About Francesco Miceli

Hello! My name is Francesco and I'm a civil engineer specialized in EPC (that is, "turnkey") wind farms projects. I'm currently based in Hamburg, Germany and I'm developing several interesting project all around the world - southern Europe, LATAM and various other countries. If you want to contact me please don't leave a comment in the blog (I don't check them very often) - you can use the contact form. You can write me in English, Spanish and Italian. To find a (somewhat concise) description of my non-wind business activities you can visit my webpage - www.francescomiceli.com If you want to know more about my work, here you can download my CV - www.windfarmbop.com/CV_Francesco_Miceli.pdf Hope you like the blog! Francesco

Automatic cost estimator - the Holy Grail of BoP

Yesterday I had the pleasure to drink an overpriced coffe (2,90€ for an Espresso? Really?) with my good friend José Ramón. He told me that I’m not writing on the blog often enough so I’ve decided to make an effort and find some time to write this post.

The subject I have selected is an evergreen topic, the Holy Grail of BoP – the possibility to create a tool that could calculate quickly the cost of the BoP of a wind farm.

There is already a good amount of material on the subject online, for instance this website of the University of Strathclyde (Glasgow) that present a model created in collaboration with SgurrEnergy (now part of the Wood Group's Clean Energy business).

You can download the tool from their web or from this link for your convenience: BoP estimator tool

I have decided to take it as starting point to show why the task is not so easy and probably me and the other engineers in the team will not be substituted by an Excel file anytime soon.

The ultimate purpose of such models is to pick a small number of input (to make the tool usable) without losing to much in accuracy. The guys at the University decided to go for an extreme simplification and selected only 6 inputs:

  1. Number of WTGs
  2. Turbines Rating
  3. Km of new roads
  4. Km of existing roads
  5. Km of cabling to substation
  6. Km if cabling to grid

That is a very, very extreme oversimplification.

For instance, the model doesn’t keep into account the topography of the area (flat, hilly, mountainous) or other relevant factors (poor soils, inundable areas, etc.) and link the cost only to the rated power of the turbine. As a consequence the calculation of the crane pads cost show a big dispersion in prices (from 5.000 to 42.000 Pounds) and a very low R Squared value (0.26 – that is, the model isn’t explaining the correlation).

Additionally, the model doesn’t consider any monetary input but the output is monetary. I believe it’s rather hard to accept this simplification: for instance, around 50% of the price of cables is in raw materials like copper, that have a high volatility. This could easily bring a multi million inaccuracy.

Also, there is no such a thing as a standard substation – and this is why we have very good electrical engineer in the team. Even without considering the peculiarities of the local grids (something hard to ignore when they are weak, like in Australia) different customers have also different needs. A customer interested in business certainty will ask for redundancy in the substation – 2 main transformer instead of one, emergency “cold” spare transformer, etc.

Same for the foundations: there are currently so many technical solution in the market (precast, with rock anchors, braced, P&H, etc.) that it is really hard to find a correlation between wind turbine MW and foundation cost. There is so much money in foundation and so much pressure on prices that project specific foundations nowadays are the norm, not the exception.

Circular economy: use of wind turbines blades as combustible and mix material for cement production

One of the future challenges of wind energy is to find a solution to recycle old blades from decommissioned wind turbines. In this post I will try to summarize several possible alternatives and to describe in detail what I think is currently the best option: to use them as a component for the production of cement.

The development of this interesting technical solution started in 2005 when one of the largest wind turbine manufacturer asked LafargeHolcim (a global cement producer) to use the decommissioned blades in cement production plants.

In 2008 Geocycle (the business unit of LafargeHolcim specialized in use of waste to produce cement) launched the full scale development of the solution in the Lägerdorf cement plant, in Northern Germany.

The recycling start in the wind farm, where the blades are cut in 10 meters long pieces using a mobile cutting technique that reduce the generation of fine dust humidifying the area. The resulting water is collected and brought to the recycling plant together with the other materials.

The pieces are then transported by train or truck to a pre-processing plant.

Here the blades are cut before in segments with a length around 1 meter and subsequently shredded in smaller pieces, with a final length of some millimetres.

Metals, both ferrous and not ferrous, are separated automatically from the material flow by magnet and eddy current magnet devices.

Finally, the crushed blade dust is mixed with a humid substrate material made of other residues such as plastic labels and miscellaneous packaging materials. The purpose of this substrate is to homogenize and bind together the blade dust. The material composing the substrate are wastes from other processes such as paper recycling.

All process steps in the pre-treatment plant are fully automated and performed in a controlled atmosphere. This guarantee maximum levels of occupational health and safety.

The end product is put in the cement plant.

In the pre-calciner (an element of the cement plant that optimize fuel consumption) the resin matrix is used as an alternative fuel, substituting coal, petroleum coke, heavy fuel oil or natural gas.

At a temperature around 900 °C the resin burn transforming the blade fibers into ashes. These ashes are then going together with the rest of the calcinated raw materials - usually a mixture of limestone and clay - in the sintering zone of the cement kiln to produce the clinker.

Finally the clinker is ground into a fine powder together with a small amount of gypsum creating the final product, cement.

The cement produced using wind turbine blades is indistinguishable in terms of quality from the standard product obtained without using the blades. It can be sold in the market or used again in wind farms, for instance in wind turbines foundations, closing the loop.

This solution provide alternative raw materials for cement production, reducing the need for quarrying, stone crushing and transportation. The ash of a wind turbine blade consists mainly of silica (SiO2) and calcium oxide (CaO) and due to this substantial amounts of natural resources like carbonate rock (limestone) and clay (usually in the form of sand) can be saved.

Additionally, it has to be noted that this solution contribute to savings of fossil fuel: the resin of the blade provide energy (around 12 MJ/Kg). This value is approximately the half of hard coal:  therefore each tonne of rotor blade substitute around half a tonne of hard coal.

It’s important to note that this solution has been indicated in 2011 as the best option available in a Position Paper subscribed by the European Composites Industry Association and other relevant category associations.

The test phase was successfully concluded in 2009 and the solution has been made commercially available since 2010.

I see several interesting benefits associated with this technique:

  1. It is capable of dealing with large amounts of materials, since the cement plants can handle thousands of tons of material.
  2. It address as much as possible geographically local opportunities, because it’s operated by LafargeHolcim, a multinational company active worldwide with over 2000 operation sites.
  3. It is feasible right now, because it is a commercially mature solution currently used in Germany and neighbouring countries. Hundreds of blades have already been recycled.
  4. It is cost-effective, because is the second cheapest alternative after landfilling.
  5. It address the totality of the materials of the blades, because the resin matrix is used as combustible, the fibres become part of the cement and other elements (e.g. metallic components) are separated upfront in the pre-processing unit.
  6. It follow strict environmental and safety standards. This solution has been developed in Germany (one of the first country to ban the landfill of wind turbines blades) and it has been certified by third party bodies.

The final cost is function of several parameters, such as the number of blades to be recycled, the distance of the wind farm from the nearest cement plant and the need to adapt the pre-processing facility.

Therefore, final cost will depend on scale factors, logistic costs and pre-treatment costs.

There are several other alternative. I will try to summarize them in the next paragraphs.

Landfill: Blades are sectioned into pieces of suitable dimensions and placed in a landfill. Transportation and preliminary shredding is needed.

Here no material recovery is possible and some national and local legislation impose high landfill taxes or complete bans on composites material with high organic percentage such as the blades.

Additionally, legislation worldwide is expected to become stricter acting as a driver for the development of alternative solutions and the creation of a recycling market, either with an extended ban or with a sharp price increase for landfill of blades.

Mechanical processes: Blades are reduced in size and separate into powder and fibrous fractions via cutting, crushing, shredding, grinding, or milling processes.

Finer pieces are sorted and used as fillers or reinforcements or as fuel for thermal waste processes.

This is currently done on a limited commercial scale and theoretically it has a wide range of potential applications for material recovery.

However, from an economical point of view it is competing with the lower market cost of alternative virgin fillers (calcium carbonate, silica).

Additionally, it has been observed a reduction of the mechanical properties of the resulting material (lower stiffness and strength). For instance, concrete made using shredded blades instead of crushed stones has unsatisfactory mechanical properties.

The only viable mechanical process currently operating in the market, presented as the competitor solution, is the one offered by Global Fiberglass Solutions, detailed in the following paragraph.

Mechanical processes - Global Fiberglass Solutions (GFS) pellets: The blades are transformed into small pellets that are sold under the brand name of “EcoPoly Pellets”. They are usable in injection mold and extrusion manufacturing processes and they are made from a customized blend of wind turbine blade materials.

EcoPoly Pellets are made to order for final customers based on the requirements of the customer’s own manufacturing process. GFS has initiated distribution options for pellet and has already found several wind farmers owner willing to provide decommissioned blades as input material for pellet production.

I believe that this technique it is currently available only in the USA.

Thermal processes - incineration: Blade sections are incinerated at high temperatures – usually over 800 °C.

Organic substances are combusted and converted into non-combustible material (ash), flue gas and energy. This process is usually combined with energy production and heat recovery.

The ash could also be used as a substitute for aggregate in other applications recovering the material or landfilled. However, no economically viable uses have been found for the ashes (beside the proposed use as input for cement production).

The advantage is that there are already numerous incineration plants available and that it can be done at attractive prices.

The disadvantage is that no large parts can be incinerated, making necessary a preliminary shredding.

Additionally, blades have a high ash content (around 50% to 65%) that needs to be landfilled afterwards with the associated transport and landfill tax costs.

Finally, burning blades create residues that can cause problems in the gas filtering systems.

For all these reasons this solution has been discarded.

Thermal processes – Pyrolysis: Blades are sectioned into suitable dimensions and decomposed using heating ovens in an inert atmosphere (450-700 °C).

Material is recovered in the form of fibres which can be reused in other industries. For instance, in pilot tests they have been used for glues, paints and concrete. Other products of this process include syngas (that can later be combusted for electricity and heat recovery) and char (recycled as fertilizer)

Under certain boundary conditions the resin matrix can be transformed in an oil “Pyrolysis Oil”.

The problem with this technique is that significant amount of energy are needed to activate the pyrolysis, impacting the overall environmental value of the solution.

Furthermore, available laboratory tests show a degradation of the properties of the glass fibers and no secondary market has been found.

Additionally, test have been performed only at laboratory scale so it is considered a solution with a low technological maturity.

Thermochemical processes – Solvolysis: Chemical solvents (water, alcohol, acid) are used to break the resin matrix bonds at elevated temperatures (300-650 °C) and pressures conditions.

Among all the solvents currently tested, water appears as the most commonly used. In this case the process is called hydrolysis.

Fibre materials recovered have a similar strength and could be reused in other applications. The resins as well can be separated and combusted for energy recovery

As far as I am aware test have been performed only at laboratory scale, mainly with focus on carbon fibres composites.

Moreover, due to the low market price of glass fiber, no secondary market is existing.

Lightning protection of wind turbine blades

Image copyright of Elsevier & X.Bian. Published on “Numerical analysis of lightning attachment to wind turbine blade”

I have received a question from a reader regarding blades damaged by lightning.

Specifically, the blade has been damaged before commissioning.

At first sight the consequences could be less significant than usual, above all if the main crane is still on site and there is a set of spare blades available as happens frequently in big wind farms.

I would also guess that the turbine supplier will have to absorb the cost unless the risk was already transferred to the customer.

I would also like to elaborate a bit more on the topic as lightning is a frequent cause of damage to wind turbines (specifically to the blades, as they are hit in around 75% of the cases).

Lightning are created by the electric field between the bottom of the clouds (negative) and the ground (positive).

The potential difference is significant (some MV). However, due to the distance, the average electric field is weak.

As the electrical charges at the bottom of the clouds accumulate a “downward leader” (a channel permitting the flow of negative charges) start moving toward the ground.

If this stepped leader is somewhere nearby a wind turbine (or another similar structure) the second phase of the phenomenon may start: an “upward leader” from the blade connecting with the downward leader and closing the circuit.

This is the instant where the “return stroke” start and you usually see the majority of the light.

Coming back to the blades the standard technical solution consist in embedding a copper receptor connected to the ground at approximately 1 meter to the tip. This receptor has a diameter of at least 50 mm (pretty much like the copper cable used in the earthing of the foundations).

The problem is that this solution doesn't works always: sometimes the lightning hit another point of the blade. Even if the surface of the blade is supposedly non conductive it has been observed that, due to the presence of pollution and water, it can behave like a conductor.

RUTE precast modular wind turbine foundation

Some days ago I have been contacted by Doug Krause, founder of RUTE - a green start up proposing an interesting solution for the wind turbine foundation.

Taking inspiration from the technology used in bridge construction RUTE is proposing a system of post-tensioned beams connected to a central hub. Each beam has an anchor system connecting it to the soil, and the foundation is delivered to the wind farms in around 20 elements.

Among the benefit the fact that the system is modular, less prone to quality problems (it is manufactured in specialized plants) and, at least in principle, reusable after 20 to 30 years for a new foundation: the lifetime of the components is over 40 years.

Decommissioning is also probably easier with such structure, at least compared with a standard shallow foundation.

Installation times can be cut as well – as it is delivered hardened it is ready for installation in few days from the start of the works.

I had a look at the technical specifications and I have seen that the bottom of the excavation is at the same depth of the standard solution, so no savings here. I have also noticed that in some situation soil substitution could be needed.

I have seen in their website that there is already a full-scale prototype built, so it’s much more than a concept. It has been installed at the Palmers Creek Wind Farm (Minnesota) on a 2.5 MW GE turbine with a 90 meter hub height.

Addenda (10 June 2019): I've received an email from the founder of the company. I post it here for the benefit of all readers.

Thank you Francesco for noticing RUTE.
That's a picture of our rock anchor, bulb T girder, model TG. The one we built in Minnesota is a box girder style, BX Foundation. It behaves just like an inverted T, spread foundation.

RUTE's biggest value to the BOP contractor is time. So most of the foundation works can happen off the project books and schedule. So a project can close finance and be erecting towers the same month. We'll hope to prove that claim this year.

Apart from the main BOP driver, the facility owner can run a pro forma out 30 years, or 40 years, the normal term of the land lease. And in those cases a foundation with bridge design, like ours, lasts well past 40 years. That's just a function of the post-tensioning which keeps the concrete in permanent compression. So there's an order of magnitude less fatigue damage than conventional reinforced concrete.

I can share some pictures from inside the foundation. You can walk around inside it and inspect.

Best Regards,
Doug, RUTE

Wind farm optimization algorithms

I have always been amazed by the number of published papers, master thesis and documents focusing on the use of algorithms to optimize the layout of a wind farm. Some of them were proposed more than 25 years ago, showing a continuous, sustained interest in the topic.

I guess that the reason for such abundance is the stimulating difficulty of the problem and the fact that there are huge investments behind a wind farm.

From a mathematical perspective the problem is complex due to the type of variables involved, both discrete (you can have 30 or 31 turbine but not 30.5) and continuous (for instance, the length of cables). Additionally there are strong links between variables (for instance higher turbines = higher tower and foundation cost) so finding the “sweet spot” that maximize earnings is not a simple task.

Generally speaking, these algorithm try to maximize the profitability of the investment, usually expressed in terms of Net Present Value (NPV). Basically they compare the value of all expenditures during the life of the project “in today money” with all the earning “in today money” using a certain discount rate for cash flows in the future.

Expenses belong to two categories, capital expenses (CAPEX) and operational expenses (OPEX), while net earnings are function of the amount of power produced, the price of electricity and the electrical losses.

Therefore even a simplified model should try to minimize these expenses:

  • Wind turbine
    • Model (power curve)
    • Tower
    • Installation
  • Civil works
    • Foundations
    • Roads
  • Electrical works
    • MV cables
    • Substation
  • Operation & Maintenance

While maximizing the production, a mainly a function of:

  • Wind
  • Wind shear (of the speed of the wind increase with height)
  • Wake effect (how turbine interact with each other creating turbulences)

The interaction between all these variables is what makes the problem interesting.

To give a few examples,

  1. Packing the turbines densely in a small area will lower the cost of roads and cables but will create huge production losses due to the turbulences inducted by the turbines upwind.
  2. Using a higher tower should increase the production – unless the wind shear is low, in which case the additional tower and installation costs would off weight the benefits
  3. A certain position could be extremely productive – but it could be very far away from the substation (increasing the electrical losses ) or on the top of a steep hill (increasing the earthworks cost)

Additionally you have to decide the level of complexity of the model. For instance the foundation cost can be considered as:

  • A lump sum, equal for all turbine models. Under such assumption, you would see a benefit decreasing the number of turbines but not switching to a different WTG model.
  • A function if the wind turbine model (greater loads = greater foundation).
  • A function of wind turbine model, geotechnical parameters of the soil and unit cost of concrete of still. This latter option, although more precise, would probably make the model very difficult to handle.

I believe that a reasonable compromise between complexity of the model and quality of the result can be achieved using nested algorithms as proposed by these researchers.

In the first steps, only the variables related to the turbines (power curve, wind resource, availability and cost) are considered. Once the turbine model and the layout are fixed the civil and electrical works can be considered, defining the optimum position of the substation (to minimize cable length) and the shortest roads connecting the wind turbines.

How much does it cost a wind turbine?

Onshore wind turbines - price per MW (Millions Euro)

The easy answer to this question is “Today it costs less then yesterday. And probably tomorrow it will cost less than today”.

Today (April 2019) the average price is around 700.000€ per MW – that is, expect to pay around 3 ML€ for a 4 MW wind turbine. That’s a huge reduction when you consider that some years ago the easy to remember formula was 1MW = 1ML€

If you are working in the wind industry you are probably aware of the huge pressure on wind turbine prices, driven by several factors and resulting in turbines cheaper than ever.

It is interesting to observe that, in the current market condition where wind turbines are very cheap, the majority of the main wind turbine manufacturers are reporting very solid order intake figures. However, net profit is still elusive and EBIT margin are very low.

For instance, Vestas reported 9.5% for 2018 while Siemens/Gamesa 7.6% (pre PPA and I&R costs) for the same period, with a guiding range for 2019 between 7% and 8.5%.

It looks like manufacturers are having more luck in the maintenance side of the business: margins there are significantly better.

One of the consequences of this situation is that several players are leaving the market (Senvion declared bankruptcy some weeks ago) and the consolidation of the sector continue: there are rumours about a possible purchase of Suzlon (heavily indebted) by Vestas, while Enercon absorbed the Dutch manufacturer Lagerwey some time ago.

In case you are wondering about the origin of the figures in this post I’ve taken the numbers for this post from the official annual statements of Siemens/Gamesa and Vestas and not from my friends working there 😉

Wood towers for wind turbines

I always believed that wood towers for wind turbines were a solution possible only in small, domestic WTGs (somewhere around 10 kW to maybe maximum 100 kW). There are several example available, for instance this product of InnoVentum.

Well, I was wrong: I see that some years ago (2012) a Vensys 77 1,5 MW turbine has been installed on a 100 meters tower. That is quite a number: a 77 meters rotor is considered small for today standard, however it fully qualify as a “utility scale” solution.

This full scale prototype followed a 25 meters test tower built by the same companies some years before.

Developed by 2 German companies (TimberTower & TiComTec) it has been built near Hannover. The foundation is standard (concrete) and the connection between the tower and the foundation is made trough 4 meters long steel rods.

With a somehow unusual octagonal cross section the tower diameter is comparable to a standard concrete or steel solution. I see however that other geometries are possible (hexagonal or dodecagonal).

The life span of this solution looks similar to the steel alternative (20 years). Unfortunately I haven’t been able to find information regarding the cost. For the sake of clarity it is not 100% wood – few steel elements are used inside the tower.

 

Wind farm project management: PRICE2 vs PMP

In another post I have discussed my experience with the PMP certification and its (somehow weak) relationship with wind farm project management.

In a nutshell, several processes and concepts are not directly applicable given the peculiarity of our industry. Said that, I believe that the PMP has a value, because it explain in detail a lot of tools and ideas that are used daily – for instance dependency types in Gantt charts, how to handle risk management or the “stakeholder management” concept.

Some days ago I have made another PM certification, the British de facto standard PRINCE2. The main reason for that has been my curiosity to see another point of view on a very broad topic such as project management.

Both certifications are trying to live together and differentiate themselves:

  • PMP define itself as a “standard” and comes with a mountain of processes and techniques.
  • PRINCE2 define itself as a “methodology”, coming with models and templates but very few techniques.

My personal impression is that the role of the Project Manager in PRICE2 is less relevant compared to the PMP idea of the same role – basically he has some decision margins, but he’s somehow squeezed between the Project Board making the big decisions and the teams doing the actual job. As soon as one of the tolerance levels is exceeded, he need to escalate the problem to the Project Board (that in the wind industry, if it exist, is usually called “Steering Committee” or something similar).

This image is strikingly different from the one that my mentor Luis Miguel gave me when he told me that, in the infancy of the wind industry, the PM was basically “the God of construction site”. Possibly the image is a bit strong but it makes the concept crystal clear.

Said that I also had the feeling that PRINCE2 was much easier to follow in the definition of the workflow, with less processes (seven) clearly linked between them and few key concepts (“Themes” and “Principles”). Understanding the relationship between the 49 processes of PMP is not that easy: I had to print them in a huge A0 and spend a lot of time staring at it to make a sense of them.

An interesting argument that I want to mention against PRINCE2 is that it is “unfalsifiable”, in the scientific sense defined by Karl Popper. This basically means that PRINCE2 has to be tailored (that is, adapted) to the specific project to work properly. If something goes wrong, you cannot proof that the problem is PRINCE2 (because possibly the tailoring you have made was wrong).

In conclusion I would recommend both certification to people interested in knowing more about project management, even if some tools, techniques and processes are not used in wind farms construction.

Concrete tower assembly in Chile

 

An interesting video on the use of concrete towers in Chile. Among the benefits of this solution the creation of local jobs (several hundred for factory) and the increase of local content (the amount of goods and services provided locally, an important parameter in some tenders).

Concrete towers are especially cost effective when the hub height is over 100 meters. Additionally they are less prone to price variation - steel prices, at least in Europe, dropped in 2016 to rebound in 2018.

Finally transport cost are usually lower, at least if the factory is located near the wind farm as it is usual.

Long term instrumented monitoring of wind turbine foundations cracks evolution

Fiber Bragg grating sensors (Image copyright fbgs.com)

Some weeks ago I have discovered that, as I am currently enrolled as a university student (getting “slowly but steady” a second degree in Economics) I have full access to the Elsevier database.

This is an enormous amount of information, including all the best scientific papers and technical articles published by industry journals.

I am using this possibility to learn more and stay updated on several niche topics that I found interesting – from recycling of wind turbine to bird strikes to foundations pathologies.

Browsing the database, I recently stumbled upon an interesting article published by Jack McAlorum et al. from the University of Strathclyde (Glasgow).

The paper is called “Deterioration of cracks in onshore wind turbine foundation”.

The authors instrumented an octagonal slab foundation (sometimes called “star foundation” or “wall foundation”) to monitor the evolution of existing cracks.

I already wrote a couple of posts on foundation cracks: they can be due to a variety of root causes, such as:

  • Design mistakes
  • Errors in the composition of concrete mix
  • Extreme temperatures
  • Errors in the execution of the wind turbine (for instance, concrete poured in different batches creating construction joints)
  • Failures due to the use of an embedded can (this is a frequent failure reason for older wind turbines)

The paper does not specify the reason for the cracks. However, as typical, the most severe cracks were in the side of the wind turbine facing the wind (as the concrete is in tension there).

What it is interesting is the fact that the behaviour of the foundation has been monitored for a very long period (over 9 months) and under standard operating conditions. This is very unusual: while other key component of the turbine like the gearbox are constantly monitored and the data is collected trying to detect problems and predict failures, I have never heard of such monitoring for the foundation.

Additionally it is interesting the type of sensor used: instead of standard accelerometers or strain gauges the researchers used a strain sensor based on fibre-optic called “fibre Bragg gratings” (FBGs).

Basically it is a sort section of optical fiber treated in a way that some specific wavelengths are reflected and some are transmitted. They can be used as a strain sensor because when they are deformed the transmitted and reflected wavelengths shift, allowing a calculation of deformation.

Cracks can evolve with 3 different displacement type:

  1. Opening (the crack becomes wider)
  2. Sliding (one face of the crack slides on top of the other)
  3. Tearing

Through the monitoring period no significant evolution of cracks was observed. Basically, the wind turbine owner was lucky: cracks did not deteriorate and no intervention was needed.

Unfortunately, the cost associated with the monitoring are not shared, so it is difficult to make a business case (cost of immediately repairing the cracks with grouts or epoxy resins vs. cost of monitoring to see if the intervention is needed).

I also see that this solution only allow monitoring visible cracks. This is a strong limitation, as several failures originate in a non-visible area of the foundation.

Said that the idea is certainly interesting and useful, above all considering that some turbine are kept in operation for a very long time, even exceeding the design life of the foundation (usually 20 years).