منابع انرژی موج: آب و هوا موج و بهره برداری

In identifying the most convenient zones for harvesting wave energy, it is natural to be attracted by the areas where we find the highest mean energy values. The obvious examples are the storm belts. A more careful analysis reveals that for practical use other factors need to be taken into account. Some of the main ones are the energy spread in frequency and direction, and its seasonality, without discussing the cost of the structure basically related to the conditions to be withstood. This reveals that other areas, in particular the equatorial ones, can be conveniently used, and be possibly advantageous from various points of view. Based on the results of the ECMWF ERA-Interim reanalysis and of altimeter data, we have carried out a comparative analysis between two locations with opposite characteristics, in the North Atlantic and in the Equatorial Pacific respectively. The quantified results confirm that less energetic, but more regular and less extreme, areas have a potential comparable to that of the classically considered storm belts.

Harvesting wave energy from the ocean is obviously a subject of interest. Taking for granted the present level of related technology (e.g., [1], [2], [3] and [4]), it is necessary to establish which are potentially the most promising areas. At a first glance, it is natural to associate these areas to the parts of the ocean where we find the highest levels of wave energy. In this paper, we show that a deeper analysis of the situation is required for an optimal choice, both from the point of view of production and for the related economic analysis. There are many challenges involved in the practice of wave energy harvesting. Some of them are technical, because the nature of wave energy is oscillatory, while standard technologies for electricity production involve rotational or linear generators. Wave energy converters (WEC) are conceived for carrying out this transformation. Others challenges are environmental, because wave energy does not come in a regular form. A normal sea state is composed by the superposition of a number of monochromatic waves. In order to convert energy efficiently, ideally an optimal WEC should be able to interact with all of the small and large wave components. In practice, from a more realistic point of view, WECs are restricted to work in specific ranges of frequencies and directions (e.g., see Ref. [5]). Other challenges involve the harsh environmental conditions at sea. WECs are exposed to corrosive saline water and to strong forces inherent to the water motion. In comparison to air for wind energy, the water density is three orders of magnitude larger and its associated energy is proportionally higher. However, the forces on the mechanisms and the related construction costs increase as well, generally with a power >1. For this reason, WECs cannot operate under strong wave conditions. Whenever a high sea state is expected, the device has to stop operations and protect itself, going in the so-called survival mode. On the other hand, WECs cannot operate if the energy is too low. A minimum of energy is necessary to start-up the system. These aspects naturally affect the performance, and therefore also the economical return of a related project. Apart from the WEC technological complexity and the variety of concepts developed to convert wave energy into electricity, several issues affecting WEC operation can be associated with the wave climate, which is the focus of this study. The advantage is that at present many environmental variables are understood with a very good degree of confidence. Wave variables in particular, are routinely monitored from space and forecast by numerical models. In addition, meteorological centres like the European Centre for Medium-Range Weather Forecasts (ECMWF, Reading, U.K.) archive data over long periods of time. In this paper we carry out a comparative evaluation of wave energy resources, taking into account aspects, like those mentioned above, that can be linked to the wave climate. Namely, we focus our attention on the distribution of energy over frequency and direction, and on the start-up and survival conditions. These considerations are accounted for by straightforward parameterizations established from the perspective of the wave climate. The paper is organized in six sections. In Section 2 we describe the main data sources, in Section 3 the general background for the calculation of the wave power is briefly presented, in Section 4 we define the statistics required for the description of the wave climate in the selected areas, and Section 5 contains the evaluation of the wave resources considering the above aspects. The output is translated into standard economical parameters like the annual production and the capacity factor to make the link with engineering applications. Finally in Section 6 a brief discussion summarizes our main findings and conclusions.

We make a comparative assessment of the wave energy practically available for conversion at two locations characterized by rather opposite wave climates. In general, the economic viability of a WEC project shall depend on the balance between associated costs and benefits. Both depend on the characteristics of the local wave climate and on the capacity of the system to be installed. Without focussing on a specific device, we have preferred a more general approach that is valid, mutatis mutandis, for any device to be installed at a specific location. In addition, to avoid any consideration specific to coastal geometry, we have purposely kept our analysis in the open ocean. However, the method of analysis should be valid for any location at arbitrary water depths, provided that the data sources are representative for the location. They should come, for instance, from a nearshore wave model that accounts properly for shallow water effects (e.g., bottom friction, refraction, depth induced breaking, …), or alternatively from local wave spectral measurements. This is important to note, since many wave energy extraction applications will be located in shallow waters. To be as general as possible, we have used the installed capacity IC of the system as a variable, and explored how the annual production of electricity AP and the capacity factor CF (an indicator of how efficiently is the system used) vary when varying IC. We have used three possible scenarios, with progressively increasing and realistic restrictions. Starting with no restriction, except of course IC, we have then introduced minimum and maximum thresholds representing respectively the trigger into action and the survival conditions. Finally we have considered specific windows in frequency and direction where the system can actually operate, looking additionally into the possible reorienting capability of the device. Granted the amplitude of their range (Hz and degrees), for each location the specific choice has been optimized according to the local wave climate. The results show that the mean energy locally available, however significant, is not a sufficient indicator for a rational final decision. The spreading of energy in frequency and direction and its seasonality play a fundamental role in dictating what can be harvested by a given system and, most of all, the efficiency of the overall investment. It turns out that, granted the interest for the stormy areas, also the equatorial zone can be profitably, and in some aspects convenient for electricity production, using the local wave motion. The basic reason for this is that the higher average energy in the storm belts are due to infrequent large winter storms which can only be captured by installing larger capacity devices. However, most likely these devices will be idle in summer. Conversely, in the equatorial zone, there is a permanent and relatively constant, albeit lower, wave motion, but at low frequencies, hence with large wave power. In this regard, the average wave power as a mapping indicator is misleading. Other indicators can be found that are more objective. For instance, Barstow et al. [36] presented two. The first is the ratio of the minimum monthly mean power to the annual mean power (their figure 4.6). This indicator quantifies the seasonal variability and it is therefore associated to the economic advantage of the project. The second is the ratio of extreme wave height to the mean wave height (their figure 4.8). This indicator gives an idea about the ratio of construction costs over revenues. It should be mentioned that these two indicators show a picture of the global resource that is in opposition to that brought across by the average power. However, given the complexity of wave energy, an objective and meaningful assessment can only be made by taking into account the most relevant aspects of that complex system. Purposely, and because of the uncertainty in the related figures, we have not considered the economical aspect of the problem. On a parallel, but different subject, the cost of an offshore wind energy system is currently quoted at 2500 Euro/kW (or ∼3000 US$/kW, Brian Holmes, personal communication). However, there are so many feed-in tariffs on green trading, renewable energy certificates, etc., that any figure becomes a little nebulous. In any case, it is clear that the costs of construction and maintenance of a structure increase with the severity of the conditions of operation and, much more so, with the ones to withstand for survival. The implications for the two areas we have considered are obvious. Our purpose was not to force a choice between the two opposite situations. Many other factors control the choice of where to operate, first of all what is available in the seas surrounding a specific country. We meant to show how to interpret the local wave climate, not only for its power, but also for its distribution in frequency, direction and time, to derive a better idea of the true potential in a certain zone. It turns out that, besides the classically considered storm belts, other areas can be similarly attractive simply because the lower energy level is compensated by a greater potential efficiency due to wave characteristics more uniform in time and to the lack of severe conditions, which is of course advantageous also from the economical point of view.