with increased temperatures which may cause leakage and degradation of the quality and lifespan of the cell. However DSSC are still at the beginning of their development cycle and it is a prototype for a series of photovoltaic devices which will exploit this technology.

Recently developments have been made in creating solid state DSSC’s, discarding the need of a liquid electrolyte [10]. One solution is the replacement of the redox electrolyte with a solid p-type semiconductor interpenetrating the noncrystalline TiO2 film. However to create an interpenetrating network of two conducting solids is a great task, but prototypes have been built that offer promising results.

Although DSSC’s are the most efficient excitonic solar cells available, they still cannot match the efficiency of solid state models, but DSSC is a young technology and there is much room for improvement. The incorporation of quantum dots into DSSC is one possible route of improvement. Quantum dots allow the conversion of a single photon into multiple excitons, which can greatly increase quantum efficiency. The theory behind multiple exciton generation (MEG) is already greatly studied and understood in the solid state solar cells .

Multiple Exciton Generation in solid state solar cells

Recently developments have been made in creating solid state DSSC’s, discarding the need of a liquid electrolyte [10]. One solution is the replacement of the redox electrolyte with a solid p-type semiconductor interpenetrating the noncrystalline TiO2 film. However to create an interpenetrating network of two conducting solids is a great task, but prototypes have been built that offer promising results.

Although DSSC’s are the most efficient excitonic solar cells available, they still cannot match the efficiency of solid state models, but DSSC is a young technology and there is much room for improvement. The incorporation of quantum dots into DSSC is one possible route of improvement. Quantum dots allow the conversion of a single photon into multiple excitons, which can greatly increase quantum efficiency. The theory behind multiple exciton generation (MEG) is already greatly studied and understood in the solid state solar cells .

Figure 7: Illustration of carrier relaxation/cooling of hot charge carriers. An electron from the bottom of the valence band (VB) is excited to the top of the conduction band (CB). The electron and hole must lose energy through phonon vibrations in order to relax to the bottom of the CB and to the top of the VB. Adapted from Figure 1, Reference [13].

Thermalization of excess carrier energy by electron-phonon scattering is a major loss in solar cells, which strongly limits efficiency of conventional solar cells. However, there are methods of harvesting this energy, and potentially achieving quantum efficiencies greater than one. Impact ionization or multiple exciton generation (MEG) is a process where a single photon may create more than one electron hole pair [13].

If a photon whose energy is twice the band gap excites an electron above the conduction band [Fig 8. a], then the electron can decay down to the bottom of the conduction band and release photon whose energy is greater than the band gap [Fig 8 b][13]. This photon may in turn excite a second electron to the conduction band and create a second electron hole pair [Fig 8. c]. This can be thought of as the reverse of the Auger effect, where during a high energy electron transition, energy is released and this energy is sufficient to ionize a second electron in the same atom instigating the release of an Auger electron.

Figure 8: Demonstration of Multiple Exciton Generation (MEG) by Impact Ionization (II). (a) A photon with energy greater than twice the band gap energy excites an electron high into the conduction band. (b) The excited electron relaxes to down to the bottom of the conduction band, releasing a photon with energy greater than the band gap energy. (c) An electron in the valence band absorbs the photon and is excited to the conduction band, resulting in the creation of two electron hole pairs, from the one initial high energy photon. Adapted from Figure 1, of Reference [13].

MEG cannot contribute to enhanced quantum yield in present bulk solar cells, because the wavelength of photons required to excite an electron to twice the bandgap, is in the ultraviolet side of the spectrum, which is beyond the spectral range of the sun. Also to ensure the excess electron energy is not dispersed at heat, the rate of phonon-electron interactions must not exceed the rate of spontaneous photon emission [13].

However within quantum dots, the level of phonon-electron interactions is radically reduced. The probability of multiple exciton generation is greatly enhanced due to increased coulombic interactions caused by carrier confinement. Also for indirect band gap materials, momentum need not be conserved during energy transmissions. According to the Heisenberg Uncertainty Principle, ∆x∆p ≥ ħ/2. So as we increase confinement in three dimensions, we decrease the uncertainty in position to zero, and hence the uncertainty in momentum increases to infinity [13]. This uncertainty in momentum renders a negligible difference in the k value between the conduction band minimum and valence band maximum for indirect bandgap semiconductors, which will in turn deliver a more efficient formation of one or more electron hole pairs upon the absorption of a photon, effectively converting indirect bandgap material to direct bandgap. MEG is a process which is of great scientific interest and of major importance to the development of future efficient solar cells.

Ireland's Energy Budget

Here we wish to discuss Ireland’s current annual energy consumption. We are particularly interested in the sources of our energy, and we wish to examine the relative percentages that the many energy sources contribute to our over all energy supply.

We will then study the quantity of energy is consumed per year, and similarly, study what percentages of the overall energy is consumed in each sector.

Our henergy supply is measured in terms of Megatonnes of Oil Equivalent (Mtoe) which equals 4.1868x10¹⁶ Joules. The following information was obtained from the 2011 report “Sustainable Energy Authority of Ireland, Energy in Ireland 1990 – 2010” [14]

Ireland’s Annual Energy Requirements

Figure 9: represents the supply of energy to Ireland over the period 1990- 2010. Figure adapted from Figure 2 Reference [14]

In 2010, energy usage reached 14.8 Mtoe, however, this is a drop from from its peak of 16.5 Mtoe in the height if the economic boom, approaching 2008.

Fossil fuels still accounted for 95% of Ireland’s Total Primary Energy Requirement (TPER). However, this percentage is gradually falling as we will see when analyzing the individual sources.

• • Our coal and peat demands have been decreasing since 1990, and now only accounts for less than 13% of our TPER, as opposed to 33% in 1990.

  • In 1990, oil accounted for 60% of our energy supply. This figure has also fallen to approximately 50% of our energy TPER.

  • Natural gas, which is a much cleaner source of fuel, has been increasing since 1990, and in 2010 accounts for 32% of Ireland’s TPER.

  • Renewable energy sources such as wind and hydro-electric power has increased by 305%, since 1990, however, this still only accounts for 1.6% of our of our TPER.

Summarizing this investigation, it is clear that Ireland is becoming more energy and environmentally conscious, as the percentage of our gross annual energy supply which is attributed to renewable energy sources is increasing. In fossil fuels, we are increasingly relyant on natural gas, a cleaner fuel source, and less on oil, peat and coal. However, 95% of our fuel supply is still attributed to non-renewable fossil fuels. This is an issue which we must address.

Ireland’s Annual Energy Consumption

Figure 10: Diagram displaying Irelands annual energy consumption within each sector of the economy between the years 1990 – 2010. Figure adapted from Figure 3 Reference [14]

• • Between the years 1990- 2010, Ireland’s total energy requirements increased by 55%, however, we can see that energy requirements have since fallen by 10% since 2008, for economic reasons.

  • Energy used for Transport increased by 123% over the past 20 years. In 2010, approximately 4.9 Mtoe was used for transport, this is approximately 33% of Ireland's entire energy usage.

  • Energy used in Industry has fallen since 2008, but stands at 19% in 2010.

  • Residential energy has been slowly increasing over the past two decades, but currently stands at 28%.

Energy used in the commercial and public services show a similar trend as residential energy, and rests at 18% in 2010. This is possibly on the grounds that residential and public services energy are mainly used to maintain housings and workplaces, and supply with light and heat etc. The winters if 2009 and 2010 proved much colder then in recent history, so it was expected that energy usage in these sectors would increase.

It is also predictable that energy usage in transport and industry were to dramatically fall over the past two years, as with the current recession, there are less and less businesses in operation.

Ireland's Renewable Energy Requirements/Predictions

Figure 11: Demograph of (a) the fuel sources and (b) fuel consumption in Ireland in 2010.

These figures will stand as out best approximation of our energy requirements in years to come. It is not plausible that our energy requirements will remain static over the coming years, however, it would prove very difficult to predict the future of our energy requirements based on this data. As a result of the financial crisis of the late 2000's, amalgamated with increasingly extreme winters, we saw major changes in almost all trends of our energy distribution. It would be unwise to extrapolate this data and attempt to continue this trend, based on only two years of inconsistent circumstances.

As we are amidst an unpredictable epoch of economic recession, any prediction of our further energy usage will strongly depend on peoples personal prediction on our climb to financial stability, and future weather predictions.

Ireland's Solar Capabilities

Figure 12. Illustration rrepresents the average annual sunshine recorded by Met Eireann between 1961 and 1990. Figure adapted from Reference [15]

Figure 13 displays the average irradiance received monthly in J/cm² for selected Irish weather stations. Figure adapted from Reference [15]

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Met Eireann provides detailed statistical data on the magnitude of solar energy recorded at its weather stations across the country, amid the time period from 1961 – 1990. For maximum efficiency, we will select data from the weather station which receives the greatest amount of sunlight annually. Unfortunately, there does not appear to be a complete set of data for Rosslare Weather station in the South East, so we will use the data collected at Dublin Airport and amend our results. From the above graph, Dublin received approximately, 1,400 – 1,450 hours of sunshine annually. Wexford receives 1,500 – 1,550 hours, about 7% more sunshine. We will increase our results in Dublin Airport by this fraction to estimate the maximum amount of solar energy we can produce in Ireland.

Figure 13: Average Sunshine received monthly at Dublin Airport Weather Station. Data acquired from Reference [16]

The above chart calculates that on average, Dublin receives 1438.42 hours of direct sunlight annually. So now I will combine this data, to try and give us an Idea of the average solar radiation received in Dublin daily.

Figure 15: Screenshot of the Google Maps tool [17] which allowed us to measure the area of buildings. Above marks the outline of (a) The Gymnasium, and (b) the Engineering Building. Image property of Google Inc.

FIgure 16: The Area of each buildings exposed roof surface and the potential power which could be harvested is displayed. A plan view of the college grounds are displayed with each building colour coded. Image property of Google Inc.

Following the above data, we see that DCU has approximately 32414.2m² of potential space to install solar panels. This is the equivalent of a single square solar panel 180m in width. Receiving 3.3774x10⁹ J/m², and with photovoltaic cells working at approximately 18% efficiency, we deduce that we conceivably, could produce 1.97x10¹³ Joules, or 0.000470661 Mtoe every year.

To put this into perspective, in order to accumulate enough energy to render out burning of peat unnecessary, 1,697 similar sized organizations would have to invest it harvesting solar energy.

The Sustainable Energy Authority Ireland (SEIA) estimate that the average Irish household consumes 5000 kWh of electricity every year, which equates to 1.8x10¹⁰ Joules.

This suggests that DCU has the potential, to supply 1096 houses with electricity annually.

Cost of Photovoltaic Cells

We have now seen that DCU has the potential to harvest 1.9706x10¹³ Joules every year. With 1438.42 hours, or 5,178,312 seconds of sunlight per year, we can produce on average 3.8x10⁶ J/s.

A recent post in The Guardian, states that the price of installation of solar panel in 2011 has fallen to about $1.50, or €1.1515 per Watt [19]. This alludes that in order to install our system, with current prices, it would amount to €4,382,018.

Following recent trends in prices, the price is likely to drop to $1 / €0.76769 per Watt in 2013, leaving the total installation costs at €2,917,222.

A report in The Washington Post [20] claims that prices have plummeted by 40% over the past year, due to advances in technology creating more efficient photovoltaic cells, and also due to Chine heavily investing in photovoltaic research and development.

Figure 16: DIsplays the dollar cost per Watt of solar photovoltaic cells in the US. Image adapted from Reference [20]

Grid Parity

Grid Parity is a term used to describe when it becomes equal or cheaper to generate electricity locally, that it would to purchase it from the electrical grid. Grid Parity is a highly important benchmark when considering renewable energy sources.

At present, renewable energy sources, such as solar, wind, hydro are all inefficient processes, and it is much cheaper to produce electricity via combustion of fossil fuels. Only when we reach grid parity, will renewable energy exist as credible competitor to burning fossil fuels.

A leading UK solar energy company, Solarcentury, predicts that grid parity may reach as early as 2013 for residential homes in the UK, at 17 - 18p per Watt, however, grid parity for commercial use may not hit untill 2018 [21]

A recent report in the Washington post, Ecofys, a leading consultancy in renewable energy, and carbon efficiency systems, also predicted grid parity to hit the UK in 2018 [20].

The Photovoltaic Association Australia claims that they have already reached grid parity, and due to recent drops in cost of producing solar panels, and the intense Australian sun, solar energy has become so cheap that they now produce electricity for the same price that is charges by the electricity grid [22]

Conclusions

To ensure we have energy sources in time to come, we must start now. We must act now, and use energy wisely.

Not only must we concentrate on more innovative methods of producing efficient clean energy, but we must considerations on conserving energy and use it wisely.

This report focuses on the pros and cons of solar energy as a "Renewable Stratagy for Ireland"

Photovoltaic cells are becoming increasingly efficient and affordable, however, me mustn't rely on solar energy for our energy requirements. Sources such as wind, tidal, hydro, geothermal, biomass, fusion etc. must all be exploited.

As a case study, Dublin City University on a local scale can harvest a significant amount of electricity, and therefore reduce our carbon footprint if it every unobstructed roof area had photovoltaic cells installed. However would be at a cost of millions with todays prices. In the near future, it would be more sensible to install cheaper, more efficient cells. Current photovoltaic cells have a lifespan of approximatly 25 years, so future cells may even have an ever increasing life span.

Before we reach grid parity, it is going to be cheaper to purchase electricity for the electric grid, produced via the combustion of fossil fuels that produce it ourselves via renewable sources. However, once we reach grid parity, it should be in every single individuals interest to install photovoltaic cells in the household, in a fight to so reduce both cost, and reduce long term harmful effects on our planet.

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