European legislation is forcing towards the creation of new buildings with nearly zero energy consumption and the refurbishment of old buildings in order to reduce their energy use (Directive 2002/91/EC, Directive 2010/31/EU). Torcellini et al., 2006 [1] have reported on various definitions of zero energy buildings. The authors reported four definitions as follows: net zero site energy, net zero source energy, net zero energy costs and net zero energy emissions. A net zero energy emissions building, according to them, produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources. Marszal et al., 2011 [2] have presented a review of definitions and calculation methodologies for zero energy buildings. The authors stated that the zero energy building concept requires a clear and consistent definition and a common agreed energy calculation methodology. They discussed various issues and approaches which should be clarified in order to facilitate a consistent and common agreed definition of zero energy building. Current published literature, according to the author’s knowledge, has not included so far the embodied energy in the calculation of zero energy buildings. Vourdoubas, 2016 [3] has reported on the creation of zero CO₂ emissions residential buildings due to operating energy use in Crete, Greece. The author estimated, for a typical residential building in Crete, Greece, its annual CO₂ emissions due to operating energy use at 84.55 kgCO₂ per m². Realization of a small residential building with zero CO₂ emissions due to energy use in Crete, Greece has been reported from Vourdoubas, 2017 [4] . The author reported that renewable energy sources including solar thermal energy, solar-PV energy and solid biomass were used covering all the heating needs in the building and offsetting its annual grid electricity consumption. The cost of the required renewable energy systems corresponded at 10.77% of its construction cost and it was equal to 1.65 € per kg CO₂ saved annually in the building. Tselepis, 2015 [5] has implemented various case studies, including households, concerning electricity generation with solar-PVs in Greece according to the net-metering regulations, indicating that it was profitable.

A review of current trends of operating versus embodied emissions in buildings has been published by Ibn-Mohammed et al., 2013 [6] . The authors stated that in order to mitigate climate change, buildings must be designed and constructed with minimum environmental impact. Total life cycle emissions from buildings are due to operating and embodied energy use. Considerable efforts have been made to reduce operating emissions from buildings but little attention has been paid to embodied emissions. Therefore, a critical review on the relation between operating and embodied emissions is necessary in order to highlight the importance of embodied emissions. Estimation of energy consumption and CO₂ emissions due to housing construction in Japan has been presented by Suzuki et al., 1995 [7] . Energy consumption at 3 - 10 GJ/m² (833 - 2777 KWh/m²) has been found depending on the type and construction of the building and its CO₂ emissions during the construction stage varied between 250 - 850 kg/m². Guellar-Franca et al., 2012 [8] have investigated the environmental impacts of the UK residential sector. The authors reported that over a period of 50 years, 90% of the global warming potential of the residential buildings is due to their operation and only 10% is due to their embodied carbon. Syngros et al., 2016 [9] have reported on embodied CO₂ emissions in building construction materials in Hellenic dwellings. The authors analyzed CO₂ emissions corresponding to construction materials for four typical dwellings in Greece, estimating the average CO₂ emissions at 777 kg CO₂ per m². Over a life span of 50 years, the annual embodied CO₂ emissions corresponding to the construction materials were 15.54 kg CO₂ per m². The authors also stated that the emissions were mainly due to construction materials and the share of electro-mechanical installations was below 2%.

Karimpour et al., 2014 [10] have reported on minimizing the life cycle energy of buildings. The authors stated that in mild climates embodied energy in buildings can represent up to 25% of the total life cycle energy. In the future when operating energy in buildings will be reduced due to the construction of nearly zero energy buildings, the ratio of embodied energy to total life cycle energy will be increased. The need for an embodied energy measurement protocol for buildings has been reported by Dixit et al., 2012 [11] . The authors stated that studies have revealed the growing significance of embodied energy in buildings. However current estimations of embodied energy are unclear and vary greatly. The authors recommended an approach to derive guidelines that could be developed into a globally accepted protocol. Cellura et al., 2014 [12] have reported on the operating and embodied energy of an Italian building. The authors emphasized the key issue of the embodied energy of the building which is particularly important in the case of low energy buildings. They also pinpointed the difficulty in defining the reference area in the building, including service and unheated zones, and the absence of an internationally accepted protocol for that. Dixit et al., 2010 [13] have presented a literature review regarding the identification of parameters for embodied energy measurements in buildings. The authors stated that current methods of estimation are inaccurate and unclear. They concluded in a set of parameters that differ and which cause variation and inconsistency in embodied energy estimation. Berggren et al., 2013 [14] have reported on life cycle energy analysis in buildings. With reference to net zero energy buildings, where on-site renewable energy generation covers the annual energy load, the authors analyzed the increase of embodied energy compared with the decrease of operating energy. They concluded that: a) in the last decades the embodied energy in new buildings has slightly decreased, b) the relationship between embodied energy and life cycle energy use is almost linear, and c) the relative share of embodied energy to life cycle energy use has increased. A methodology for life cycle building energy rating has been developed by Hernandez et al., 2011 [15] . Apart from the operating energy consumed during its operation, usually the embodied energy consumed during the construction of the building is not taken into account. For buildings with “zero net energy use” during its operation, the only life-cycle energy use is the embodied energy. In a case study of a detached house in Ireland, the authors have estimated the embodied energy at 1000 KWh/m². A review on the energy use in the life cycle of conventional and low energy buildings has been reported by Sartori et al., 2007 [16] . The authors analyzed 60 cases found in the literature which showed that operating energy is represented by the largest part of total energy demand. They stated that there is a linear relation between operating and total energy use, concluding that low energy buildings are more energy-efficient even though their embodied energy is slightly higher. Ramesh et al., 2010 [17] have presented an overview on life cycle analysis of buildings. The authors analyzed the results of 73 cases from 13 countries including residential and office buildings. They found that operating energy corresponds at 80% - 90% of the life cycle energy use and the rest corresponds to embodied energy. They estimated the life cycle primary energy requirements at 150 - 400 KWh/m² y for residential buildings and 250 - 550 KWh/m² y for offices. The authors also observed that low energy buildings perform better than zero operating energy buildings in the life cycle context. Adalberth et al., 2001 [18] have reported on life cycle assessment, regarding environmental impacts, of four multi-family buildings located in Sweden built in 1996. The authors stated that environmental impacts during the operation of the buildings contributed at 70% - 90% of their total life cycle impacts. Since environmental impacts are directly related with their energy consumption, they suggested that design and construction of energy-efficient buildings using low emissions energy results in minimizing their environmental impacts. A report on life cycle energy and environmental performance of a new university building has been made by Scheuer et al., 2003 [19] . The authors conducted a life cycle assessment of a 7300 m² six- story building located in the University of Michigan campus. The primary energy intensity over a life cycle of 75 years was estimated at 1171 KWh/m² y and the operating energy accounted at 94.4% of its life cycle energy use. Ramesh et al., 2013 [20] have reported on a case study of life cycle energy analysis of a multifamily residential house in India. The authors estimated that operating energy in the house corresponded at 89% of its life cycle energy use and the remaining 11% corresponded to embodied energy. The primary energy consumption was estimated at 288 KWh/m² y and the authors concluded that building-integrated photovoltaics were promising for the reduction of life cycle energy use. Fay et al., 2000 [21] have reported on a case study of life cycle analysis of buildings in Australia. The authors stated that as operating energy in buildings becomes lower due to efficiency improvements, embodied energy becomes more significant. They also reported that while a net zero operating energy building is now achievable, a zero life cycle energy building is likely to be more difficult.

The aim of the current study is to investigate the possibility of creating zero CO₂ emissions grid-connected residential buildings due to operating and embodied energy use with reference to the island of Crete, Greece. Appraisal of operating and embodied energy needs in the building is conducted first, followed by sizing the electricity generating solar-PV systems in order to zero the overall CO₂ emissions due to both operating and embodied energy use. The current study indicates the way that renewable energy technologies can be used in residential buildings in order to zero their overall carbon emissions due to operating and embodied energy use with reference to the island of Crete, Greece.

The following equations were used:

X1 = energy consumed for space heating in the building (KWh/m² y)

X2 = energy consumed for hot water production (KWh/m² y)

X3 = energy consumed for lighting (KWh/m² y)

X4 = energy consumed for the operation of various electric appliances (KWh/m² y)

Xoper = energy consumed during the operation of the building (KWh/m² y)

Xemb = embodied energy in the building (KWh/m² y)

Xli.cy. = life cycle energy use (KWh/m² y)

a = percentage of embodied energy to life cycle energy use (dimensionless number)

z = generated energy from a solar-PV system (KWh/year per KWp)

C.O.P. = coefficient of performance of a heat pump (dimensionless number)

P = Nominal power of a solar-PV system (KWp)

Xoper = X1 + X2 + X3 + X4 (1)

Xli . cy . = X1 + X2 + X3 + X4 + Xemb (2)

Xli . cy . = Xoper + Xemb (3)

Xemb = a ∗ Xli . cy . = a ∗ [ Xoper + Xemb ] = a ∗ Xoper + a ∗ Xemb (4)

Xemb = Xoper ∗ a / ( 1 − a ) (5)

In the case that electricity is not consumed for space heating and for hot water production in the building, then

P = ( X3 + X4 + Xemb ) / z   ( KWp / m 2 ) (6)

In the case that a heat pump is used for space heating and electricity is not consumed for hot water production in the building, then

P = ( X1 / C . O . P . + X3 + X4 + Xemb ) / z   ( KWp / m 2 ) (7)

A part of the grid electricity is generated by non-CO₂ emitting fuels like renewable energy sources and nuclear power. In this case the solar-PV system in the building should offset only the grid electricity generated by fossil fuels.

If Y% is the share of grid electricity generated by non-CO₂ emitting fuels, then Equations ((6) and (7)) can be written as follows:

P1 = ( ( 100 − Y ) / 100 ) ∗ ( X3 + X4 + Xemb ) / z (6a)

and

P2 = ( ( 100 − Y ) / 100 ) ∗ ( ( X1 / C . O . P . ) + X3 + X4 + Xemb ) / z (6b)

where P1 and P2 are the nominal powers of the required solar-PV systems adjusted for the share of non-CO₂ emitting fuels used in grid electricity generation.

Energy consumption per sector for a residential building located in Crete, Greece is presented in Table 2.

It should be mentioned that the high share of energy use for space heating in the residential building, although the climate is mild, is due to the fact that most buildings constructed before 2010 in Greece are very poorly thermally insulated. For the following estimations, it has been assumed that the embodied energy in the residential building with a life span of 50 years is equal to 15% of its life cycle

(Vourdoubas, 2016).

energy use (Ramesh et al., 2010 [17] ). This is an average value reported also in other studies concerning life cycle consumption in buildings.

Setting in Equation (5), Xoper = 170 KWh/m²y, and a = 0.15, then Xemb = 30 KWh/m²y

Setting in Equation (6), X3 = 20.4 KWh/m²y, X4 = 27.2 KWh/m²y, Xemb = 30 KWh/m²y

and for Crete, Greece, z = 1500 KWh/y per KWp, Y = 0.18

Annual electricity generation from steady (non rotated) solar photovoltaics installed in Crete, Greece is approximately 1,500 KWh per KWp.

It is estimated that currently in Crete, Greece, 18% of the annual grid electricity is generated from renewable energy sources mainly wind and solar-PV energy.

then P = 0.0 52   KWp / m 2 and P1 = 0.0 42   KWp / m 2

Therefore the nominal power of the required solar-PV system which will generate annually and inject into the grid the amount of electricity used for lighting, for the operation of electric appliances and for annual compensation of the embodied energy of the building over a period of 50 years, is 0.042 KWp/m². For a residential building with a covered surface of 100 m², it is 4.2 KWp.

Assuming that the solar-PV system will generate annually only the electricity used for the operation of the building (Xemb. = 0), then Equation 6(a) gives P1 = 0.026 KWp/m². For a residential building with a covered area of 100 m², the nominal power of the required solar-PV system is 2.6 KWp. The estimated additional nominal power of the solar-PV system, which is 1.6 KWp, will generate and inject into the grid, over a period of 50 years, electricity equal to the embodied energy in the building.

When a heat pump is used for space heating in the residential building, its electricity consumption is higher compared with the previous case. The nominal power of the required solar-PV system in order to zero its CO₂ emissions due to life cycle energy use is estimated from equation 7. Assuming that the C.O.P. of the heat pump is 3 and according to Table 2, X1 = 107.1 KWh/m²y, X3 = 20.4 KWh/m²y, X4 = 27.2 KWh/m²y, Y = 0.18 and Xemb = 30 KWh/m²y, then:

P = 0.0 76   KWp / m 2 and P1 = 0.0 62   KWp / m 2

Therefore the nominal power of the required solar-PV system which will generate annually and inject into the grid the amount of electricity used for space heating with a heat pump, for lighting, for the operation of electric appliances and for annual compensation of the embodied energy of the building over a period of 50 years, is 0.062 KWp/m². For a residential building with a covered surface of 100 m² it is 6.2 KWp, which is higher than in the previous case. Assuming that the solar-PV system will generate annually only the electricity used for the operation of the building (Xemb = 0), then Equation 7(a) gives P1 = 0.046 KWp/m². For a residential building with a covered area of 100 m², the nominal power of the required solar-PV system is 4.6 KWp.

The results of the above-mentioned estimations are presented in Table 3.

Equations (6(a), 6(b)) and 5 indicate that the nominal power of the required solar-PV system in both cases depends on the share of the embodied energy to life cycle energy in the building, (a), and from the share of grid electricity generated from non-CO₂ emitting fuels, (Y). In Table 4 the power of the solar-PV system for different values of a is presented and in Table 5 the power for different values of Y.

¹Cost of the solar-PV system = 1500 €/KWp.

¹P1, Power of solar-PV when electricity is not consumed for space heating and for hot water production in the building. ²P2, Power of solar-PV when a heat pump is used for space heating and electricity is not consumed for hot water production in the building.

¹P1, Power of solar-PV when electricity is not consumed for space heating and for hot water production in the building. ²P2, Power of solar-PV when a heat pump is used for space heating and electricity is not consumed for hot water production in the building.