Methods and tools to evaluate the availability of renewable energy sources

Abstract

The recent statements of both the European Union and the US Presidency pushed in the direction of using renewable forms of energy, in order to act against climate changes induced by the growing concentration of carbon dioxide in the atmosphere. In this paper, a survey regarding methods and tools presently available to determine potential and exploitable energy in the most important renewable sectors (i.e., solar, wind, wave, biomass and geothermal energy) is presented. Moreover, challenges for each renewable resource are highlighted as well as the available tools that can help in evaluating the use of a mix of different sources.

Introduction

As we all know, there is basically only one source of energy for us, living on the Earth: the sun. The power it irradiates on our planet is estimated to be about 175,000 TW, four orders of magnitude more than the power we use even in our energy intensive times.

The energy we have received and continue to receive from the sun is converted in many different ways by the dynamics of our planet and of its atmosphere: the high temperatures below the crust are due to its original activity; the presence of hydrocarbons in the soil, to ancient photosynthesis; winds and waves to the present thermal differences.

Thinking to a horizon of few tens of years, the current solar activity and the primordial heat left inside the planet may be assumed as constant and thus represent the unique renewable sources of energy for mankind.

We have however several different ways for transforming this energy into forms that are more suitable for our everyday use. The mechanical energy of winds, water and waves can be converted into electricity so that it can be easily shipped far from the source (and we are not forced any more to bring our grain to the windmills as centuries ago). Biomass resources, which are the product of biological processes induced by solar light, can be burned to produce heat (to be used either as such or again to produce electricity) or chemically or biologically processed to generate usable fuels. The sunlight can be used directly to produce heat in a more usable form or can drive electron movements in silicon cells to produce electricity. A renewable energy source, freshwater, has been indeed the first way of producing electricity and has been extensively studied and exploited all over the world since more than one century. This is why it will not be further analysed in this paper.

All the options we have to extract energy from solar activity enjoy the advantage of being sustainable (they can be replicated in time, at least over a horizon of several years) and to alter only marginally the carbon balance of the planet's atmosphere, because the production, use, and decommissioning of conversion plants involve some emission that is normally small in comparison to those involved in the production of the same energy by fossil fuels. The use of fossil fuels on the contrary is both unsustainable (they are present in the Earth in finite quantities) and increases the amount of CO₂ in the atmosphere by releasing the carbon absorbed by vegetation millions of years ago and presently stored into the soil.

On the other hand, all the renewable forms in which we exploit the sun energy are characterized by being spatially distributed and lacking the huge reservoirs of fossil fuels or freshwater, that can easily compensate for the time differences between offer and demand of energy. So the exploitation of these sources of energy is somehow more complex, and they are sometimes referred as “intermittent sources”.

Their spatial distribution also means that their exploitation is closely linked to the peculiar characteristics of the local environment and, in turn, it may have environmental impacts distributed on a wider area.

A characteristic they share with fossil energy sources is the impossibility of converting and exploiting all the energy which is potentially available. We can thus distinguish three different values:

• potential energy, that is the gross energy of the source (e.g. that of wind at a given location);

• theoretical energy, that is the fraction that can be harvested by the energy conversion system (e.g. the solar radiation collected by a certain surface of solar panels);

• exploitable energy, the fraction that can be used taking into account criteria of sustainability related to logistic, environmental and economic issues (e.g. the heat produced by a biomass fueled plant).

These definitions may be interpreted in a slightly different way for different applications. In many cases, for instance, the electric output of a plant can be considered as representing the exploitable energy. However, if we are talking of an offshore wind farm, 20 km from the rest of the grid, perhaps we want to compute the electric energy net from the (non-irrelevant) losses on the underwater connecting cable.

Despite the technological and logistic difficulties, the attention toward these renewable forms of energy is steadily increasing all over the world, due to the urgency to act against climate changes induced by the growing concentration of carbon dioxide in the atmosphere. The recent statements of both the European Union and the US Presidency pushed in this direction.

As an example, the European Union set an overall binding target of a 20% share of renewable energy sources in energy consumption and a 10% binding minimum target for biofuels in transport to be achieved by each Member State by 2020. Reaching this target will need a consistent proactive attitude of all governments, since in 2006 renewable energies were estimated at 6.92% of the primary energy consumption of the EU countries, and at 14.6%, mainly hydropower, of the electricity production.

This is why it is worth to revise methods and tools presently available to determine potential and exploitable energy in the most important renewable sectors, as done in this paper.

In the next sections, we will thus survey the state of the art of evaluation approaches for solar, wind, wave, biomass and geothermal energy, with attention to the site specific environmental characteristics, but without dealing with the final conversion step. Though this must be kept in mind because it sometimes influences the amount of exploitable energy, a review of possible conversion devices and processes would go far beyond the scope of this paper.