Creation of 100% renewable energy systems in various countries and areas has been reported by many authors. The possibility of having 100% renewable energy systems in Denmark by the year 2050 has been reported [1] . The authors considered that the target of 100% renewables combined with energy savings in the country by 2050 could be achievable. For electricity generation the main energy sources foreseen by 2050 are wind energy and biomass, and for heat production biomass, heat pumps and solar thermal systems. In the transport sector they foresee that more electric vehicles and trains will be used. The possibility of creating a 100% renewable energy system for Ireland has been reported [2] . The authors studied various scenarios aiming to cover all the energy needs of the country with renewable energies. In these energy scenarios the main energy source used was biomass followed by wind energy, solar thermal energy, wave and tidal energy. Transportation was fueled by electricity, hydrogen and bio-fuels. The authors highlighted the importance of very efficient energy systems like co-gene- ration of heat and power, heat pumps and district heating. The possibility of achieving 100% electricity supply in Portugal has been reported [3] . The authors stated that the energy system in Portugal in 2006 was dependent on fossil fuels at 86%. However the share of renewables in gross electricity production varies between 20% and 35% depending on hydropower production. A 100% renewable energies scenario is based mainly on hydro and wind energy followed by solar energy and biomass. The main electricity storage foreseen is in pump storage systems followed by hydrogen storage and batteries.

Transformation of various islands to 100 % renewables has also been reported and implemented. Energy planning in the Croatian island of Mljet with the use of appropriate software has been studied [4] . The authors investigated the possibility of using local renewable energy resources like solar and wind energy combined with hydrogen storage to increase the penetration of renewable energies in the local system. Studying different scenarios, they indicated that decentralized energy generation in Croatian islands could minimize the use of fossil fuels, also reducing their carbon footprint. The increased use of renewable energy resources in the island of Porto Santo in Madeira, Portugal has been reported [5] . The authors investigated the possibility of using indigenous, intermittent renewable energy resources in the island in order to reduce the use of fossil fuels in it. They reported that the island could be transformed to 100% renewable electricity with the use of solar and wind energy. Electricity storage is needed and they proposed the use of H₂ produced with water electrolysis. H₂ could be used for power generation via fuel cells as well as for transport fuel. The authors stated that reversible hydro and batteries could be used for electricity storage. Higher penetration of renewable energy sources in the island of S. Vicente, Cape Verde has been reported [6] . The island has high wind energy resources and its topography is suitable for using a reverse hydro system for electricity storage. The authors found that wind energy combined with a pump storage system could cover a high percentage of the electricity demand in the island. Wind energy could also be used simultaneously for water desalination covering a large part of the water requirements in the island. A report regarding 35 European smart islands and projects has been published [7] . A smart island is described as an island that creates local sustainable growth, offering a high quality of life to the local people and equipping public power supply and transport systems with the smartest technologies. According to this report there are currently 32 island systems in Greece which are not interconnected to the mainland grid. They are electrified with thermal plants using diesel oil as fuel. The report indicates that in the future solar-PV and wind energy could be used to cover their electricity needs. Renewable energy solutions for islands have been reported [8] . The authors stated, with reference to the programme “Renewislands” that the global development of renewable energy technologies can assure sustainable power supply for islands. To overcome the limitations of intermittent renewable energies, like solar and wind, they proposed the production of H₂ for electricity storage and the use of fuel cells. Electricity generation from renewable energies in small islands has been reported [9] . The author reported that most islands around the world today are dependent on fossil fuels for most of their energy needs, especially for transport and electricity generation. However there are many successful cases of island electrification through renewable energies. Tech- nical and economic analysis of a wind powered pump hydro-storage system in the island of El Hierro, Canary Islands, has been reported by [10] . The authors used an appropriate model to size a wind farm combined with a pump storage system in this island. Their results indicated that the annual wind energy penetration in the electricity grid could be at 68.40%. They also stated that this system was unable to compete economically with the current operating thermoelectric plant.

The possibility of utilizing renewable energies in Gavdos Island has been reported [11] . The authors stated that a hybrid solar-PV and wind power system could cover all the electricity demand and additionally solar thermal energy and solid biomass would provide all the heat demand. The present status and prospects of RES in Greek islands has been reported by [12] . The author stated various applications of solar and wind energy in Greek islands indicating that hybrid systems with electricity storage in pump hydro systems are suitable for local energy generation in them. Energy statistics and renewable energy potential in Greece has been reported [13] . The author stated that in December 2014, 84% of the energy generation in non-interconnected islands came from thermal power plants, 13% from wind parks and 3% from PV stations. She also stated that pump storage systems are the cheapest and most convenient way to store electricity. The application of renewable energy resources in the Greek islands of the South Aegean has been reported [14] . The authors stated various applications of renewable energies in this area, concluding that the Hellenic islands of the south Aegean present a high potential for application for renewable energy sources which should be further developed. The sustainable management of renewable energy installations and their impact with reference to the small Greek islands has been reported [15] . The authors stated that small Greek islands constitute sensitive ecosystems and this fact imposes limitations on the development of energy generation plants including renewable energies. A wind power fuel cell hybrid system on the non-interconnected Aegean island grid has been reported [16] . The hybrid system is comprised of a wind turbine, an electrolyzer, a H₂ storage unit and a fuel cell. The authors stated that part of the energy generated by the wind turbine could be stored in the form of H₂ and then it could be delivered for consumption at constant power through the fuel cell. Optimization of the energy production system in the Dodecanese islands, Greece has been presented [17] . The authors investigated the exploitation of renewable energies, particularly solar and wind energy, in the Dodecanese islands having an autonomous electric grid. Examining three different scenarios they estimated the optimum penetration of renewable energies in the electric grid. They concluded that solar-PV could generate 5% of the total electricity needed, wind turbines 40% and thermoelectric plants the remaining 55%, reducing the current electricity cost by 7.6%.

Energy consumption in Mediterranean island hotels has been reported [18] . The authors stated that in Balearic islands hotels energy consumption varies between 7 up to 77 KWh per night spent (n.s.) with an average of 15.4 KWh/n.s. Similar studies in Greek islands have estimated the energy consumption from 5 to 25 KWh/n.s. with an average at 15.4 KWh/n.s. The possibility of creating zero CO₂ emissions residential houses and hotels due to energy use in Crete has been described [19] and [20] . The author has indicated that the combined use of solar thermal energy, solar-PV energy, solid biomass and ground source heat pumps could result in zero carbon emissions buildings due to energy use in the island. The possibility of increasing the sustainability of small-scale hotels in Greece has been reported [21] . The author suggested that following an energy audit the hotel owner could invest in various sustainable and cost-effective energy technologies which would increase its environmental sustainability, resulting at the same time in financial benefits.

The purpose of this study was to investigate the possibility of zeroing the CO₂ emissions in Gavdos Island due to energy use. Gavdos Island has been chosen due to its following characteristics:

a) It is a small island with few permanent residents and many tourists during the summer period,

b) Its electric grid is isolated from the grid of Crete,

c) There are abundant renewable energy resources which could be used for electricity and heat generation with reliable and cost effective technologies, and

d) Due to the small size of the island its transformation to a zero CO₂ emissions island is technically easy and economically feasible. It could be used as a demonstration example of a sustainable energy island in Greece.

The current energy consumption in the island has been estimated as well as the fuels used including fossil fuels and renewable energies. Renewable energy technologies which are locally available, mature and cost-effective are reported. Technologies which could be used for covering all its energy needs replacing fossil fuels have been indicated.

Gavdos Island is rich in solar and wind energy. Small quantities of solid biomass are also available in the island. The intermittent nature of solar and wind energy necessitates the use of an electricity storage system. Existing renewable energy sources in Gavdos could be used for:

1) Electricity generation,

2) Space heating and cooling,

3) Hot water production, and

4) Charge of electric batteries which could be used in electric vehicles.

This is presented in Table 2.

In order to use only local renewable energy resources for covering all the ener- gy needs in the island, it is necessary to:

a) Replace the LPG used for cooking with electricity,

b) Use electric cars with batteries instead of conventional cars,

Source: P.P.C.

Source: Own data.

Source: Own data.

c) Use electric bicycles for transportation or conventional cars using bio-fuels,

d) Use the abundant solar thermal energy for hot water production, and

e) Permanent inhabitants in the island will utilize local biomass or individual heat pumps for space heating in the winter.

The replacement of vehicles using conventional fuels with electric vehicles is currently costly and it could be achieved gradually in few years. Solar thermal energy, solid biomass and heat pumps are currently used in Gavdos.

Renewable energy technologies which could be used for covering all the energy needs in Gavdos Island are presented in Table 3.

The required renewable energy technologies which could be used in Gavdos Island and would result in an island with zero carbon footprints are well proven, mature, reliable and cost-effective. All of them have been used commercially in many cases with good results. However the use of electric cars with batteries in the island needs consensus among the permanent residents and political will from the local authorities restricting the use of conventional cars from visitors. However this could be achievable under some conditions. Water electrolysis for hydrogen production with the use of solar and wind energy is achievable. Hydrogen could be used either for electricity generation with fuel cells or for transportation with electric cars equipped with fuel cells.

However, since this technology is currently neither mature nor cost-effective, it has not been considered for application in Gavdos Island. This could change though in the near future.

In order to estimate the annual electricity requirements in the island, the following assumptions have been made.

a) Electricity will be used for cooking in residential buildings and in commercial shops, replacing LPG. It will be used also for lighting and the operation of various electric devices,

b) Solar thermal energy will be used for hot water production,

c) Solid biomass will be used for space heating in the residential buildings during the winter,

d) Annual electricity consumption would be at 1000 KWh per permanent inhabitant,

e) Each tourist may remain in the island for five (5) days on average,

f) Each tourist will need 10 KWh per night spent on the island [18] ,

g) An electric car consumes 0.1 KWh per Km [22] . Annual electricity consumption for transportation in the island would be 5000 KWh, and

h) Although it is not discussed in this paper the implementation of energy efficiency measures in households and enterprises is considered a prerequisite for the zeroing of the carbon footprint in Gavdos Island.

It is considered that these assumptions are realistic and they could be used for the purposes of the current preliminary study.

Therefore the annual electricity consumption by the permanent inhabitants would be 154,000 KWh, for transportation 5000 KWh and from tourists 700,000 KWh, in total 859,000 KWh. Assuming that during the use of diesel oil for electricity generation, 0.27 kg CO₂ per KWh are emitted, it is concluded that the use of a solar-PV system generating all the required electricity in Gavdos Island could result in annual CO₂ savings of 231.9 tons.