Bright future for PV solar power - this article outlines why PV-engine hybrids are the optimal solution for the variations in solar irradiance.

Bright future for PV solar power

Variations in solar irradiance, which cause sudden peaks and troughs in photovoltaic (PV) power generation, present a challenge to grids and operators. Wärtsilä’s Smart Power Generation provides grid-scale solutions for this challenge and, by now entering the solar business, Wärtsilä can offer a one-stop-shop solution for both island mode and off-grid locations. This article outlines why PV-engine hybrids are the optimal solution for the solar challenge.

Although solar power is already growing exponentially, analysts say the biggest boom is yet to come. According to Bloomberg New Energy Finance, the installed base of large-scale solar photovoltaic (PV) power systems will grow from about 100 GW to 450 GW between 2015 and 2025. A large share of that growth is predicted to come from emerging markets in Africa, the Middle East, Latin America, and Asia.

Wärtsilä’s existing power plant portfolio consists of an installed base of 60 GW across 176 countries. Moreover, the company is already well established in some of the regions in the world that have been identified as having the best potential for solar, including Africa, the Middle East, Latin America and Southeast Asia. As the only truly global player in the segment, Wärtsilä specialises in the delivery of utility-scale projects with a full Engineering, Procurement and Construction (EPC) scope.

Wärtsilä is now taking the know-how acquired from building hundreds of engine-based power plants in more than 100 countries into the solar market. It will continue to focus on customers with larger-scale PV requirements, including utilities, independent power producers (IPPs) and industrial customers. With a portfolio that includes standalone PV, PV-engine hybrid, and retrofit hybrid solutions, Wärtsilä’s primary target market will be solar plants in the >10 MW range.

In addition to delivering full EPC for solar PV and PV-engine hybrid plants, Wärtsilä can also provide support in the form of project development and arranging financing. Wärtsilä Development and Financial Services (WDFS) is a global team with extensive experience of developing and co-developing IPP projects, now also including standalone PV, PV-engine hybrid and hybrid retrofit plants. WDFS can take development risk and equity positions, as well as providing a range of development and financial services.

How to overcome fluctuations?

While the fuel-saving, emission-cutting advantages of solar speak for themselves, the main drawback of the technology remains its inconsistent capacity, caused by the lack of irradiation at night or on cloudy days. The best way to balance these fluctuations is to invest in PV-engine hybrid technology, as opposed to standalone solar PV.

Standalone solar is a simple, robust technical solution that requires minimal maintenance and allows power plant operators to produce electricity with no fuel costs. While standalone solar may be a good option if the plant feeds into a local network, or a grid that is large enough or has flexible generation installed to cope with variations in production from the solar plant, the vast majority of power plants require a balanced flow of electricity and, in these instances, standalone solar is not ideal. This is due to the fact that solar power plants only produce energy during the day when the sun is shining. With no back-up power generation or storage solution, night-time production is zero.

Reliable solution

A PV-engine hybrid from Wärtsilä consists of a solar PV plant and a Wärtsilä Smart Power Generation power plant, with multiple internal combustion engines that can run on any gaseous or liquid fuels, including biofuels. The key advantages of PV-engine hybrid plants are their ability to achieve the same reliability as a normal thermal plant, making them an interesting solution for small grids and island mode or off-grid locations.

The engine and solar PV units of the hybrid are synchronised, with the solar modules taking priority but receiving back up from the engines. Whenever there is sufficient solar irradiance for the PV modules to produce electricity, the engines are ramped down or stopped, only to ramp up again when clouds cover the sun or once the sun has set. As a result, no fuel is consumed for the MW-hours produced by the solar plant but the engines are always there to provide the all-important baseline.

New build or retrofit?

PV-engine hybrid projects can either be implemented as new builds – where the engine plant and PV plant are both new – or as retrofits, whereby a solar PV plant is added to an existing engine plant.

The feasibility of both retrofits and new builds depend on the same economic logic: although building the additional solar-PV plant incurs higher Capital Expenditure (Capex), this can be cancelled out by the reduced Operational Expenditure (Opex) resulting from the reduced fuel consumption.

A hybrid plant will become economically feasible once the lifetime value of the fuel saved by the PV plant exceeds the total investment in the PV plant. If the fuel cost is above that breakeven point, the PV-engine hybrid is more feasible than the comparative thermal plant on its own. Furthermore, hybrids also make it possible to leverage the synergies from shared Opex and Capex, by utilising the same interconnection point, substation, transmission lines and staff.

Smart Power Generation is the perfect solar complement

Wärtsilä Smart Power Generation is ideally suited to provide the baseload power in a PV-engine hybrid plant because a system of this kind requires engines that are ready to spring into action instantaneously. Unlike traditional power plants based on gas turbines and/or steam turbines, with less flexible capabilities, the engines in a Wärtsilä Smart Power Generation plant are flexible and very quick to start, ramp up and down, as well as load-following. This is key because solar irradiance and wind can come on very quickly and drop off suddenly and it is therefore crucial to be able to balance that in a minimal amount of time.

In order for the engine plant instantaneously and precisely to balance the power produced by the solar PV plant, an interconnection system between the two plants is needed. In smaller systems or island mode the engine automation system is adjusted for operating in isochronous mode, only ramping up and down to keep the frequency stable when solar output varies. The automation system can also be set to maintain the MW output from the hybrid plant by regulating engine output when solar output varies. For reactive power production, the reactive power of the solar plant can be kept constant in the inverter control, while the engines balance the reactive power for the entire plant.

The technical challenges of a solar plant

A customer that has been quick to take advantage of Wärtsilä’s ability to deliver large-scale PV plants is AES Jordan, an energy company based in the Jordanian capital of Amman. In 2014, Wärtsilä delivered a Smart Power Generation plant comprising sixteen Wärtsilä 50DF engines to AES Jordan and, in April last year, the two companies signed a Memorandum of Understanding to build a solar PV farm. Wärtsilä’s EPC scope includes 52 MW of solar modules, covering an area of 81 hectares, as well as inverters, switchgear, control systems and step up transformers.
Solar plants typically require a considerable amount of space, amounting to at least one hectare per MW. As a result, the most labour-intensive part of an EPC project of this magnitude is the civil engineering, which can be quite extensive.

Another major factor in the EPC project is the electrical work required to connect the panels with the cabling, inverters and transformers.

A Wärtsilä solar PV plant consists of solar panels, electrical cabling, inverters, switchgear, control systems, and transformers. The panels are not produced by Wärtsilä but sourced from Tier 1 global solar panel manufacturers. The solar panels are mounted on a structure, which is kept up by posts rammed to the ground. Challenging factors that affect the design of the system include wind conditions and soil properties. The panels are connected in serial to increase the voltage to the right level for the inverter. Panels in series are installed in a row with the panel strings connected to an outgoing DC cable. If individual string monitoring is required, the strings of a few rows are connected to combiner boxes from where the DC power is excavated. Wärtsilä solar plants are made up of a modular set-up of optimally laid out rows where the panels are connected to central inverters. Optimal cabling layout and voltage levels are key parameters for the solar plant design.

Wärtsilä is able to deliver two types of solar panel systems. The first, which is most common, is a fixed tilt mounting system, referring to solar panels that are mounted in a fixed position to the sun. The second option is a single-axis solar tracker solution, whereby the solar panels are mounted on an axis that changes its inclination during the day to ensure that the panels are always positioned in an optimal angle to the sun.

The principal tracking system is single axis tracking, where the panels follow the sun from east to west during the day. The tracking system can increase the energy yield from the panels by some 15–20 per cent or even more in some cases. The drawback is that the space required increases by more than 50 per cent per installed MWdc.

PV-hybrid feasibility study

A recent feasibility study by Wärtsilä has assessed the economic rationale for retrofitting an existing engine power plant with a solar PV plant to create a PV-engine hybrid.

The study examines a theoretical 70 MW engine power plant in the Philippines, operating on Light Fuel Oil (LFO) and retrofitted with a 20 MW solar PV plant, over a 25-year time period. It uses the historical five-year average fuel price of LFO to calculate the fuel savings, and thereby the payback period of the investment into the PV plant. The discounted payback period of the project was found to range from approximately six to nine years, depending on the capacity factor (irradiation levels) assumed for the PV plant. The highest level of irradiation resulted in the shortest payback period, whereas lower irradiation resulted in lower PV generation and, therefore, a longer return on investment.

The feasibility study shows that a retrofit PV plant is an economically feasible investment that will pay itself back well within its lifetime – although the payback time depends on the future cost of fuel. For plants running on LFO, or diesel, the payback period is shorter due to the higher value of the fuel, compared with using heavy fuel oil (HFO), for example.

There are three key factors that drive the feasibility of PV-engine hybrids. They are, as follows: Capex, where lower Capex improves feasibility; energy yield, where feasibility is improved by a higher yield; and, finally, fuel prices, where a higher cost of fuel increases the value of fuel savings and hence the speed of return on investment.

Current market trends favour the future of PV and PV-engine hybrids. As the capital cost of solar PV continues to decrease, while efficiencies and yields improve, payback periods are getting shorter, making PV and PV-engine hybrid plants increasingly viable.

These days, the solar PV panels account for some 50% of the total EPC cost of a solar plant, whereas just a few years ago, they represented as much as two-thirds. As the cost of solar panels continues to decrease, while their efficiency gradually improves, solar PV and PV-engine hybrid plants are only going to become more competitive in the years to come.

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