Original Research Article
The tidal energy potential of the Manukau Harbour, New Zealand

https://doi.org/10.1016/j.seta.2014.07.001Get rights and content

Abstract

A renewable source of electricity generated near the demand in Auckland might help defer investment in grid infrastructure, avoid grid transmission losses and help achieve the target for renewable energy mix in New Zealand. The tidal energy at both ends of the entrance channel to the Manukau Harbour is calculated based on navigation chart current speed data. Calculation is done using the kinetic energy flux method, and a farm method calculation is used for comparison purposes.

The peak current speeds are modest: 1.35 and 1.80 m/s. These give estimated extracted electricity results of 7.2 and 7.9 GW h/year for flux method and 13.3 and 11.2 GW h/year for farm method. This electricity could supply about 900–1400 New Zealand homes and is similar to the generation output of a small wind farm. Factors affecting feasibility for a future tidal energy project in the Manukau Harbour include New Zealand electricity demand changes, technology maturity, and environmental effects – especially on rare dolphins. The study concludes that the resource is sufficient to support a small-scale distributed generation project after technology costs reduce. A strong case is made for investment in another more detailed study in 2017.

Introduction

Twenty years ago the only form of tidal energy for electricity generation was a barrage, which harnesses potential energy like a hydro dam. More recently interest has turned almost entirely towards harnessing the kinetic energy of tidal flows using principles similar to wind energy and the term tidal energy has come to have that form as its principal meaning. Internationally, the United Kingdom government is leading the way in supporting tidal and wave energy development, the latest example being grants towards commercial scale demonstration projects. Active research is ongoing in the United States, Canada, Europe and Japan. It may be another ten years or more before tidal energy reaches the technical and commercial maturity of wind energy. The current limitations for tidal energy are a lack of proven economic solutions for array design, installation and maintenance as well as a lack of ability to accurately anticipate the local environmental effect of extracting energy from tidal currents in any given location.

In New Zealand, 74% of electricity in 2010 was generated from renewable sources – 56% from hydro, 13% from geothermal, 4% from onshore wind and 1% from biomass sources [1]. The New Zealand government is committed to having 90% of electricity generation from renewable sources by 2025 [2]. While there is still scope for geothermal, onshore wind and small hydro, a marine energy (tidal or wave) project can potentially contribute towards this goal. Tidal energy would also reduce vulnerability to low hydro generation during drought.

Long Run Marginal Cost (LRMC) of competing power projects are a major factor in investment decisions. Table 1 shows the assumptions used by the NZ Electricity Authority for modelling about possible future new generation investment [3], [4]. The 2010 report says that marine generation could see some major improvement in the LRMC of tidal energy in the future due to demonstration projects in the UK.

The same Generation Expansion Model (GEM) was used in a 2010 study [5]. The chart in Fig. 1 shows how marine energy projects only appear after lower cost geothermal, hydro and wind projects have been built. In the years since that 2010 study, lower than expected growth in electricity demand has resulted in most power companies placing many of their generation projects in abeyance [6]. This means that a future developer of a marine energy project may need to wait even longer than the years indicated in Fig. 1 before the supply/demand economic balance is favourable.

Auckland is New Zealand’s largest city, and its economy attracts migrants from many Pacific Islands and from Asia. For the next 30 years Auckland’s population is forecasted to grow at 1.4% per year of which one third is forecasted to be from migration. This is a higher rate than other regions of New Zealand, which have an average 0.8% per year forecasted growth rate [7]. This means that Auckland’s share of residential electricity demand growth due to population growth will be larger than other parts of New Zealand.

The Manukau Harbour has tidal currents that may be worth exploiting for energy, and has the advantage of being near Auckland’s large electricity load. Auckland’s electricity demand in 2010 was about 10,000 MW h, which was 25% of the total New Zealand demand [1]. As shown in Fig. 2, the New Zealand transmission grid brings most of Auckland’s electricity from generation plants south of Auckland. In 2010 transmission losses were 3.4% for all of New Zealand and distribution losses were 5.1% [1]. Electricity generation from the Manukau Harbour would have benefits of avoiding transmission losses, avoiding part of distribution losses and deferring investment in upgrading the capacity of transmission lines south of Auckland.

New Zealand has yet to see its first completed tidal energy project. Two projects have gained resource consent for an initial device but have not moved forward for some time. The first regional resource assessment of marine energy for the purpose of informing government policy was published in 2008 by Power Projects Limited [9]. That study excluded harbours from consideration due to a focus on open ocean sites by a government research body funding the work, a gap that partly motivated this study. A second motivation is that the scope of the 2008 Power Projects Limited study did not include many factors that affect the commercial feasibility of a tidal energy project for a project developer. This study uses a kinetic energy flux method in clear steps to calculate the tidal energy at both ends of the entrance channel to the Manukau Harbour and identifies important factors that affect commercial feasibility for a project developer. A logical next step for this and other New Zealand locations with tidal energy potential are:

  • gather field data for current speeds and bathymetry;

  • use a numerical model such as in Plew and Stevens [24] extended to include 3D simulation of current speeds and optimising device array size and layouts; and

  • conduct a detailed techno-economic feasibility evaluation such as in the EPRI System Level Design, Performance and Cost reports [11].

Environmental effects are important for any electricity power plant project. In New Zealand an Assessment of Environmental Effects (AEE) is required in order to gain resource consent to build the project under the Resource Management Act 1991 (RMA). Both the initial assessment and the ongoing monitoring represent a significant cost, the consent application process can be uncertain and lengthy and failure to gain consent stops the project from proceeding. For a tidal energy project, the important environmental effects to assess are those on birds, fish, marine mammals, other sea life, plants, sediment and the overall ecosystem. Both effects upon and opinions from the local community, other users of the harbour and local iwi (Maori) need to be included in a proposal that explains how all adverse effects and concerns can be avoided, remedied or mitigated.

In 2006 an AEE for the Manukau Harbour was published as part of a project to expand State Highway 20 (SH20) [12]. This was a storm water management assessment, but it would be a useful starting point for any future proposal for a tidal energy project. The SH20 AEE refers to “regionally sensitive receiving environments such as estuarine environments and Coastal Marine Area (CMA)” [12, p. 6] and this would also be important for an AEE for a future tidal energy project proposal.

For a future proposal for a tidal energy project in the Manukau Harbour, assessing effects on dolphins would likely be the most critical part of the AEE. Hector’s and Maui’s dolphins are endemic to New Zealand and are the world’s smallest dolphins. Hector’s dolphins live in waters around Cook Strait and the coastal South Island. Maui’s dolphins are a nearly identical sub-species that live only in waters off the northern North Island west coast and are considered to be one of the world’s most rare dolphin species [13]. A map of sightings in Fig. 3 shows that the area of most sightings is just south of the entrance of the Manukau Harbour.

Collating research into the environmental effects of marine energy devices was the focus of Annex IV on the Ocean Energy Systems (OES) international technology initiative of the International Energy Agency (IEA) in 2010–2013 [14]. Studies about the effects on fish and marine mammals were a particular feature of the collated research work, but much work is still needed to improve understanding of these effects.

Finally, technology maturity is a significant factor for a tidal power project developer. The majority of funding by industry and by the UK government has been for a group of developers of heavy, expensive equipment designed to survive the harsh conditions at locations with peak current speeds of about 4 m/s [15]. A second group of technology developers are designing less expensive equipment suited to locations with peak current speeds of less than 3 m/s and it is this group that may be more economically feasible for a project in the Manukau Harbour. Two examples are Verdant Power in the United States [16] and Pulse Tidal in the UK [17], whose devices are shown in Fig. 4.

The year 2017 is when the UK Renewables Obligation programme stops accepting new tidal generation projects for eligibility to receive the support of five Renewable Obligation Certificates (ROCs) per MW h generated until 2037 [20]. Since projects take at least two years to complete, by 2015 tidal device firms should begin to price their technology for post-2017 projects to be competitive without the ROC subsidy in the UK and in markets outside the UK, including in New Zealand.

Section snippets

Methodology

Over the past ten years two methodologies have been widely used for calculation of tidal energy potential of a location: the kinetic energy flux method and the farm method [9], [21], [22], [23], [24]. Both can be applied using only published tidal current speed data from navigation charts [21] or in more expensive and detailed studies that gather site-specific current speed and bathymetry field data combined with 2D or 3D hydrodynamic modelling of current speeds at all parts of the location [10]

Calculation

The STEM model makes use of the Formzahl number or F number for a location, defined as a ratio:F=(K1+O1)(M2+S2)where K1 and O1 are the two most important diurnal tidal constituents and M2 and S2 are the most important semi-diurnal constituents. A value of F = 0.045 was used in STEM for in the Manukau Harbour, based on a measurement in Bell et al. [32]. STEM assumes K1 and O1 are the same and assumes the KU2 constituent is 5% of the MU2 constituent. Using the peak current speeds for spring and

Results

The results for total kinetic power and estimated extractable electricity are shown in Table 3. A small distributed generation project of about 1 MW mean output technically feasible at both locations. Depth-averaged power densities for Manukau A and Manukau B locations were 0.24 and 0.52 kW/m2 respectively.

The annual electricity demand of a New Zealand home in 2010 was 8.1 MW h, or 8100 kW h, on average [1] and is a helpful metric to understand power generation project size. The number of homes

Discussion

This study shows that the Manukau Harbour has enough tidal energy to make a small distributed generation project of about 1 MW mean output technically feasible, and in 5–10 years the economic feasibility of such a project will improve. Investment in more detailed research and assessment of project feasibility for a site in the Manukau Harbour should occur in 2017. That study can deliver reliable results by doing four things that were beyond the scope of this study;

  • gather field data for current

Acknowledgements

This work was carried out for a Master of Energy thesis at the University of Auckland. The lead author would like to thank his subject matter thesis advisor Dr. Craig Stevens, Principal Scientist, National Institute of Water and Atmospheric Research (NIWA) and Department of Physics, University of Auckland. The lead author also thanks Dr. Jack Hardisty, University of Hull, for providing his tidal current speed model STEM, Dr. Adrian Croucher, University of Auckland, for providing data extracts

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