Δρ. Βασίλειος Ψυλόγλου Εντεταλμένος Ερευνητής, Ινστιτούτο Ερευνών Περιβάλλοντος και Βιώσιμης Ανάπτυξης, Εθνικό Αστεροσκοπείο Αθηνών Συγκριτική μελέτη των χαρακτηριστικών κατανάλωσης ηλεκτρικής ενέργειας για τις πόλεις της Αθήνας και του Λονδίνου ΤΕΙ ΠΕΙΡΑΙΑ, Τμήμα Φυσικής, Χημείας και Τεχνολογίας Υλικών, 21 Οκτωβίου 2009.

Factors that Influence Energy Consumption Human Activities - Habits Prevailing Meteorological Conditions (Air Temperature & Rel. Humidity, Wind, Solar radiation Intensity) Data Availability Total Hourly Residential & Commercial Energy Consumption (kWh), spanning the period from January 1997 to December 2001 Source: Public Power Corporation of Greece, National Grid Transco of UK Mean Hourly Values of Meteorological Parameters (Air Temperature & Rel. Humidity, Wind, Solar rad. Intensity), for the same period Source: National Observatory of Athens, Greece United Kingdom Meteorological Office.

Daily Mean Air Temperature, Normalized Energy Consumption and GNP/capita for Athens, Greece, for the period GWh / GNP per capita Peaks in energy demand are linked with extremes in temperature

Daily Mean Air Temperature, Normalized Energy Consumption and GNP/capita for London, UK, for the period GWh / GNP per capita Seasonal trend in energy demand is apparent in both cities influenced by the prevailing weather conditions

Daily Mean Air Temperature and Normalized Daily Total Energy Consumption (GWh/GNPpc), for Athens, Greece, for the year Energy consumption presents a clear WEEKLY VARIATION with MIN values on Sundays and MAX values close to midweek. This pattern is occasionally interrupted on isolated holidays falling in the middle of the week, as well as during Christmas and Easter vacations and also in August for the Athens case only.

Daily Mean Air Temperature and Normalized Daily Total Energy Consumption (GWh/GNPpc), for Athens, Greece, for the year Easter Holidays Summer Holidays 15 th of August The festive Christmas period Increase in energy consumption mainly due to the extensive use of air-conditioning Lowest temperatures MAX values in energy consumption Transient season periods

Daily Mean Air Temperature and Normalized Daily Total Energy Consumption (GWh/GNPpc), for London, UK, for the year Easter Holidays The festive Christmas period Warmer period with no need of air-conditioning Lowest temperatures MAX values in energy consumption

Daily, Weekly, Monthly Variation of Energy Consumption Monthly Seasonal Variation Index (MSVI) Ε ij : monthly energy consumption for month i in year j Index for month i in year j MSVI ij = E ij / E j Ε j : monthly average consumption for year j The use of the MSVI index has the advantage that different levels of Energy consumption can be compared !

MAX values of energy consumption are related to the appearance of the lowest temperatures. Need for space-heating. The major part of the population of the greater Athens area is on summer vacation. Increasing trend in electricity demand, mainly due to extensive use of air-conditioners. Need for space-cooling. Increased energy requirements during the festive Christmas period.

MAX values of energy consumption are related to the appearance of the lowest temperatures. Need for space-heating. Increased energy requirements during the festive Christmas period. Similar behavior with an exception: the lack of need for air-conditioning use keeps energy demand in low levels.

Daily Seasonal Variation Index (DSVI) Ε ijk : energy consumption for the particular day k Index for day i, of week j, of year k DSVI ijk = E ijk / E jk Ε jk : mean daily consumption for week j, of year k Daily, Weekly, Monthly Variation of Energy Consumption The use of the DSVI index has again the advantage that different levels of Energy consumption can be compared !

Electricity consumption also decreases on the holidays that fall between Monday and Friday and also on the days that follow a holiday or are placed between two holidays (due again to the inertia) Energy consumption is lower during weekends (especially during Sundays) due to the reduced economic activity. Lower levels of energy consumption are also present on Mondays because of the inertia caused by the reduced economic activity during the weekends

Daily Seasonal Variation Index (DSVI) – Seasonal Characteristics Daily, Weekly, Monthly Variation of Energy Consumption Energy consumption is higher during week-days in JULY. This reflects the fact that during weekends in JULY, people are away from the offices/houses and tend to go outdoor or out of the city boundaries. The opposite holds true for JANUARY. In JANUARY we have higher values during week-ends.

Hourly Seasonal Variation Index (HSVI) Ε ijk : electricity consumption for a certain hour Index for hour i of the average 24-hours period, of month j, in year k HSVI ijk = E ijk / E jk Ε jk : mean monthly energy consumption for month j in year k Daily, Weekly, Monthly Variation of Energy Consumption The use of the HSVI index has again the advantage that different levels of Energy consumption can be compared !

During the working hours of the day, there is an extensive use of electricity both for household (cooking, heating) and business (office heating, server and PC usage) needs. During the late afternoon and early evening hours, there is a second max due to the use of lighting and heating/cooking using additional heaters or air-conditioners. In London, energy demand is kept constantly high during office hours, i.e. from around 9am to 7pm with two less pronounced maxima, one around midday and the second in the early evening hour.

Hourly Seasonal Variat. Index (HSVI) – Seasonal Characteristics Daily, Weekly, Monthly Variation of Energy Consumption In Athens, during JANUARY, there are two distinct maxima: one close to midday (cooking, heating, office needs) and another in the evening (lighting, extra heating, home entertainment) as most people stay indoors. During JULY, there is a midday max which occurs at the same hours as the January one, and an evening max which occurs much later and is much less pronounced. In the late afternoon/early evening hours, there is a decrease in demand reflecting the fact that most people like to stay outdoors and return home late. In London, during JANUARY there is only one distinct peak in the evening hours. In JULY, the evening peak disappears, but demand is kept high throughout the daylight hours and especially around midday.

Relationship between Energy Consumption and Air Temperature, for Athens, Greece. The relationship between energy consumption and air temperature is NOT linear. On the contrary, presents two maxima and one minimum around 22 o C. However, there are certain limits beyond which energy consumption does not increase further. Around 22 o C there exists an area where energy consumption shows no sensitivity to air temperature. Outside this area, consumption increases with the increase or decrease of air temperature (space-heating/cooling needs).

Relationship between Energy Consumption and Air Temperature, for London, UK. London, UK, also exhibits a NON linear relationship between air temperature and energy consumption BUT with only one minimum and one well defined maximum. The minimum values of energy cons. appear to be around 16 o C and this is the temper. for CDD calculations. Above 16 o C, energy levels for London tend to rise slightly but below this temperature energy levels increase significantly (space-heating needs).

Conclusions This work presents a comparative study of energy demand and thermal comfort levels for two cities: Athens, Greece and London, UK, covering the period For both cities energy demand peaks in winter but for Athens a second significant peak is apparent in the summer, which is not found in London. The winter peak is associated with low temperatures in both cities In Athens, the summer peak is linked to high temperatures and increased air-conditioning use Both cities exhibit lower energy demand levels during public holidays and weekends (especially Sundays and summer weekends) The hourly variation of energy demand levels has two maxima in Athens (around midday and evening) but is constantly high throughout office hours for London. The main advantage of MSVI, DSVI, and HSVI indexes use is that different levels of energy consumption can be compared !

Acknowledge : This work was supported by EU projects ENSEMBLES and CIRCE THANK YOU FOR YOUR ATTENTION

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