Ocean economy refers to the economic activities that take place in the ocean, receive outputs from the ocean, and provide inputs to the ocean. Blue economy refers to a sustainable ocean economy, where economic activity is in balance with the long-term capacity of ocean ecosystems to support this activity and remain resilient and healthy. Blue economy policy framework is an operational policy agenda to foster economic growth and development in ocean spaces, while ensuring that the ocean’s natural assets continue to provide the resources and environmental services on which human well-being relies[1]. Sustainable energy provision is fundamental to the transition to a low-carbon economy, and the basis for progressing towards sustainable development globally. It is critical in ensuring progress in areas such as food, water, health, gender equality and poverty alleviation.

Causes of rapid growth in Renewable Energy Production include rising cost of fossil fuels (finite resources), increase in demand for electrical energy, environmental issues caused by conventional sources and reduced costs of renewable energy generation installations.  According to Renewables 2018 global status report[2] 178 GW of renewable power generation capacity added in 2017 globally. New solar photovoltaic generating capacity alone was greater than additions in coal, natural gas and nuclear power combined. Renewable energy accounted for 85% of all new EU power installations in 2017: 23.9 GW of a total 28.3 GW of new power capacity.

Renewable energy sources from the ocean include ocean thermal energy conversion (OTEC), wave energy, tidal energy and offshore wind energy. The growing demand for wind energy offshore installed in recent years has been motivated primarily by reduced space available for installation of wind farms on land and advantages such as better energy production, reduced turbulence intensity, higher capacity factor and absence of size limits. OWFs are expected to become a major source of energy globally in the future. Currently, major utilization of OWFs is visible in Europe and the United States.

Europe leads in offshore wind energy with its first OWF installed in Denmark in 1991. Europe installed 16.8GW (15.6GW in the EU) of gross additional wind power capacity in 2017 bringing the total net installed capacity to 168.7 GW:153 GW onshore and 15.8 GW offshore. With these developments, wind energy is now the second largest form of power generation capacity in Europe, closely approaching gas installations [3]. 15.6GW of new wind power capacity during 2017, an increase of 25% compared to 2016 annual installations. 12.484 GW was onshore, and 3.154 GW were offshore. Onshore installations grew 14.3% while offshore grew 101% compared to 2016. As a result of these developments, wind energy now accounts for 18% of EU’s total installed power generation capacity. Using a technical feasibility method based on technologies available in 2005, it was found that Mozambique, Tanzania, Angola, South Africa and Namibia – have potentially large off-shore wind energy resources[4]. The study also found out that Somalia, Sudan, Libya, Mauritania, Egypt, Madagascar, Kenya and Chad have large on-shore wind energy potential. In Kenya, a Swedish firm, VR Holding AB had in 2016 expressed interest in building a 600MW wind farm in the Indian Ocean waters bordering Ras Ngomeni in Malindi. However, the government turned down the request citing lack of a framework for renewable energy projects of that scale besides low demand for electricity in the country. As a result, the investor moved to Tanzania.The bar chart given in Fig. 1 represents the world wide wind energy installation at present and its estimated future growth[5].

The capacities of offshore wind turbines range from less than 1 megawatt (MW) up to around 5 MW, with turbines of up to 7 MW being tested (European Wind Energy Association, 2015)

Design and Construction Considerations in Offshore Wind Farms

An OWF comprises of offshore wind turbines, a collector network system, step up substation and a transmission grid. Most of the OWFs use 20 kV or 33 kV voltage level for interconnection of individual wind turbines and then stepped up to appropriate voltage level to feed power through cables to the grid. Large wind farms are composed of many megawatt range wind turbines for a large aggregate power potential. All OWFs currently in operation are radially connected to the onshore electric grid through AC or DC submarine cables.

2.1 OWF Installation

The tower structures including wind turbine-generator set in OWF are installed in the seabed. Due to technical reasons, it is difficult to anchor the tower structure directly on the seabed in deep water, where higher potential for generation exists. In the past, the development of OWF with fixed support platforms were based on the experiences gained from onshore wind turbines installation. However, in such fixed platform types, the high frequency excitation due to rotating blades and tower flexibility causes resonance to occur at its natural frequencies. As a result, may significantly shorten its fatigue life when the water depth increases [8].  In the year 2009, the first floating offshore wind turbine was installed by Statoil-Hydro and Siemens near the port of Bergen, Norway. Since then, many floating offshore wind turbines have been installed.  The floating supported structures have greater flexibility of construction and installation procedures and can be easily removed from the OWF system. Fig 2 shows the different offshore wind turbine foundations. 

2.2 Power evacuation and Grid interconnection of Offshore Wind farms

Power evacuation from the OWFs to the onshore grid system is achieved either through Medium voltage/High voltage alternating current system or through a HVDC transmission system. Figure 3 and 4 shows the general configuration of HVAC and HVDC power evacuation topologies for OWFs.

Subsea power cables have a high capacitive shunt component. When a voltage is applied onto a shunt capacitance, capacitive charging currents are generated. These charging currents increase the overall current of the cable reducing the power transfer capability of the cable (thermally limited).The power transfer capability for a specific cable decreases with its shunt capacitance. Like the capacitors, the shunt capacitive component of the cables generates more reactive power and charging currents depending on three factors: The length (magnitude of the shunt capacitive component), the applied voltage and frequency of the applied voltage. The length is determined by the location of the wind farm and it cannot have big changes. The transmission voltage is directly related to the current and the wind farms rated power.

Thus, the main variable which can be changed is the frequency.  Consequently, there are two different types of transmission system configurations; AC and DC. In DC (zero frequency) there are not charging reactive currents. DC configurations do not generate charging currents or reactive power due to the capacitive shunt component of the submarine cable. This is a huge advantage in comparison with AC configurations. However, since the normal power generation, transmission and distribution is AC voltage, and then AC/DC converters technology and the transmission voltage characteristics are the main features of HVDC topology. The transmission is through mono-polar voltage (a SWER) or bipolar (two cables). The bipolar HVDC transmission system can evacuate more power to shore, improve reliability and redundancy.  The AC/DC converters are either LCC (Line commutated converters) thyristor based or VSC (Voltage source converter) based on switching devices with the capability to control their turn on and off.

 

2.3 OWF control and operation

Voltage and frequency control

The various AC machines such as double-fed induction generators (DFIGs), wound rotor induction generators, synchronous generators (SGs) or permanent magnet synchronous generators can be used with variable speed wind turbines. DFIGs are the most widely used due to their overall low cost, modular, compact and standardized construction. Several techniques have been proposed for voltage and frequency control. Static synchronous compensator (STATCOM) is a popular choice for reactive power compensation for voltage control at the wind farm. In other systems control is achieved using classical voltage and frequency droop characteristic to achieve well shared active and reactive power. DFIG connected to the grid via HVDC supplies constant voltage and frequency at stator terminals irrespective of the shaft speed.

 

2.4 Protection and security of OWFs

According to new grid codes in countries with large penetration of wind energy generation, wind farms are required to remain connected to grid even during faulty conditions for a certain period and continue to feed active and reactive power. Wind farm protection system is divided into different protection zones such as wind farm area, collection system, interconnection system and the utility area.  The generator is protected via its circuit breaker having a breaking capacity of 2–3 times of generator rated current. Electric fuses protect the local step up transformer. The collector feeder is protected as a radial distribution feeder by use of over-current relays. The main collector bus, grid   power transformer and integrated transmission system are protected by relays. The submarine cables are protected through either current differential or distance relays with overcurrent relays as backup protection. The voltage sag experienced due to faulty conditions in turn significantly reduces the delivered active power to the grid.  Thus, excess mechanical power leads to increase in rotor speed. Now, the control scheme acts to prevent over speed and protect power converter. Subsequently, failure   in    coordination   or mal-operation of relay may be experienced due to the changes in fault current profile. In addition, submarine cables may cause mal-operation due to the different impedance characteristics and configuration of cable segments. These scenarios should be considered appropriately in design for selecting and setting of their protective devices.

 

2.5 Grid code compliance for OWFs

The increased penetration of wind farm into grid has led to several challenges in its operation and therefore enforcement of stricter grid codes. The grid codes require limits on voltage total harmonic distortion (THD) level at PCC, FRT capability, in addition to participation of wind farms in regulation of frequency and voltage control. Though the grid codes of different transmission system operators may differ, but should at least include the FRT capability, reactive power capability, power modulation capability, frequency response capability and various power quality related characteristics.

 

2.6 Condition and Structural Health Monitoring of OWFs

Condition monitoring (CM) information in utilities is utilized to detect incipient faults, and thus enable to perform more effective and efficient maintenance scheduling. From the operational point of view, any prospective maintenance policy based on CM information should have clear economic benefits; otherwise the initial outlay for the CM system and associated costs are not justified. A significant amount of economic profit exists in operation of OWF. However, this depends on turbine-generator availability. The structural health monitoring (SHM) including turbine generator sets and associated auxiliary devices has proven as one of the most efficient methods to improve the availability of turbine-generator set.

 

2.7 Economics and ecological considerations in Offshore Wind Farms

The key consideration in OWF development is their economic benefit. The cost of electricity in wind farm is influenced by economic depreciation, operation and maintenance cost, tax paid to authorities, energy storage components, etc. The cost analysis also depends on the intermittency factor due to variation in wind speed. Further, land rate, royalties and profit of the wind energy path, incentives, subsidies, production tax credits in the content of installation of wind energy are also to be taken into account for evaluation of wind farm economics. The capital cost of wind energy has progressively decreased, leading to gain in momentum for its utilization on offshore regions. For electricity generation from offshore wind energy, grid connection costs are the major component.

 

Opportunities in Grid Integration of Offshore Wind Farms

OWFs present many opportunities for Kenya and Africa. These are enumerated below:

Wind is typically much faster, stronger and, steady over bodies of water than over land, resulting in increased power potential and efficiency. There is also space availability in the ocean for installation of the turbines without interfering with populated areas.

OWFs provide renewable energy and they do not emit environmental pollutants or greenhouse gases.

The offshore wind industry has an “additional employment effect” due to the higher cost of installing, operating, and maintaining offshore wind turbines than land-based ones.

As global commitment to renewables increases in the future, more attention is likely to become focused on the immense stores of energy in the ocean.

  1. Challenges in Grid Integration of Offshore Wind Farms and Mitigation measures

Due to their unique features, OWFs present a number of challenges in their design, operation and   maintenance with regard to grid integration. The challenges and possible mitigation measures are summarized in Table .

 

Conclusion

In this paper, the design and construction features of offshore wind farms have been reviewed. The power evacuation and grid integration of OWFs has also been highlighted. Opportunities that stand to accrue as a result of grid integration of OWFs have been highlighted. Due to their unique characteristics, OWFs present some challenges with respect to their grid integration. Some of the challenges have been highlighted and possible mitigation measures against them have also been suggested.

 

A pilot project could be initiated to evaluate the viability of OWFs in Kenya through a Public private partnership to develop a test bed that will enable gathering of operating experience, demonstrate technical reliability and verify commercial feasibility.

Table 1: Challenges and Mitigation Measures in OWFs integration

No. Challenges Possible  Mitigation Measures
1. Limited experience in system operation with increased penetration of Renewable Energy Generation including OWFs Review the international practices and implement capacity building programs to fulfil the gaps

Develop roadmap for development of local capacity.

2. Current Grid code do not specifically address specific characteristics of offshore wind and its transmission connection Grid code modifications to be in line with the  characteristics and capacity of OWFs

Review the need for a separate grid code or modifications to suit OWFs

3. Current Planning standards do not address reliability standards for OWFs Develop OWFs offshore planning standards

 

4. HVAC subsea transmission cables experience limitations in terms of the feasible transmission distance and high power losses and resonance problems. This is caused by capacitance effects. Use HVDC transmission system for longer distances and higher power rated OWFs.
5. Wind generation is characterised by generation variability, intermittency and low fault Level. Hence integration of OWFs in a power system will pose operational challenges Invest in detailed analysis and understanding of the technical challenges and corresponding mitigation measures to be undertaken through proper planning, design and prudent operation of the grid.
6. Operational challenges in integration of OWFs to the national grid include:- Frequency instability, Lack of generation reserves, Inaccurate load forecasting and scheduling of renewable plants. Conduct research into the impact of grid integration of OWFs on voltage and frequency stability, forecasting and scheduling methods for RE plants.

(ii)Increase spinning generation reserve to mitigate against variability of power from OWFs

7. High operation and maintenance costs due to both   the size   and the locations of OWFs. Invest in condition monitoring systems to enable scheduled maintenance and thereby reduce breakdowns.
8. Installation of OWF pose a threat to the life of birds, affects the sea shore habitat, causes barrier in migration routes, and disconnection of ecological environment such as roosting and feeding sites. Careful siting of windfarms to avoid important bird, fish and other aquatic migration corridors.

 

Leave a Reply