1. Introduction

The average total energy consumption per capita in the 28 European (EU-28) member states in 2015 was 2.13 Mtoe/PC and particularly in Greece 1.51 Mtoe/PC [1], [2]. The average electricity consumption per capita was 0.46 toe/PC (5,395 kWh/PC), in the EU-28 and 0.40 toe/PC (4,677 kWh/PC), in Greece. The mean conversion factor of electricity into primary energy (primary/electricity), based on the Greek energy balance data for the period 2005-2015 [1], [2], is estimated at about 2.46, with a minimum of 1.81 observed in 2015.

The total energy consumption in Greece in 2015 was 16.44 Mtoe (191,162 GWh), 25% lower than the record energy consumption of 2007 that was 21.96 Mtoe (255,380 GWh) [1], [2]. During this period (2007-2015), the total energy consumption dropped constantly (Fig. 1); also did the energy consumption in all sectors (industry, transportation, buildings, etc.) [1], [2]. In the EU-28 Fig. 2), the total energy consumption in 2015 was 1,083 Mtoe (12,589 TWh), 7.8% lower than the record energy consumption of 1,174 Mtoe (13,649 TWh) in 2007 [1]. Buildings (residential and of the service sector) contribute significantly to the total energy consumption in every EU-28 member state. In 2015 this share corresponded to 38.9% in the EU-28 and 38.2% in Greece [1], [3].

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Fig. 1. Evolution of final energy demand per sector in Greece (1990-2015) [1], [2].

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Fig. 2. Evolution of final energy demand by sector in EU-28 (1990-2015) [1].

Moreover, the share of residential buildings on the total final energy consumption is 25.3% in the European countries and 26.8% in Greece [1], [3].The residential sector was responsible for 16.8% of the total greenhouse gas emissions in 2014 in Europe, that is 605 Mt CO₂ (out of 3,603 Mt CO₂) and 21.5% in Greece, that is 17.7 Mt CO₂ (out of 82.5 Mt CO₂) [2], [3].

In Greece, the annual total energy use in the residential sector dropped from 5,329 ktoe in 2007 to 3,763 ktoe in 2013 and rose to 4,401 ktoe in 2015, i.e. by 29.4% and 17.4%, a reduction strongly correlated with the Greek government debt crisis of 2009 [2]. As shown in Fig. 3, the highest decrease (approximately 64% in 2013) was for diesel oil, attributed to its significant cost increase in 2013 and 2014 by 50%, due to taxation. At the same time, the consumption of natural gas and biomass in residential buildings increased slightly during 2011-2015; however, this increase was much lower than the decrease of diesel oil consumption during the same period [2].

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Fig. 3. Annual actual energy consumption per fuel for residential sector in Hellas (kWh.m⁻².yr⁻¹), in 2007-2015 [2].

The main energy sources in Greek households are diesel oil and electricity (Fig. 3); in 2007 diesel oil consumption accounted for 50.5% of the total energy use, i.e. about the double of electricity consumption. This picture changed dramatically in 2013 for the first time, when diesel oil consumption corresponded only to 26.2% of the total energy use, while the share of electricity rose to 42.2%. However, the actual electricity consumption in Greek households did not change significantly over time and is about 17,800 GWh per annum [2]. The use of renewable energy sources (RES) in Greek households increased between 2011 and 2013; biomass holds the largest share, exceeding 10,000 GWh in 2012 (Fig. 3).

Correspondingly, in 2015 (Fig. 4), the EU-28 households covered their needs using mainly natural gas (35%) and electricity (25%) (97.4 ktoe and 68.4 ktoe respectively, while the total energy consumption was 257.15 ktoe) [3]. The highest percentage per fuel use in households is 58.1% for RES in Montenegro, 71.3% for natural gas in the Netherlands, 77.9% for derived heat (district heating) in Iceland, 82.6% for electricity in Norway, 38.2% for oil products in Ireland and 33.0% for solid fuels in Poland [3].

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Fig. 4. Distribution (%) of annual actual energy consumption per fuel for residential sector for EU-28 (left) and Greece (right), in 2015 [3].

Many literatures focus on the energy consumption of residential buildings, actual or estimated and on CO₂ emissions [4], [5], [6], [7], [8], [9], [10], [11]. The building energy consumption in less developed countries has already exceeded that of the developed and the need for promoting building energy efficiency policies is evident [4], [5], [6], [7], [8], [9]. Residential sector corresponds to the higher percentage of energy consumption in the building sector, while the development and urbanization are associated with increased electricity use in buildings that will significantly increase primary energy demand [5], [8], [9], [10]. European countries and the USA consume mainly natural gas and electricity, China and India biomass and Japan consumes oil [5], [8]. The residential sector in Japan and Canada present the world highest specific energy consumption (expressed in kWh.m⁻².yr⁻¹), while Russia and the USA follow [4], [6], [7]. The residential sector in the USA, China and the EU-28 corresponds to the higher energy consumption per capita [6]. In 2011 the residential sector corresponded for 27% of the global energy consumption and to 17% of the global CO₂ emissions [10], [11].

1.1. Energy Performance and Classification of Residential Buildings

The European Commission tried to reduce the final energy consumption in the residential sector by adopting the European Directive 2002/91/EC on the energy performance of buildings (EPBD) and the EPBD recast (Directive 2010/31/EC) [12], [13]; both directives aimed at improving the energy efficiency of the building stock in Europe. These directives provided the member states with the basic guidelines for defining the minimum requirements and specifications for buildings; they also imposed the energy audit and the Energy Performance Certification (EPC) of buildings. The EPCs distribution per energy label (A, B, C, D, >D) for the residential buildings among the EU-28 shows that most buildings in Europe rank within the energy label D and less performing, but most of the new buildings rank within the energy labels B or A [14].

In Greece a new Regulation for the Building Energy Performance (REPB) [15], in line with the European Directive 91/2002, became effective in 2010. It replaced the Regulation of Building’s Thermal Insulation (TIR), effective since 1979 that regulated only the overall heat transfer coefficient values of the building elements and the building envelope [16]. The Greek residential buildings constructed before this first regulation, TIR, have no insulation. About 13% of the total number of Greek residential buildings in 2016 dispose an EPC.

The REPB imposes minimum energy performance requirements for buildings and classifies them into nine energy classes (A+, A, B+, B, C, D, E, F, G) by comparing their energy performance with that of a reference building. The reference building is similar to the examined one, but meets minimum requirements [15]. New and renovated buildings must be at least in class B, with some technical or operational exceptions for renovated buildings. In residential buildings this energy classification considers the primary energy consumption for space heating, cooling, and domestic hot water (DHW). The Greek residential buildings constructed before 1980 in compliance with the regulations of that time, are usually classified into lower classes (G, F, E or D) depending on the current situation of the building envelope and their electrical and mechanical equipment [17], [18]. Similarly, residential buildings constructed after the TIR and before the REPB, are usually classified in classes ranging between E and C, depending again on their current situation [17], [18].

According to the data of the Ministry of Environment and Energy [19] and based on information derived from the EPC issued up to March 2016, 99.5% of residential buildings constructed before the TIR, 94.5% after the implementation of the TIR (1980-2010) and 57.3% constructed or renovated according to the REPB (after 2010), are all classified in energy classes between C and G (Table 1).

Table 1. Percentage (%) of buildings certifications per energy class constructed or renovated before 1980 (before the TIR), 1980-2010 (according to the TIR) and after 2010 according to the REPB [18], [19].

Regulation / Construction period	Primary Energy Consumption*	Percentage (%) of buildings per Energy Class
	(kWh•m−²•yr−¹)	Α+	Α	Β+	Β	C	D	E	F	G
No regulation (before 1980)	175-525	0.0	0.0	0.1	0.5	5.0	11.5	15.6	21.9	45.4
TIR Regulation (1980-2010)	120-350	0.0	0.1	0.4	4.0	27.4	32.8	18.0	7.8	9.6
REPB Regulation (after 2010)	50-270	0.1	0.7	7.5	34.5	37.6	14.6	3.4	0.9	0.8

⁎

Space heating, cooling and DHW.

The European energy regulations succeeded in lowering the energy consumption of residential buildings [10], [18], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. There was also a positive effect on buildings’ energy performance and reduction of greenhouse gases (GHG) emission, through the implementation of appropriate policies and action plans [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]. However, significant differences between the actual energy performance and the one calculated by models (i.e. according to European Directive for Energy Performance Buildings – EPBD, etc.), have been observed [6], [45], [46], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66]. The comparative assessment of the differences between actual and calculated specific energy consumption (kWh.m⁻².yr⁻¹), needs to take into account several parameters, such as the building construction type [37], [39], [43], [58] building elements (envelope materials, electromechanical (E/M) systems etc.) [55], [57], [58], [59], [60], climate, occupancy profile (hours per day, day per week, internal climate conditions etc.), behavior of the occupants (biological, psychological and socioeconomic parameters, physical environment influences etc.) [54], [63], [64], [65]. It is not always easy to quantify the effect of each of these parameters, on the energy consumption of a building.

Monitoring of the actual energy consumption and detailed documentation of the quality or characteristics of the building stock are essential for the best estimate of the expected benefits, under various energy saving scenarios [36], [37], [38], [39], [43], [46]. However, the results of many studies are strongly biased by the absence of reliable statistical data, whereas the international research confirmed the gap in the availability of such information on a global scale [4], [5], [6], [7], [37], [41], [42], [43], [44], [45], [46], [47], [67].

According to the Eurostat, the final average annual specific energy consumption for all types of residential buildings in the EU-28 was about 158.8 kWh•m⁻²•yr⁻¹ in 2014 (Fig. 5), 10.5% lower than the recorded annual specific average energy consumption in 2011, that was of about 184.0 kWh•m⁻²•yr⁻¹ [68]. The final energy consumption however, differs among countries. For example in 2014 (Fig. 5), the lowest was recorded in Malta (45.6 kWh•m⁻²•yr⁻¹), Portugal (66 kWh•m⁻²•yr⁻¹) and Cyprus (65 kWh•m⁻²•yr⁻¹), and the highest in Romania (281.6 kWh•m⁻²•yr⁻¹), Latvia (273.3 kWh•m⁻²•yr⁻¹) and in Estonia (261.6 kWh•m⁻²•yr⁻¹), values significantly higher than the EU-28 average. The average annual specific energy consumption in Greece was 112 kWh•m⁻²•yr⁻¹, in 2014, 31.4% lower than the record average energy consumption of 2011 and 25.9% lower than that of 2012 [68].

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Fig. 5. Annual specific final energy consumption for residential buildings (kWh.m⁻².yr⁻¹) for the EU-28, in 2014 [68].

This paper presents the results of the analysis of the available data related to the energy and technical characteristics of the existing Greek residential building stock, including the actual energy consumption. The data used here were based on primary data of the Hellenic Statistical Authority [69], [70], [71], [72], [73], [74], the Ministry of Environment and Energy [2], the Eurostat [1], [3], the Hellenic Electricity Distribution Network Operator S.A. [75]; results of similar studies [18], [37], [60], [76], [77], [78], [79], [80], [81], [82], [83], [84] were also taken into account. The potential for energy conservation and CO₂ reduction, for the Greek residential sector, assuming that the REPB will be applied in all non-insulated buildings, were also estimated [15]. The percentage of space heating energy conservation, for all climate zones and the various residential buildings types, is estimated based on the Heating Degree Hours (HDH) method.

2. Building Stock of Greek Residential Sector

According to the 2011 census [70], buildings with a residential or mixed type, but mainly of residential use, were about 79.1% of the building stock in Greece, corresponding to 3,246,008 buildings. According to the REPB, Greece is divided in four climate zones (Fig. 6), according to the number of calculated Heating Degree Days [15], [85], [86]. From the 3,246,008 buildings, 20.8% belong in climate zone A, 47.3% in B, 27.3% in C and only 4.6% in zone D.

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Fig. 6. Climate zones according to Hellenic Regulation on the Energy Performance of Buildings [18].

About 64.7% of the dwellings are used as main residences having a continuous occupancy throughout the year. The remaining are summer residences, either for rent or for own use. The total number of dwellings in 2011 was 6,371,901 of which 4,122,088 were main residences [70] and only these were taken into account in this work.

The share of the various buildings’ types varies depending on the construction year, the newest being in their majority apartment buildings (Fig. 7).

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Fig. 7. Evolution of building construction by residential building type and construction period in Greece [70].

3. Energy Characteristics of Greek Residential Buildings

The building census of 2011 [69], [70] revealed also some qualitative technical characteristics of the building envelope (Table 4). The bearing structure of residential buildings is mainly reinforced concrete, brick and concrete blocks, as well stone. The share per structure type differs among climate zones and depends on the available materials in each region and the construction traditions (Fig. 8). Also, 57.2% of the residential buildings have an inclined roof, mainly with ceramic tile cladding. However, more than 75% of buildings in zones C and D have inclined roofs and about 70% in climate zone A have flat roofs.

Table 4. Number and percentage (%) of residential buildings, per type of building surfaces element, for 2011.

Total Number of Residential buildings: 3,246,008
Residential buildings	Number	Percentage (%)	Residential buildings	Number	Percentage (%)
Bearing structure made of			Roofs type
Reinforced concrete	2,048,168	63.1	Flat roof	1,389,450	42.8
Bricks/concrete blocks	565,772	17.4	Inclined roof	1,856,558	57.2
Stone	561,634	17.3	Tiles	1,699,248	91.5
Wood	12,743	0.4	Cover sheets	113,710	6.1
Metal	9,107	0.3	Other material	43,600	2.3

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Fig. 8. Number of Greek residential buildings per type of bearing structure and climate zone.

The first Regulation [16] of Building’s Thermal Insulation (TIR) became effective in 1979, which provided requirements for the overall heat transfer coefficient of the building elements and the building envelope, depending on the climate zone. The next regulation [15] for the building energy performance (REPB) effective since 2010, is in line with the European Directive 91/2002 and has imposed stricter requirements for the overall heat transfer coefficient and the thermal performance of building envelopes by 14% and up to 43%, depending on the climate zone [15], [17], [18], [87], [88], [89]. However, a high percentage of dwellings constructed after 1980 do not have any type of insulation, mainly in the walls (Fig. 9). The 2011 census [70] revealed that only 22.4% of the Greek dwellings (23.8% of the main residences) have thermally insulated walls and 42.7% (48.5% of the main residences) have double glazed windows, (Table 5).

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Fig. 9. Number of dwellings per type of thermal protection element and construction period [70].

Table 5. Number and percentage (%) of Greek dwellings, per type of thermal protection of their envelope, as of 2011 ^[70].

	Total dwellings		Permanent dwellings		Empty dwellings
Envelope type	Number	Percentage (%)	Number	Percentage (%)	Number	Percentage (%)
Total dwellings	6,371,901		4,122,088		2,249,813
Double glazed windows	2,723,017	42.7	1,999,510	48.5	723,507	32.2
Thermally insulated walls	1,428,545	22.4	982,007	23.8	446,538	19.8
Thermally insulated roof or/and floor	492,577	7.7	302,651	7.3	189,926	8.4
None insulation	2,903,594	45.6	1,681,365	40.8	1,222,229	54.3
Unknown type	1,547,185	24.3	1,156,065	28.0	391,120	17.4

The share of buildings without any type of thermal protection (double glazing or insulation), is estimated of about 45%-55% in climate zones A, B and C and 25% in climate zone D [69], [70].

Regarding space heating (Table 7), 67.7% use diesel oil or natural gas fired central heating systems (individual or collective), 20.3% use other space heating systems (local low thermal power systems, electric heater, heat pumps, heaters etc.) and 12% do not have or use any heating [70]. The above share differs between climate zones and building use (main residences or unoccupied buildings). Climate zone A has the higher percentage, 25.4%, of unheated dwellings; this percentage is higher, about 40%, for the unoccupied dwellings in the same climate zone. The share of dwellings per space heating system also varies, depending on their construction year (Fig. 10). 70% of dwellings constructed after 1960 have a central space heating system (individual or collective), while this percentage reaches 90% for dwellings constructed after 1990.

Table 7. Percentage (%) of dwellings, per type of heating system and climate zone, for 2011.

	Zone A	Zone B	Zone C	Zone D
Total buildings	983,929	3,400,315	1,747,392	240,265
Central heating system (autonomous or not)	45.1%	74.2%	67.5%	71.1%
Other heating system	29.1%	15.5%	24.3%	21.9%
Without heating system	25.9%	10.3%	8.2%	7.0%
Total of permanent (inhabited) buildings	556,790	2,245,581	1,157,768	161,950
Central heating system (autonomous or not)	57.5%	82.8%	76.7%	79.6%
Other heating system	28.5%	13.0%	21.8%	19.5%
Without heating system	14.0%	4.2%	1.5%	0.9%
Total of empty buildings	427,139	1,154,734	589,624	78,316
Central heating system (autonomous or not)	29.8%	57.5%	49.3%	53.4%
Other heating system	31.0%	20.5%	29.4%	26.8%
Without heating system	41.9%	22.0%	21.3%	19.8%

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Fig. 10. Percentage (%) of dwellings, per type of heating system and building year construction, as of 2011^[5].

4. Energy Use in Greek Residential Buildings

The energy consumption per type of fuel depends on the availability of several fuels per climate zone. Natural gas is not available in climate zones A and D, as well in a lot regions of climate zone B and C. Also, the consumption of biomass is higher in the coldest climate zone D and in the semi-urban and rural regions [2], [70], mainly due to its lower cost compared to other sources.

The analysis of available energy consumption data [1], [2], [69], [70] shows that the Greek dwellings use:

a)

primarily diesel oil for space heating in all climate zones, followed by biomass mainly in climate zone D,

b)

primarily electricity for DHW production in zones B, C and D, followed by solar energyand by diesel oil mainly in zones C and D (Table 9). Especially in zone A, the DHW production uses mainly renewables and electricity follows (Table 9).

Table 9. Percentage (%) of main residences per energy source that use for space heating and domestic hot water, per climate zone, in 2011.

	Climate Zone
	A	B	C	D
Total permanent inhabited dwellings	564,599	2,247,314	1,168,944	141,231
Source of energy for space heating
Electricity	16,4%	8,5%	6,0%	2,4%
Natural gas	0,0%	8,7%	13,3%	0,0%
Diesel Oil	51,9%	71,4%	65,2%	62,4%
Biomass	9,1%	2,5%	9,1%	14,5%
Other energy	8,5%	4,5%	4,8%	19,7%
None energy source (no heating)	14,1%	4,4%	1,6%	0,9%
Source of energy for Domestic Hot Water
Electricity	41,8%	54,1%	45,5%	41,2%
Natural gas	0,0%	2,0%	10,7%	0,0%
Diesel Oil	3,8%	7,0%	18,2%	17,9%
Solar energy	50,3%	34,9%	20,8%	26,7%
Other (biomass etc.) / None energy source	4,1%	2,0%	4,7%	14,2%

The thermal energy consumption data in Table 10 are weighted average values for the main inhabited residential dwellings, as of 2011, insulated or not, single-family houses or apartments, consuming oil and natural gas for space heating. Non-insulated buildings, particularly in the colder climate zones C and D [73], [74], [81] usually present much higher thermal energy consumption than the average of the corresponding climate zone (Table 10). Similarly, many new buildings present much lower thermal energy consumption. Generally, single-family houses have a higher specific thermal energy (kWh•m⁻²•yr⁻¹) than the apartment buildings [4], [6], [30], [33], [28], [54], [69], [70], [71], [72], [73], [74] because of their higher A/V ratio value. The cooling needs are very low and are usually covered by local heat pumps (split units), operated occasionally. Even in zone A, the cooling needs are limited to the days with very high ambient temperature.

The space heating thermal load Q_heating (kWh/yr.) of a building can be easily estimated by the HDHs, with sufficient precision, according to Eq. [1]

with U_m the maximum allowed overall heat transfer coefficient of the building elements (W•m⁻²•K⁻¹), Α the total area of the building elements surrounding the conditioned spaces (m²), V_air the fresh air intake (infiltration and/or natural/technical ventilation) per hour (m³•hr⁻¹), c_p the specific heat of air (1,0 kJ•kg⁻¹•K⁻¹), ρ the air density (1,204 kg•m⁻³) and F_othe coefficient of intermittent operation of space heating system [90].

The value of actual HDHs obviously differs among climate zones, but it is also related to the occupancy profile of the buildings (space heating operation time). Taking into account the mean HDHs value per climate zone and for three typical occupancy profiles for Greek residential buildings, namely the 24-hrs, 18-hrs (15:00 pm to 9:00 am) and 14-hrs (17:00 pm to 7:00 am) [91], the thermal (space heating) energy consumption (kWh_th•m⁻²•yr⁻¹) was estimated for 144 residential building types (Fig. 11). These are combinations of twelve different floor plans (rectangular, angular etc.) and one up to twelve stories’, corresponding to a range of A/V ratio values. Also, they are considered to have openings with low airtightness and non-insulated external envelope opaque elements. The setup indoor temperature is 20°C. Space heating is provided via a diesel oil fired boiler having an efficiency of 87%, with partially insulated distribution network. It was found that the energy consumption is almost proportional to the A/V ratio for all occupancy profiles and climate zones (Fig. 11). The highest A/V values correspond to the single-family houses and the lowest to the 12-storey apartment buildings.

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Fig. 11. Annual space heating energy consumption of non-insulated residential buildings (kWh_th•m⁻²•yr⁻¹), per A/V ratio, per climate zone and for three occupancy profiles: (a) 24-hours, (b) 18-hours and (c) 14-hours.

The estimated (Eq. [1]) annual space heating thermal energy consumption for non-insulated residential buildings (Fig. 11) is much higher than the weighted average actual thermal energy consumption of all residential buildings (Table 10). This difference is much higher for buildings with high A/V ratio (mainly single-family houses), for all occupancy profiles, even for the 14-hours profile (Fig. 11.c).

It should be noted that, in case of energy performance calculations using the national calculation tool for building energy performance “TEEKENAK” (the REPB regulation calculations software) and the National Technical Guidelines based on the quasi-steady state monthly method EN ISO 13790 [86], [87], [88], the space heating thermal energy consumption of the 144 residential buildings is estimated, in many cases, to about 10-25% higher than that calculated by Eq. 1, even for the most energy demanding scenario of 24-hrs occupancy profile. The highest differences are observed for the apartment buildings; for single-family houses the difference is smaller. However, in some cases in climate zones A and B, the situation is reversed, i.e. the HDHs results are higher than those of the EN ISO 13790 method. The above percent difference becomes much higher considering that the energy performance calculation, according to the EN ISO 13790, takes into account the building internal gains, including the overall solar gains by the building elements (opaque and transparent). These solar gains reduce space heating requirements by 20% to 45%, depending on the type (single-family or apartment building), on the existence of insulation, on the percentage of transparent elements and the available solar radiation in the region.

Generally, the actual energy consumption of Greek residential buildings is directly related to the occupants’ behavior, as well as to construction and financial parameters like the:

•

actual occupancy profile of buildings during the day;

•

operating profile of electromechanical systems; space heating or cooling systems, DHW production system, electric devices etc.;

•

energy performance of space heating or cooling systems, which depends on their age and the maintenance;

•

thermostat control for the space heating and cooling systems operation;

•

actual state of the structural and electromechanical elements of building;

•

total floor area and the possibility of space heating or cooling of part of the building;

•

energy sources used, including renewable energy systems (solar collectors, photovoltaics panels, etc.);

•

available energy sources, as the biomass in rural areas;

•

energy cost and the subsidies on fuel cost, especially in the coldest climate zones;

•

energy cost in islands, which is higher due to transportation costs;

•

financial situation of occupants that has an impact on their energy consumption habits.

The specific actual energy consumption (kWh•m⁻²•yr⁻¹) of the Greek residential sector has been recorded or estimated in several studies [37], [73], [74], [76], [77], [78], [79], [80], [81], [82], [83], the results of which are quite close with our findings for 2011 (Table 10).

The National Hellenic Statistical Service completed a survey on the energy consumption of 6,550 households, for the period 1987-1988 [73]. It was found that the specific annual energy consumption (kWh•m⁻²•yr⁻¹) varied, depending on the floor area; it was high in buildings with small floor area and vice versa. The average annual total energy consumption was estimated at 149.4 kWh•m⁻²•yr⁻¹, of which 115.2 kWh•m⁻²•yr⁻¹ were thermal energy and 34.2 kWh•m⁻²•yr⁻¹ electricity. Also, during 2011-2012 the Hellenic Statistical Authority (as this service was renamed) completed another survey for the energy consumption on 3,600 households [69], [74]. This study revealed that the 64% of energy is consumed for space heating, 6% for DHW production, while space cooling and lighting corresponded to low percentages.

Indicative results from a sample of 176 residential buildings (53 single-family and 118 apartment buildings) revealed that the annual actual average thermal energy consumption ranged from 70 to 155 kWh•m⁻²•yr⁻¹ and that the electricity consumption ranged from 34 to 37 kWh•m⁻²•yr⁻¹[81].

A previous similar survey for the complete Greek building stock [89] provided the actual energy consumption for residential buildings constructed before 1980 and from 1980 to 2000; it also the predicted the energy consumption for the year 2010. It was found that the mean predicted total (thermal and electric) energy consumption for 2010 ranges from 94.3 – 161.6 kWh•m⁻²•yr⁻¹ for the single or double-family houses, and 80 – 149.2 kWh•m⁻²•yr⁻¹ for the apartment buildings, depending on the climate zone, with higher values in zone D [37], [52], [53], [89].

The weighted average energy consumption in Greek buildings for 2011 (Table 10), is expected to drop in the coming years, since the contribution of diesel oil in space heating was significantly reduced (Fig. 3). This reduction is due both to the Greek government debt crisis and to the increase of taxation of diesel-oil used for heating purposes, to the same amount of that for transportation (as an additional means for fighting against diesel oil smuggling). At the same time a percentage of electricity users (estimated to about 10% in 2016) canceled their contracts with power supply utilities, because of additional charges applied on electricity bills [92].

5. GHG and CO₂ Emissions of Greek Residential Buildings

The environmental footprint of buildings is usually quantified through indicators related to CO₂ and total GHG emissions, which depend on the actual energy consumption. The indicators, kg CO₂_eq/kWh and total GHG emissions differ among the various energy sources; in Greece the higher values are those of electricity [1], [2], [3]. CO₂eq emissions related to thermal energy production are estimated to about 0.264 kg CO₂/kWh_th for diesel oil and 0.196 kgCO₂/kWh_th for natural gas [16]. However, this indicator, which refers on the final electricity consumption, varies depending on the renewable energy penetration, as well as on the percentage of net imports of electricity [1], [2], [3]. In 2011 it was about 0.974 kgCO₂/kWh_el, in 2014 it dropped to 0.817 kgCO₂/kWh_el and in 2015 it was reduced even further, to 0.699 kgCO₂/kWh_el.

According to the available data from the Eurostat and the Hellenic Ministry of Energy and Environment [1], [2], [3], it is estimated that during the period 1995 – 2015, the residential sector accounted for, on average, to the 35% of the total final electricity consumption in Greece. This percentage corresponds to annual average CO₂ emissions of 16.47 MtCO₂eq and to overall GHC emissions of 16.53 MtCO₂eq [Fig. 12]. During the same period, the residential sector corresponded on average to 32% of the total (thermal and electric) final annual energy consumption in Greece, corresponding to 23.98 MtCO₂eq emissions of CO₂ and to 24.19 MtCO₂eq emissions of overall GHC [Fig. 12].

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Fig. 12. Evolution of CO₂ and GHG annual emissions in the Greek residential sector, for the past 26 years (1990-2015).

Buildings can contribute in mitigating climate change, since their CO₂ emissions can be substantially reduced [89], [93]. A significant part of these savings can be achieved by reducing life cycle costs, and thus the CO₂ emissions [93]. According to several national or regional studies estimating CO₂ mitigation potential, the building stock has the highest share of negative- and low-cost greenhouse gas reduction potential among all sectors [93]. Also, compliance with standards, demand-side management programs and mandatory energy and environmental labelling, seem to be some of the most cost-effective instruments, all achieving energy savings at negative costs for society [94].

The building sector burdens the environment through direct CO₂ emissions due to energy consumption, and indirect CO₂ emissions from the energy required for production of elements and systems related to their energy performance, as well as from the disposal, recycling, and decomposition of the above. In this framework, as well as from the obligation to comply with the Kyoto Protocol to reduce CO₂ or total GHG emissions, the European Commission put in effect the Delegated Regulation (EU) No 244/2012 [95], i.e. a common methodology framework for calculating cost-optimal levels of minimum energy performance requirements for buildings and their elements. The life cycle embodied energy analysis results, in order to estimate the net cost benefit of several measures related to building energy performance, as well to help policy-makers justify political and economic actions [37], [52], [53], [89], [96], [97], [98], [99].

6. Energy Conservation and Management in Residential Buildings

The energy conservation in residential buildings depends on the condition of their structural elements and service equipment and the possibility of improving it by cost-effective interventions. When applying the minimum requirements of the REPB [15], [17], [18] on the 144 non-insulated residential buildings, assuming the less energy demanding occupancy profile, that of 14-hrs (Fig. 11.c), the potential of thermal energy conservation for space heating seems to be high (Fig. 13). It should be noted that the average overall heat transfer coefficient U_m for non-insulated residential buildings ranges between 2.1 and 2.8 W.m⁻².K⁻¹, while according to the REPB, it ranges between 0.58 and 1.2 W.m⁻².K⁻¹ (including eventual thermal bridges). The maximum allowed U_m values of buildings according to the REPB depend on the climate zone and the A/V ratio, the highest U_m values corresponding to the lowest A/V ratio values (i.e. those of apartment buildings) in all climate zones.

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Fig. 13. Annual space heating energy conservation potential (kWh•m⁻²•yr⁻¹) as a function of the area/volume (A/V) ratio, when the REPB is applied.

The annual space heating energy consumption for 144 residential buildings complying with the minimum requirements of the REPB is also calculated using the HDHs (Eq. 1). The average percentage of thermal (space heating) energy conservation per climate zone is estimated about 63% (58%-72%) for zone A, 67% (61%-76%) for B, 68% (63%-78%) for C and 70% (65%-80%) for D, with the higher values corresponding to single-family houses. These percentages correspond to 50 up to 350 kWh.m⁻².yr⁻¹ of space heating thermal energy savings, depending on the climate zone and the A/V ratio (Fig. 13). The highest energy conservation values are obtained for single-family houses (high A/V ratio), especially for those located in the coldest climate zones C and D. The same percentage of energy conservation is obtained for the other two occupancy profiles, those of the 24-hrs and 18-hrs. However, the potential of energy conservation (kWh•m⁻²•yr⁻¹) for any building type needs to be further analyzed in order to take into account to the actual habits of the occupants and the existing conditions of the building construction. Therefore, the energy upgrade of buildings is not always energy and/or financially efficient.

The mean actual energy conservation (kWh•m⁻²•yr⁻¹) of residential space heating (Table 13) is much lower than the estimated energy conservation based on the HDHs and the 14-hrs occupancy profile (Fig. 13), where in many cases is less than a half. This difference is due mainly to the occupant habits and occupancy profile of buildings which, in many cases, is much lower than 14-hrs per day or the indoor temperature may be lower than 20°C.

The most efficient electricity savings measure in buildings is the replacement of the electrical boiler for DHW production, by solar collectors. This is a cost-effective measure resulting up to 50% electricity conservation for DHW, depending on the DHW consumption profile and the climate zone [18], [31], [37]. In Greece, the use of 1 m² flat plate solar collector per person saves about 150-300 kWh annually, depending on the climate zone [89], [99].

The use of solar collectors, for covering at least 60% of the DHW production needs, is the only minimum requirement of REPB [15], which exploits a renewable energy source (RES). However, in the framework of the ED 2010/31 [13] and the requirement for Nearly Zero-Energy buildings (with very low energy needs that should be covered mainly by RES), RES should also be also used to cover electric and thermal needs, other than the DHW.

Relevant studies for various Greek regions show that the use of solar collectors for space heating is financially sound in the case of very well thermal insulated building envelope, if there is adequate space for their installation [99], [100]. The calculation of the minimum solar flat plate collector area needed to completely cover the space heating requirements of a building is based on the load of the month during which solar irradiance is minimum. It is found that this is about 1 m² of flat solar collector for an annual thermal load of about 135-150 kWh•m⁻²•yr⁻¹ [99], depending on the available solar radiation of the location. Also, in the case of the most energy demanding 24-hrs occupancy profile and for a 100% space heating loads coverage for a building designed according to the REPB, the minimum area of a flat solar collector, in relation to the required oil boiler capacity (kW), is estimated to be about 1.8 m²/kW for the zone A, 2.1 m²/kW for B, 2.7 m²/kW for C and 3.2 m²/kW for zone D.

On the other hand, the use of photovoltaics (PV) to cover electricity needs seems to be very efficient both in terms of energy savings and payback period [101], especially when the PV arrays are connected to the local electric grid (net-metering system). The minimum required PV area depends on the available solar irradiance, ranging in Greece between 1450-1800 kWh•m⁻²•yr⁻¹ on a horizontal surface and is estimated about [101]:

(a) 5.8-4.8 m² per MWh of total annual electricity consumption, in case of grid connected silicon PVs and b) 4.2-3.5 m² per kWh of total electricity daily consumption, in case of autonomously operating silicon PVs system (with batteries).

The location a PV array as well as solar collectors, require, for the optimum exploitation of solar radiation in Greek regions, about at least 2 m² of free land per m² of PV or solar collector area.

In view of the Zero- or even of the Low Energy Buildings necessity in the forthcoming years, the integration of Renewable Energy Sources or Energy Harvesting technologies [102] in buildings and the interaction of the building and the “smart electricity grid” seem to be mandatory. One effective way to achieve these goals is the incorporation of PV- or conventional Electric Vehicles’ (EVs’) charging points of adjacent parking lots into the operation of buildings. The building management systems (BMS), the automation control systems or Internet of Things (IoT) technologies [103], [104], [105], [106], [107] can also contribute significant in energy conservation and thermal comfort in residential sector, enabling the smart city projects and initiatives.

7. Discussion and Conclusions

The building energy conservation is an important action towards the compliance with the Kyoto Protocol, asking for reduced CO₂ or total GHG emissions. In this framework, many studies related to the energy efficiency refurbishment of buildings, according to the EPBD [13] evaluated several interventions and economic measures for many European countries [8], [9], [10], [18], [20], [21], [22], [23], [24], [25], [26], [27], [29], [31], [32], [34], [35], [37], [39], [41], [42], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [68], [89]. Also, other studies have evaluated the differences between the final actual and the estimated expected energy consumption in buildings, since the occupancy profiles and behavior of the occupants affect strongly the final energy consumption in the residential sector [6], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [73], [74]. The results of these studies cannot be easily compared, because of the different assumptions and data used. The minimum technical requirements according to national regulations (U-value, heating efficiency etc.), type and design of building construction, type and cost of energy, climate conditions, operation profile of buildings, etc., differ significantly between the European countries. However, there are many common conclusions about the energy efficiency refurbishment of residential buildings. Thermal protection of the building envelope mainly for buildings located in colder climates (i.e. at northern latitudes), is very efficient in terms of energy and in most of the cases also financially, under the condition of optimum insulation thickness and airtightness of openings (windows and doors). On the contrary, the use of combined or not solar systemsin residential buildings may not be cost effective in all European climates since it depends not only on the availability of solar radiation, but also on the local investment and energy costs. Similarly, the use of high performance electromechanical (E/M) equipment is not cost effective in some cases and depends on the local investment and energy costs, as well as on the operation profile of buildings and the required automation. On the other hand, there are low cost interventions that are very efficient both in terms of energy savings and financially, as the regular maintenance of the heating boiler, heat pumps, air handling units, etc. as well as the installation of thermostats in heated or cooled spaces.

Most of the above studies are usually based on estimations of the expected energy consumption. However, buildings with high energy performance (optimum thermal protection, high efficient systems etc.) do not appear to correlate with less energy consumption (space and water heating, space cooling etc.), even when the type or the size of the building is taken into account [4], [6], [9], [14], [18], [19], [37], [43], [54], [57], [59], [61], [62], [63], [73], [74], [81]. This is due to the fact that the actual operating profile of the buildings varies and affects the final energy consumption. Considering the above assumptions, the benefits of any regulation on the energy performance should be examined in terms of the expected lower actual energy consumption.

A previous study on the total Greek building stock determined the priorities for energy conservation measures (ECMs) [[37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52]–[53], [89], [108]], including residential buildings, in order to reduce the environmental impact of CO₂ emissions, through the implementation of a realistic and effective national action plan. Different energy conservation scenarios and their impact on the reduction of CO₂ emissions were evaluated. The specific energy savings in residential buildings range from 33% to 60% depending on the climate zone, the construction date and the building type (single-family or apartment buildings) [37], [52], [53], [89]. Some measures are financially attractive and would not require the support of any financial instruments [109]. On the other hand, some measures have a good potential, but their implementation may require specific subsidies and/or the use of other instruments.

The estimated space heating thermal energy consumption for non-insulated Greek residential buildings according to HDHs method or to monthly quasi-steady method of EN ISO 13790 (Hellenic REPB) is much higher than the weighted average actual thermal energy consumption of any type (different A/V ratio) residential building. This difference seems to be much higher for buildings with high A/V ratio (mainly single-family houses), for all occupancy profiles, even for the 14-hours profile. The actual energy consumption of residential buildings, except of the building construction and operation profile, is mainly related to the occupants’ behavior and the thermal comfort sense.

The space heating thermal energy calculations based on HDHs in Greek regions showed that the average percentage of thermal energy conservation per climate zone, when applying the REPB minimum requirements on existing old residential buildings, can reach up to 63% in climate zone A, 67% in B, 68% in C and 70% for D. These percentages could be applied to any final energy consumption, actual or estimated. However, the low actual thermal (space heating) energy consumption in residential buildings seems to reduce significantly the (notional or factitious) potential of energy conservation, which estimated by theoretical methods (HDHs and EN 13790). Thus, the actual energy consumption should be considering in case of cost-effective evaluation of any building energy intervention.

On the contrary, the use of RES, such as solar collectors for DHW and space heating, or photovoltaics for electricity production, seems to be energy and financial effective for the Greek buildings, considering the high available solar radiation and the continually reduction of initial and operational cost. The use of solar collectors for DHW production reduces up to 50% of energy consumption for DHW.