Dimitris Christakis

Abstract: This paper presents a methodology for determining the specifications of an isolated R.E.S. power production system on an environmentally sensitive ecosystem. The wind and solar power constitute the primary power generation system and diesel generators act as backup. Real wind and solar potential measurements are used. The wind atlas of the island has been constructed. The specifications of the proposed system are optimized by the life cycle cost method. The renewable energy sources (R.E.S.) total annual energy production exceeds 90%. As a result, the dependence on the diesel generator set annual energy production is limited and the system's operational cost is not practically influenced by the increasing fossil fuel prices. The introduction of a small size desalination plant for the production of drinkable water is also investigated. Both the available R.E.S. potential and the minimization of environmental impacts are considered for the siting of the equipment. The methodology of the present paper may be applied to other regions rich in R.E.S. potential, where the introduction of small size environmentally friendly isolated R.E.S. power systems is investigated. © 2008 Elsevier Ltd. All rights reserved.

Abstract: The present paper investigates the introduction of pumped storage systems (PSS) in isolated power production systems with high thermoelectric production and wind energy rejection. The introduced PSS aims at:•the maximisation of the wind energy penetration and•the minimisation of the energy production cost. In former studies, the introduction of PSS in power production systems aims at the power demand peaks saving. In the present study, the PSS storage-production procedure is accomplished without any predefined schedule. Energy is stored whenever:•wind energy is rejected and•the thermal generators that burn cheap heavy fuel oil do not operate at their nominal powers. Furthermore, the production of the thermal generators that burn expensive diesel oil is substituted during the power demand peak hours. Two case studies for Crete and Rhodes are accomplished. An iterative procedure is performed, in order to calculate the optimum pumps and hydro turbines nominal powers in both islands. The optimisation criterion is the energy production specific cost minimisation. The PSS introduction in Crete yields to almost 10% annual electricity production cost reduction. The annual wind energy rejection is nullified. The investment payback period may be less than 5 years. The PSS introduction in Rhodes leads to a 1.85% annual electricity production cost reduction. The PSS project does not exhibit attractive economic indexes. Conclusively, isolated power production systems with energy production specific cost higher than approximately 0.05€/kW h seem to be appropriate for PSS introduction, following the operation algorithm of the present paper. © 2007 Elsevier Ltd. All rights reserved.

Abstract: A power-quality (PQ) measure is introduced in this paper. The proposed measure, based on the Shannon's entropy, expresses the deviation of the examined voltage cycle from the ideal one. Two entropy concepts are introduced-the spectral and the probabilistic entropy. The two entropies, calculated for every voltage cycle, are properly combined to provide the PQ measure. The PQ measure is represented graphically on a 2-D Cartesian coordinate system. The PQ measure is calculated for the most common PQ disturbances. The graphical representation of the calculated PQ measurements leads to the definition of an area of acceptable PQ on the PQ measure 2-D diagram. The PQ of a power system may be evaluated by the proposed measure, by implementing voltage measurements during predetermined time periods in selected network nodes. The produced electricity price may be configured in terms of the measured PQ. © 2008 IEEE.

Abstract: Crete and Rhodes represent the two biggest isolated power systems in Greece. The energy production in both islands is based on thermal power plants. The annual wind energy rejection percentage is calculated for Crete and Rhodes in this paper. The rejected wind energy is defined as the electric energy produced by the wind turbines and not absorbed by the utility network, mainly due to power production system's stability and dynamic security reasons. A parametric calculation of the annual wind energy rejection percentage, in terms of the installed wind power, the power demand and the maximum allowed wind power instant penetration percentage, is accomplished. The methodology takes into account (i) the wind power penetration probability, restricted by the thermal generators technical minima and the maximum allowed wind power instant penetration percentage over the instant power demand; and (ii) the wind power production probability, derived by the islands' wind potential. The present paper indicates that isolated power systems which are based on thermal power plants have a limited wind power installation capacity-in order to achieve and maintain an adequate level of system stability. For a maximum wind power instant penetration percentage of 30% of the power demand, in order to ensure an annual wind energy rejection percentage less than 10%, the total installed wind power should not exceed the 40% of the mean annual power demand. The results of this paper are applicable to medium and great size isolated power systems, with particular features: (i) the power production is based on thermal power plants; (ii) the power demand exhibits intensive seasonal variations and is uncorrected to the wind data; (iii) the mean annual power demand is greater than 10MW; and (iv) a high wind potential, presenting mean annual wind velocity values greater than 7.5ms^-1 is recorded. Copyright © 2007 John Wiley & Sons, Ltd.

Abstract: Gait Evaluation Differential Entropy Measure (GEDEM) [1] is a new quantitative approach to evaluating the condition of a subject by monitoring the acceleration [2][3]. The approach is capable to identify medical conditions [2, 3] and is also not dependent on age, weight, height or BMI. The initial studies [4][5][6] required the walking on the ground along a straight line for 20-30 seconds. Due to the promising results of the method, work is currently carried out on a treadmill. The treadmill alleviates the constraint of long corridors, and additionally offers better control on the gait velocity, minimum deviation in the coronal plane, and less variability in the GEDEM values [7, 8]. This work investigates the relationship between the gait velocity and the GEDEM. The gait velocity is a measurement parameter that influences the gait acceleration spectrum and therefore it is expected to influence GEDEM values.

Abstract: The electricity production system of the island of Crete is far from optimum; a considerable pert of the energy produced originates from inefficient units burning expensive Diesel oil. Due to large seasonal and daily variations of power demand, future expansion of the production system cannot be realized by cost effective base load units, as these should stop and start on a daily basis. In this paper, the effect of the addition of Pump Storage Systems (PSSs) on the electricity production cost is examined. The PSSs pump water from a low level reservoir by using power produced during the nights from the base load units of the system, which burn inexpensive H.F.O.. Water from the high level reservoir will be exploited by the PSS to produce low cost energy during the peak demand hours, thus replacing operation of diesel oil burning units.