Date of Award

8-29-2010

Document Type

Thesis

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Abstract

Hybrid renewable systems represent a very reliable and economical solution to the energy supply problem for stand-alone applications. The purpose of this thesis is to analyze the relationship between the size of such systems and the amount of renewable resources available to them. Moreover, the results will identify which components of the system have a greater energy contribution as the amount of resources change.

The optimum size of a stand-alone hybrid renewable energy system was investigated over the possible ranges of insolation and wind speed in the United States. This system includes an array of photovoltaic (PV) panels, a set of wind turbines, an AC diesel generator, a bank of lead-acid batteries and an inverter. In order to evaluate the performance of this system, an AC electricity demand ranging from 10 to 100 kW peak power demand was modeled. This range was selected to describe the energy needs of most residential communities, businesses and small industrial applications. The system was optimized in HOMER by finding its minimum lifecycle cost for every combination of insolation, wind speed and peak electricity demand values.

The first results served as a validation of the expected outcome; an increase in either insolation or wind speed translated into a larger amount of energy produced by the PV array or the wind park, respectively. However, the increase in solar energy is achieved through an increase of the PV array size, whereas the increase in wind energy is accomplished using less wind turbines. The reason is that the power produced by the PV panels has a nearly proportional dependence on the amount of insolation striking them and thus, more PV panels are required to produce a larger amount of energy as insolation increases. On the other hand, the power produced by the wind turbines is directly proportional to the cube of the wind speed. This explains why as the wind speed increases, a smaller number of wind turbines will produce a larger amount of energy.

Another important result was derived by evaluating the system under very high or very low renewable resources. When very low insolation and wind speed values are available, the contribution of PV panels is greater than that of wind turbines. Conversely, with very high insolation and wind speed values, the main contribution to the system is provided by the wind turbines. The reason is that below 3.5 – 4 m/s wind speed, turbines will not typically start moving, and the only active renewable components of the system are the solar panels. With high insolation and wind speed values, the first results showed an increased performance of the wind turbines versus the PV panels.

The results for the lowest insolation and wind speed values also showed that neither PV panels nor wind turbines are capable of producing a great amount of energy. A small inverter size revealed that most of the energy was delivered in AC by the diesel generator, which works as the main source of energy for the system. The diesel generator produces electricity only when demanded by the loads. Therefore, when the generator is being used extensively, very little energy needs to be stored in the batteries for a later use. This explains the small size of the battery bank in the optimum system. Under very high insolation and wind speed values, wind turbines have a bigger contribution than PV panels in the optimum system. In fact, since wind is available during the most part of the day, more energy is readily available and a large diesel generator is not required to cover peaks in the electricity demand. In this case, the diesel generator acts as a back-up source of energy for the system.

Finally, an estimate of the optimal lifecycle and initial costs of system was provided. It was observed that as the amount of insolation and wind speed increases, both costs decrease following an almost linear trend for most of the cases.

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