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An overview on green house gas emission characteristics and energy evaluation of ocean energy systems from life cycle assessment and energy accounting studies

An analysis has been made as regards emission characteristics of ocean energy systems from life cycle assessment and scope of energy availability from energy accounting studies. Assessment tools developed and standardized were the indices like scope of Green house gases (GHG) emission per kWh power generation, percentage of CO2 saved compared to coal fired power station and the energy payback period. Emission characteristics of ocean energy systems were also compared with that from solar power, bio-fuels and wind energy systems. Four case studies were made comprising of wave energy converters, Ocean Thermal Energy Conversion (OTEC) system and tidal energy. It could be observed that CO2 emission percentage saved from ocean energy schemes were more than 95 per cent; and energy payback period varied between one year and a little higher than two years, depending on the type of the device.


INTRODUCTION
Like all other renewable energy schemes, application of life cycle analysis (LCA) are useful for the ocean energy (OE) schemes as well, to assess the degree of benefits accrued from the saving of GHG emission. Energy payback period (EPBP) estimations from energy accounting (EA) studies are also considered important criterion for evaluating the scope of acceptability of the concerned OE device for power generation. The OE types considered in the present study include all three OE systems comprising of the wave schemes, ocean thermal energy conversion (OTEC) systems, as well as of the tidal energy (Thresher and Musial, 2010) It may be relevant to add that though large scale commercial application of OE systems, other than barrage, are yet to come up; but pilot plant studies have proved successful in number of OE devices for all three OE systems. Case studies of some of the OE devices that have the potential of commercial application have been taken up in the present study. The scope of GHG emission saving capability as also of energy payback periods have been estimated from LCA and EA studies, respectively (Helius and Reinout, 2007). In the present study, the case studies taken up included-two wave energy converters (Pelamis and Wave Dragon), one type of OTEC scheme (CC-OTEC) and a proposed barrage project (Severn barrage). A brief account of LCA and EA studies of above 4 cases are appended below. Methodology adopted in LCA and EA estimations: Life time emission of GHGs expressed in g/kWh power generation of an energy device, as per LCA estimations, would be = Gi × Mi / P i + operational stage emission in g/kWh. (1) In the above equation, Gi represents the gas emission in kg/kg of the inventory items; Mi is mass of the inventory items of the device; and P l is the life time power generation of the device, expressed in kWh. Like all other renewable energy systems, in case of OE systems also the operational stage emission would be rather marginal, excepting OTEC schemes which do contribute some emission in its operational stage as well. Likewise, energy payback period (EPBP) would be = Ei Mi / Pa; (2) Where, Ei is the embodied energy of the inventory items of the device expressed in MJ/kg; Mi is their respective mass in kg, Pa is the annual power generated by respective OE devices, also expressed in MJ (Mega Joule). The data as regards Gi of inventory items was adopted from Danish model of LCA estimations as used for wind energy systems (Schleisner, 2000), estimating the emission characteristics of construction materials of OE devices as per ISO 14040 with LCA boundary conditions of 'cradle to gate' (ISO 14040, 2006

ANSF
from Bath University data source as well (Hammond and Jones, 2008), for checking up the degree of discrepancy of results (if any), since LCA has been known to be process specific and country specific (Blengini, 2008). Ei, the embodied energy data of the inventory items were also adopted from both the Danish model ( (Schleisner, 2000), and corroborated from Bath University data sources as well (Hammond and Jones, 2008

CASE STUDIES OF OE DEVICES
In order to make LCA and EA studies, both lifetime and annual power generation data of different OE devices are required to be availed, in addition to the mass of all the inventory items of the device concerned. Thus knowing the life of a device, its annual power production and inventory data-both emission characteristics and energy pay back periods can be estimated from computation of Tables 1 and 2, and equation 1; as also of Table 3 and equation 2, respectively. Four case studies as made from the above premise are appended below in Fig. 1.

LCA AND EA STUDIES OF 750kW PELAMIS TYPE WAVE ENERGY CONVERTER
Pelamis is a cylindrical type wave energy converter consisting of semi-submerged structure with cylindrical sections linked by hinged joints, as shown below in Fig.1. Its life period is reported to be around 20 years (Parker et al., 2007) with annual power generation of 2.5GWh, if placed in Ireland coast (Dalton et al., 2010). Thus its lifetime power production would be 50GWh. The distribution of mass of inventory materials of 750 kW Pelamis unit was learnt to be broadly constituting of steel: 380,000 kg and copper: 15000kg (Taylor, 2006). The life time emission characteristics of Pelamis in g/ kWh, could then be estimated employing the equation 1 and making computation of the above data with table 1, that gives mass of emitted gases in kg/kg of the inventory materials, as per the Danish model. The GHG equivalent of respective gases could also be determined multiplying the emission in g/kWh with their respective global warming potential (GWP) value; which for CO 2 , N 2 O and CH 4 are '1', '310' and '21', respectively. The results thus obtained, are shown below in Table 4. It would be evident from the above table that emission of CO 2 is only of relevance in assessment of GHG, despite its low GWP of only '1'; mainly because of its much  (Schleisner, 2000).
*Only mean values are considered.
* Only mean values are considered.  Table 3. Energy requirement in MJ / kg of the inventory materials of OE devices.

CASE STUDY OF 7MW WAVE DRAGON
Wave Dragon (WD) is an overtopping type of wave energy converter. It focuses the incoming waves towards a huge reservoir (a floating ramp) with two wave reflectors and overtopping the reservoir water to run a number of turbines by converting the pressure head of water to power generation, as shown below in Fig.2. It has been claimed that its annual power generation when placed in Wales Coast would be 20 GWH (Millar et al., 2007). Also its life is claimed to be 50 years (Tedd, 2007) thereby with life time power production of 1000GWh. Its inventory data has been shown below in

CASE STUDY OF 100MW CLOSED CYCLE OTEC
Electricity from OTEC is generated utilising the small temperature difference between warm surface seawater and deep cold seawater, usually following a Rankin cycle heat engine (Green and Guenther, 1990). OTEC however, requires power for its operations to generate the power. Hence, the terms gross energy output and the net energy availability comes up for OTEC schemes; the latter being usually 65 percent from its gross energy output, the value of which increases with larger sized OTEC plants (Vega, 1999). In case of 100MW OTEC plant, net energy may obviously be presumed to be 75% of the gross power generated. Presuming the capacity factor to be at least 30% (as observed for most of OE schemes), annual power production from 100MW CC-OTEC would be = 100*0.75*0.3*365*24= 191.7 GWh; with life time power production of 5913GWh, considered for its 30 years life period. The mass of construction materials of the above OTEC plant is shown below in Table 7. Life time emission of CO 2 estimated from the above data with computation as per equation 1, and of gases =27.37g/kWh. Bath data results yields CO 2 emission to be =26.38g/kWh. In addition to the above emission data, in case of OTEC the operational stage emission has also to be taken into account, as per equation 1; unlike the wave schemes. This needs adding up CO 2 emission from the working fluid NH 3 plus other sources of operational stage emission. The former was estimated to be 0.49g/kWh from the inventory data of Japanese researchers (Tahara et al., 2000); whereas the latter is reported to be a little less than 1g/kWh (Green and Guenther, 1990), presumed as 0.8 g/kWh-thus totalling 1.29g/kWh. Adding up the above value during operational stage of CC-OTEC, its life time emission would be = 28.47g/kWh, as per Danish model and 27.67 g/kWh, as per Bath data source. It may be relevant to add that employing data source of NIRE-LCA software, which the Japanese researchers used (Tahara et al., 2000), the value arrived at was only 24.08 g/kWh for 100 MW CC-OTEC. EPBP of 100 MW CC-OTEC, based from its net annual power production of 197.1GWh and computation with equation 2 and table 3 data, was estimated to be = 2.35 years from Danish model and 3.07 years from Bath data source.  (Elliott, 2004). Computation of all these emission data divided by its life time power generation extending 100 years, the value of CO 2 emission showed 3.01 g/kWh for Danish model and 3.26g/kWh from Bath data source. Computation of EPBP values, on the basis of annual power generation of the Severn barrage showed values of 0.53 years and 0.48 years, for Danish model and Bath data, respectively.

CRITICAL APPRAISAL OF RESULTS
It could be observed from the above results that Severn barrage project with 100 years life and high power generation capability showed minimum values of both CO 2 emission and energy payback period. In fact, these values are mainly influenced from the capability of annual power generation of the device concerned, its life period as well as of the mass and type of inventory materials of the device concerned. It is to be noted that the above stated 750 kW Pelamis if placed in Portugal coast would have produced half the power than that availed (2.5 GWh annually) from its placement in Ireland coast (Dalton et al., 2010 (Kofoed et al., 2006).
40% higher values than that estimated in the present study considered for 50 years life. Also, the discrepancy (though minor) of the results between Danish model and Bath data source, for the same device, are caused because of the varied data base of respective inventory items concerned. It may be relevant to add that the values obtained from LCA and EA studies pertained only for the respective OE device concerned, without considering the input as would accrue for power transmission from cable lines etc. However, the results prove to be important as one of the assessment tools, making comparative study of the competing OE devices for their acceptability. It has hence been considered important to compare the above results of OE systems with other renewable energy types, like solar, bio-Fuels and wind energy, as well as determining the GHG saving compared to coal fired generator. These studies have been made in the subsequent section.

COMPARATIVE STUDY ON CO 2 SAVING PERCENTAGE
CO 2 emission from coal fired generator, as obtained from LCA studies was found to be 900 g/kWh (Odeh and Cockerill, 2008). Considering this to be the 100 % emission, the percentage of CO 2 saved from the application of an energy device would be = 100 -Emission of device concerned in g/kWh* 100/ 900 (3) It has also been noted that emission of CO 2 from solar power may vary between 58g/kWh and 35 g/kWh, depending on the technology adopted (Parliamentary office post note, 2006). In case of bio-fuels it varies between 25g/kWh and 93 g/kWh, depending on the type of bio-mass used (Parliamentary office post note, 2006). Wind energy shows the least value of 6 g/kWh (Crawford, 2009). Based from the above data, GHG saving percent of different types of OE device, as determined from equation 3 are shown in Table 8 and Fig. 3, given below.

Conclusion
It could be inferred that CO 2 is the main contributor in GHG emission. Emission of CO 2 as determined from LCA studies and EPBP values estimated from EA studies, depend on annual energy production, device life, as also of mass and type of the inventory materials required. Amongst the OE systems, it is the barrage that showed maximum efficiency as regards CO 2 saving as also of achieving minimum energy pay back period. All the OE devices showed more than 95% CO 2 saving than the coal power plant. Amongst the other renewablle energy types, it is only the wind energy that showed minimum emission, whence solar power showed maximum values, with bio-fuels comparable to the OE systems.Values of EPBP varied between less than one year to two years, with only OTEC showing a little higher than two years. Though LCA studies are known to be counry specific and process specific, but results obtained from the present study, carried out using both Danish model and Bath data source, broadly conformed with each other.