A description and assessment of a small renewable energy community located in Crete, Greece is presented. The community included private residential and agricultural activities without any involvement of the public sector. Small-scale decentralized energy systems were used. Solar energy and solid biomass which are locally available covered most of the heat and electricity requirements in the community. Renewable energy technologies used include solar thermal energy, solar-PV and solid biomass burning utilizing olive tree wood and olive kernel wood. These technologies are mature, reliable, well proven in Crete and cost-effective. Existing energy systems were generating 857,877 kWh per year covering 94.46% of the current energy requirements in the community, significantly reducing its emissions at 278,494 kg CO
2 per year. The addition of a new solar-PV system with nominal power of 33.6 kWp could cover all the remaining electricity needs in the community, transforming it to a zero-CO
2 emission community due to energy use. The total installation cost of the existing renewable energy systems in the community was estimated at 0.16�per total kWh of thermal and electric energy generated annually and at 0.50�per ton of CO
2 emissions saved annually. Results indicated that the creation of the above-mentioned small local energy community is economically viable, environmental friendly and socially accepted. Therefore it could be replicated in other territories with similar availability of renewable energies, increasing their energy autonomy and sustainability.
CO<sub>2</sub> Emissions Crete-Greece Renewable Energy Solar Energy Sustainable Energy Communities1. Introduction
The necessity to mitigate greenhouse gas emissions and to decrease the use of fossil fuels has increased the efforts for utilizing renewable energy (RE) sources, wherever possible. Designing and implementing decentralized RE projects in local societies following the bottom-up approach results in many environmental, energy, social and economic benefits, promoting the target of sustainability in various territories. Recently the idea of local RE communities is supported and promoted in EU countries with various national and regional incentives.
1.1. Renewable Energy Communities
Guidelines for a sustainable energy community (SEC) have been published [1] . According to those guidelines a SEC is one in which everyone works together to develop a sustainable energy system. To do so, they aim as far as possible to be energy efficient, to use RE where feasible and to develop decentralized energy supplies. A viewpoint on community RE has been published by Walker et al., 2008 [2] . The authors investigated the differences between community RE projects and other RE installations. They considered the answer in two questions: who the project is by (process), and who the project is for (outcome), in order to identify the involvement of the local community in the project development and to categorize the project as community RE. Energy autonomy in SECs has been reported by Rae et al., 2012 [3] . The authors stated that generation and utilization of renewable energies have technical, economic and environmental challenges. However their social and political impacts should also be taken into account. Successful existing projects in islands and remote communities provide a useful indication of the challenges and opportunities which are likely to arise in future projects. Lessons from Australia regarding the transition to low-carbon communities have been reported by Moloney et al., 2010 [4] . The authors stated that many community-based programs have adopted an integrated approach, focusing on both technical and behavioral dimensions in the shift to low-carbon communities. They concluded that behavioral changes in order to decrease consumption are an important factor towards the transition to low-carbon communities. Key elements of a RE community have been presented by Carlisle et al., 2008 [5] . The authors outlined five elements needed to create a RE community, which are:
4) Utility role expansion, power generation and load management, and
5) Putting it all together for a RE community
They reported that supplying adequate clean energy to a rapidly industrialized world is one of the 21st century’s greatest challenges. Designing new communities using a RE systems approach could significantly decrease the use of fossil fuels and the emissions of greenhouse gases. A report on social acceptance of RE innovation has been published by Wustenhagen et al., 2007 [6] . The authors stated that social acceptance is an important factor which could constrain the development of RE projects. This is particularly apparent in the case of wind energy, largely due to its visual impacts on landscapes. The public perceptions for community-based RE projects have been reported by Rogers et al., 2008 [7] . The authors stated that community-based RE projects are more likely to be accepted by the public than top-down development of large-scale schemes. According to their investigations using questionnaires, the local community responded positively in the development of community-based RE projects. However the local inhabitants were not keen on developing and leading such a project. A report on sustainable energy communities (SEC) framework has been published, in 2016, [8] . It is stated that the creation of a SEC requires local knowledge, involvement of local citizens and time spent. Sustainable energy authority of Ireland can provide skills development, funding and technical support in order to design and implement a local RE project. The authors also stated that there are 28 members in the sustainable energy authority network in Ireland. An assessment of the impact of RE deployment on local sustainability has been presented by Rio et al., 2008 [9] . The authors stated that deployment of renewable energies has many positive impacts in local communities. Apart from the environmental benefits, socioeconomic benefits are also important. These include diversification of energy supply, enhanced regional and rural development opportunities, creation of a domestic industry and employment opportunities. Community innovation for sustainable energy has been reported by Hielscher et al., 2011 [10] . The authors stated that community-based initiatives are developing new energy-related consumption practices with a view to the socio-technical transition to local, renewable or lower carbon systems. They concluded that community energy approaches are multi-faceted and they are interested in achieving sustainability. Configurations between “the public” and RE have been reported by Walker et al., 2007 [11] . The authors, with reference to the deployment of renewable energies in the UK, outlined five different modes of cooperation between technology and social organization. The five modes of implementation of RE projects in the UK include: public utility, private supplier, community, household and business. Community RE projects’ ownership includes various types of partnerships, cooperatives and user-led initiatives. The barriers and incentives for community-owned means of energy production and use have been reported by Walker, 2008 [12] . The author stated that incentives for community-led energy projects include: Creation of local income, local approval and planning permission, local control, lower energy costs and reliable supply, ethical and environmental commitment and load management. Barriers to those projects include unclear legal conditions under which the projects will operate, uncertain economic and technical viability, need for extensive liaison and expert advice. The author concluded that in other EU countries, like Denmark and Austria, there is a far more significant level of community ownership regarding RE projects than in UK.
1.2. Solar Photovoltaic Electricity
An economic analysis of different supporting policies for the production of electrical energy by solar-PVs in European Union countries has been reported by Dusonchet et al., 2010 [13] . The authors stated that according to their analysis in many cases the differences between the implementation of the same support policy in different countries can give rise to significantly different results. A review on photovoltaic self-consumption in buildings has been reported by Luthander et al., 2015 [14] . The authors examined two options for increased self-consumption. The first included PV-battery systems with battery storage capacity of 0.1 - 1 kWh per installed kWp and the second demand-side management (DSM). They stated that with a battery storage system the relative self-consumption could be increased by 13% - 24% and with DSM by 2% - 15%. An energy and economic evaluation of building-integrated photovoltaics (BIPVs) has been presented by Oliver et al., 2001 [15] . The authors stated that BIPVs offer cost reductions in both energy and economic terms over centralized PV plants. They concluded that BIPVs offer the “double dividend” of reduced economic costs and improved environmental performance. A holistic approach of BIPVs in the UK has been presented by Hammond et al., 2012 [16] . The authors implemented an energy analysis, an environmental analysis and an economic appraisal. Their findings indicated that the PV system generating electricity paid back its embodied energy in 4.5 years. The economic analysis indicated that it is unlikely to pay back its investment over the 25-year lifetime. Vourdoubas, 2016 [17] has reported on the possibility of creating zero-CO2 emissions residential buildings due to energy use with reference to the island of Crete, Greece. The author stated that with the use of locally available renewable energies all the energy requirements of the residential buildings could be covered. He indicated two different combinations of renewable energy technologies (RET) which could achieve this goal. The first included solar thermal energy, solar-PV and solid biomass and the second solar thermal energy, solar-PV and high efficiency heat pumps. Tselepis, 2015 [18] has reported on the PV market developments in Greece regarding net-metering. Implementing case studies for PV installations in a household and a commercial enterprise, he indicated that these investments are currently attractive.
1.3. Solar Thermal Energy for Hot Water Production
Solar thermal markets (STMs) in Europe have been presented by the European Solar Thermal Industry Federation, 2015 [19] . According to this report total solar thermal capacity in Europe has reached 31.8 GWth and the single-family housing still represents the bulk of the STMs. Greek STMs are some of the most important in Europe and current development is mainly due to the growing tourism sector. Total installed capacity in the country has reached 3 GWth. Solar heating and cooling for a sustainable energy future in Europe has been reported by the European Solar Thermal Technology Platform [20] , who claim that solar thermal technology has the potential to cover 50% of the total EU heat demand which corresponds to 49% of the total energy demand in Europe. The required technologies are available or can be developed to cover most of this demand. Solar thermal applications are based on solar energy which is available everywhere and its use for domestic hot water production is popular in various countries. Solar thermal heating for residential applications has been reported by the IEA and IRENA Technology Brief, 2015 [21] . According to this report solar domestic water heating technology has become a common application in many countries. Solar thermal systems are mainly manufactured by local small- and medium-size enterprises which also install and maintain them.
1.4. Solid Biomass Burning for Space Heating
An overview of RE applications in Crete has been reported by Zografakis, 2005 [22] . The author stated that thermal uses of solid biomass in Crete, including applications in olive oil mills, bakeries, greenhouses, hotels and dwellings, contribute 8.5% of the total energy consumption in the island. Investigation of non-technical barriers which might prevent biomass use for energy generation has been presented by Rosch et al., 1999 [23] . The authors stated that apart from the technical problems related with biomass use for energy generation, there are various non-technical problems related with funding, insurance, administrative regulations, organizational complexity and public acceptance which hinder in many cases the promotion of biomass-to-energy projects. An overview of the biomass resource potential for energy generation in Norway has been presented by Scarlat et al., 2011 [24] . The authors stated that although the contribution of RE in Norway is among the highest in Europe, due to hydroelectricity, biomass has a low contribution to the Norwegian energy supply. They suggested that Norway should benefit from the experience gained in Denmark, Sweden and Finland regarding bio-energy use. Vourdoubas, 2015 [25] has reported on a greenhouse used for flower cultivation in Crete, which was heated with olive kernel wood, locally produced in Crete. Olive kernel wood, a by-product of the olive oil producing industry, is a cheap, renewable fuel extensively used for heat generation in the island. All the annual heating needs of the greenhouse, corresponding to 95.31% of its total energy needs, were covered with this biomass source.
The aim of the current work is to describe and assess a small RE community located on the island of Crete, Greece. Various RET used in buildings and other local activities are described together with an estimation of heat and electricity production and consumption. An economic and environmental assessment of the RE community combined with its main characteristics is also presented. The results of this study indicated that the creation of sustainable energy communities utilizing locally available RE and reliable, cost-effective technologies has many economic, social and environmental advantages.
2. Renewable Energy Community2.1. Description of the Small Energy Community in Crete
Renewable energy systems providing heat and electricity have been installed in various small, grid-connected, residential flats and two small greenhouses used for flower cultivation. The buildings and the greenhouses, privately owned, are located in a rural area in the municipality of Platanias, in Western Crete. The aim of this small energy community was to cover nearly all of its energy needs with locally available renewable energies, including solar energy and solid biomass, minimizing the use of fossil fuels. The owners of the residential flats and greenhouses were committed to this aim. The replacement of conventional fuels with renewable energies would result in a significant decrease of CO2 emissions due to energy use in the community. There are fifteen (15) residential flats in the community with a total covered surface of 795 m2. The covered area of the two (2) small greenhouses was 0.33 ha. Energy demand in the residential flats included energy use for space heating and cooling, lighting, hot water production and for the operation of various electric appliances. Energy demand in the greenhouses included energy use for space heating and cooling, lighting and the operation of various electric equipment and machinery. Local climate in mild and average monthly air temperatures in Chania vary between 10.8˚C and 15.8˚C. Energy demand in a typical residential building of this type in Crete is presented in Table 1. Energy demand has been estimated in each sector by dividing the total energy demand by a percentage which is representative for Greek residential buildings according to various studies.
The total annual energy requirements in the fifteen (15) residential flats was 135,150 kWh and their distribution in various sectors is presented in Table 2.
Energy demand in a typical residential building in Crete
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