Renewables

Marnie Surfaceblow: A Desert Solar Farm's Muddy Problem

Long-time POWER readers may remember Marmaduke Surfaceblow, a fictional character whose engineering escapades were brilliantly portrayed in hundreds of stories published within POWER magazine’s pages over more than 30 years beginning in 1948. Today, the fictional series continues through Marmy’s granddaughter, Marnie, who is an engineering wiz in her own right.

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Sometimes fixing one problem causes another or leads to a chain of failures that can be difficult to diagnose. Consider all of the upstream and downstream impacts when changing equipment and/or operations.

“Really, now, am I expected to fry my behind on that?” Marnie Surfaceblow, vice president of Surfaceblow & Associates International, stared with disbelieving horror as she aimed Maya’s thermal imager at the rental car seat. “Does that actually say 115 degrees Celsius or have I been driven mad by the heat? And since we’re in Botswana, where is this famous ‘bush tea’ I keep hearing about? Will it wake me up better than that airline, um … coffee?”

Maya Sharma, Marnie’s lead field engineer, dropped her luggage in the trunk with a heavy clunk, and sighed. “Ma’am, allow me to respond in reverse sequence to your queries. First, rooibos tea, which you call ‘bush tea’ only because you binge-watched The No. 1 Ladies’ Detective Agency on the flight, does not contain caffeine. Second, I fear you have always been mad. Third, calibrate the camera’s albedo setting; the seats are likely only 100 degrees Celsius. Finally, you sit on these, not the bare seat.” Maya brandished two beaded seat covers, tri-color patterned like the national flag, and placed them on the seats.

Not fully trusting their efficacy, Marnie settled gingerly onto the bead-covered passenger seat, and they soon were on their two-hour drive to a solar photovoltaic (PV) power station that was having, as Marnie termed it, “a wee bit of trouble.” After setting the cruise control on the motorway, but keeping her eyes glued to the road, Maya asked, “Ma’am, while the data we received appears most interesting, I am suspecting you have another reason for us accepting this project?”

“Two reasons,” Marnie replied cheerfully as she watched the dry savannah pass by her view. “Botswana is pushing plentiful PV power potential, practically predestining our presence.” Maya shook her head at the epic alliteration, while Marnie continued. “Moreover, the plant manager studied at university with my father, Guy Newcomen Surfaceblow, and is practically one of our family.”

“And your needing additional frequent flier miles by month’s end to achieve ‘Triple Platinum’ membership was coincidence?” Maya deadpanned.

After an uncomfortable pause, Marnie replied sotto voce, “They even give you titanium luggage tags with your name in gold leaf.”

Welcome to Botswana

After a perfunctory check-in with security, the duo parked in front of a small engineering building, apparently a repurposed former schoolhouse. Despite their sunglasses, the scattered incidental reflections from the PV panels surrounding them were nearly blinding (Figure 1), and exceedingly hot, spawning dust devils between the PV panels. Aligned in neat rows and rolling up and down the surrounding hills, Marnie marveled that the 20-MW plant covered just over 400 acres (162 hectares). “A 100-MW combustion turbine would take less than a thousandth the space,” she commented.

1. The sun was a “wee bit bright,” and the temperature was plenty warm when Marnie and Maya arrived in Botswana. Source: POWER

Maya nodded, “It is a sacrifice of land, weighed against the benefits of energy independence and sustainable renewable energy. The soil here was eroded from decades of poor farming practice; thus, this is a good use for it.”

The mournful susurration of the wind was interrupted by a happy cry. “My goodness, is that little Marnie, all grown up now then?! Welcome to where the sun stays to rest, passing on its blessings!”

The two ladies turned to face a tall elderly man dressed in white business attire, leaning on a cane but with a quiet strength, whose smile was a warm welcome. Three other men, dressed in similar attire but with somewhat wearily wary looks, stood behind him.

“Uncle Mo-mo!” Marnie squealed with delight, running forward to hug the older gentleman. Almost knocked off his feet, he laughed warmly, then chided, “Easy child, more than 80 years have I seen, let me see at least one more!”

Introductions were made, with “Uncle Mo-mo” being introduced as Mosegi Mosweu, the plant manager. The three men with him were the entire power station engineering team—Ashish Jain from India, Dirk Becker from Germany, and Jun Huang from China. After being hustled out of the heat and into the somewhat cooler engineering building, Marnie and Maya were led to a small conference room, with tinted and dust-coated windows overlooking the PV rows. Marnie froze suddenly, sniffing the air suspiciously, then broke into a dimpled smile. “Unc … I mean, Mr. Mosweu, do I smell not mere pedestrian coffee, but the Marmaduke Family Special House Blend?!” Mr. Mosweu smiled further, “Specially imported for your pleasure, Ms. Surfaceblow.” Marnie needed no further encouragement, quickly filling the largest mug that she could find.

Hard-to-Clean PV Panels

The ancient business ritual of the exchanging of business cards, plugging in of laptops and phones, and small talk about the weather ensued. Mr. Mosweu cleared his throat as a signal for everyone to return to character. Jun called up a PowerPoint and connected her laptop to a large flat-screen monitor.

Dirk took the floor first. “This power station has operated for two years. The reliability is adequate, but the performance mediocre. You have seen the dust? It is everywhere, and the panels are covered with it. Power output is reduced by anything from 9% to 16% between cleaning activities.”

“Sir, can you describe how your PV cleaning system performs?” asked Maya.

Dirk stared at Maya in disbelief, shook his head and raised his hands dramatically, and scoffed “Fraulein, you did not read the design documents we sent?”

Mr. Mosweu started to raise a hand, but stopped as Marnie shook her head quickly. Maya, fixing Dirk with a gimlet stare, replied steadily, “Your original plan was using demineralized well water on-site, but your aquifer is depressed from agriculture needs. After months without well water, you changed to filtered river water. It is delivered by four trucks per day, each containing 7,500 liters of water, and I estimate you require greater than 6,000,000 liters of water annually. Your water cleaning system employs water spray at two pressure levels. Both to avoid thermal shock and to take advantage of your much stronger night breezes, you operate the system very late in the evening. Likewise, the system starts at low pressure to create a light mist, then switches after five minutes to a water stream.”

Pausing for breath, Maya drank her tea, closed her eyes for a second, then stood up and pointed a finger at the German engineer. “What I am asking you, sir, is for reality. How does your system actually perform?!”

Shocked but impressed, Dirk nodded his head. “My apologies, Fraulein. You understand the system well, and certainly you also see it requires too much water. There is significant overspray of many panels, and this leads to high water costs.”

“And let us not forget the impact on the river and wildlife,” added Ashish. “Also, only some water falls from the panels to nourish the soil. Much evaporates as drift and is lost.” Turning to address Marnie, Ashish added, “Ma’am, can you help us improve our water cleaning system?”

“What about using compressed air, electrostatic, or motorized brushes?” asked Marnie, flipping through plant operations and maintenance records on her laptop.

“I have studied many methods, testing on one row of panels,” replied Ashish. “Compressed air systems have required much auxiliary power, and the dust is very abrasive. The dust has high electrical resistivity, requiring powerful electrostatic charges. These also require much power, and have damaged both our monitoring and control components for the sun tracking systems.”

“We tested rotating and sweeping brushes, and I designed a system myself,” added Dirk, shaking his head. “The abrasive ash again scores the panels, causing lost efficiency.”

“In short, my child, we seek a different way,” Mr. Mosweu concluded. “Something creative, such as that one time,” he shook a playfully chiding finger toward Marnie, “when not only did you use that mowing machine motor to power your bicycle, but you fueled it with your grandfather’s Sandpaper gin. Oh child, he was most cross, yes?”

Marnie struggled keeping her composure, a half-smile playing about her lips. “He wasn’t that upset. At my university graduation party, he took me aside and told me the lost gin was worth seeing me carry on the family tradition.”

Maya paused from plotting data on her laptop, and with a raised eyebrow said, “I am most impressed you operated a motor with a fuel comprised of greater than 50% water.”

Marnie shrugged. “That Sandpaper gin of his, well, it had a tad bit more kick than 100 proof. Remember the fire I told you about …”

“At your cousin’s Quinceañera? Yes, I’ve heard about it many times,” Maya replied quickly.

A Consistent Inconsistency

“I have read of your great works, and the family tradition is well-bourn by you, Marnie. So, you can help your old Uncle Mo-mo?” Mr. Mosweu asked with an expectant smile.

2. With the help of summer interns, Maya and Marnie produced color-coded satellite plots of the solar farm based on each panel’s actual power output versus design. Source: POWER

It was Maya who answered, but with a question. “Mr. Mosweu, and company, we have examined your panel design and performance, employing many methods, and we have doubts about some anomalies. May I?” asked Maya, already plugging the room projector cable into her laptop. The large LCD wall display showed a satellite plot of the solar fields, each panel outlined with one of several colors (Figure 2).

“Point one: not all PV panels lose power equally—see by the color-coding here. The panels suffering the greatest loss are found in this slope between these two low ranges of hills, where you have terraced the land like ghats—steps,” Maya noted. “On the more elevated plane there is some variation in power loss, but not nearly at the level seen in the downward slope and steps.”

Ashish frowned but nodded. “That is also where we have the greatest wind flow at night. You see at the bottom of the slope, where there are several 250-kW wind turbines to take advantage of that effect.”

“Those wind turbines were very obliging, whispering secrets in our ready ears that helped us find the metaphorical thieves stealing your power,” Marnie responded as she stood to pour her seventh cup of coffee.

Mr. Mosweu asked, “I am curious. We have nearly 50,000 panels, and you outlined each with colors? How was this accomplished?”

“Summer interns, fueled by free pizzas, caffeine, and high-fructose corn syrup,” Marnie cheerfully replied. Maya gave her boss “The Look,” and said, “You perhaps should not be so happy about encouraging bad dietary habits among youth.” She then sighed, advancing to the next slide while muttering quietly, “Why do I say this to my boss, whose blood is at least 50% coffee.”

A Masked Culprit

Freshly fueled with coffee, Marnie continued. “Point 2: your power loss doesn’t correlate with dust accumulation. Oh, I know, it looks like it does, but to quote Shakespeare, ‘that which appears significant may be only an illusion.’ ”

“Ma’am, that is from the anime Cowboy Bebop, not Shakespeare,” interjected Maya. “Observe these power output trends over the day,” she said while displaying multi-colored charts. “In most cases, your direct current to alternating current—DC to AC—power inverters show the typical trends of reduced output as a function of the ambient temperature. A question: why were string inverters chosen, rather than centralized inverter islands, given your peak capacity is only 20 MW?”

Dirk was please to answer. “Logistics and ease of repair,” he said. “Large inverters are difficult to bring to the site, and few are specialized in their repairs. With more but smaller inverters, we keep spares in our inventory, and our maintenance crew can replace the entire unit, which is much safer and easier than rebuilding a much larger inverter.”

“Critical choices,” added Ashish, “as inverter failures here are much greater than expected.”

Marnie nodded in agreement. “Wise decisions, and they also helped us find what should be the problem. Maya, please show our friends the trend you so cleverly noticed?”

Moving to the next slide, Maya continued. “When your system was new, typical trends of reduced power production were noted as the site temperature increased. I analyzed variables such as power output, ambient temperature, hourly cloud cover, humidity, and many other factors. Again, ambient temperature is the most significant variable—you may experience up to 10% inverter loss on the hottest days, but typically this is recovered in the late evening. Adjusting for local weather, this cycle correlates at a level of three Sigma.”

Marnie interjected with a sidebar, “Not enough to announce a new subatomic particle, such as the coffee quark, but trendy enough to require a second look. And you know I love to be trendy!”

“Only in fashion, ma’am. Otherwise, you are the antithesis of trendy.” Ignoring the faux wounded innocence portrayed by her boss, Maya moved to the next slide. “Point 3: your water cleaning system started operation almost a year late due to your dry aquifer, but most curiously, two months after the cleaning system was online, your power output on the panels in the terraced valley region declined on a much faster trend.”

“What is this?” exclaimed Mr. Mosweu. “You tell me we have made the panels dirtier by cleaning them?”

“I know we cannot tour the site until after sunset, as the temperature among the panels would be deadly, but join us for a walk in the valley and we shall discover the shadow of death to your power system,” Maya said.

Marnie, now on her 10th cup of coffee, broke the stunned silence by asking, “Could we talk to your electrical maintenance crew please?”

A Stroll Through the Valley

It was an hour past sunset, but solar fields were still uncomfortably warm. Joined by a small maintenance crew, Marnie, Maya, Mr. Mosweu, and his engineering team opened several inverter doors in the higher plains region, finding little of note. Dirk commented, “The dust cannot be stopped from entering the inverter cooling fan ductwork. Constantly changing cartridge filters was highly expensive, so we designed passive inlet air filtering with impact separators, cyclones, and vortex screens. You see they work well, as the inverter systems have little dust.”

“And as you will soon see,” added Marnie with a dramatic air, “appearances can deceive.”

Marnie led the team toward the PV panel inverter sheds in the valley region. Surprisingly, the valley sheds contained less dust than those in the plains. “And that, Uncle, is the problem. Come, feel the inverter cooling fan flow.” Indeed, as Mr. Mosweu and his team placed their hands in front of the ductwork, only a slight breeze of air was felt. Ashish and Dirk confirmed the cooling fans were still running, despite their lackluster airflow.

But just two rows further down the valley, no cooling air was flowing in any of the inverter sheds. Smiling with the confidence of competence, Marnie announced, “Have the maintenance crew open the intake ductwork and clean it out, please?”

Quick work by the site maintenance crew confirmed the cause of the blocked flow—dried mud filled the intake ducts. And such was the case with almost every other inverter shed they inspected in the valley. Wiping the sweat from his face, Mr. Mosweu sighed, “Ms. Surfaceblow, Ms. Sharma, please explain.”

A nudge from her boss prompted Maya to tell the story. “It is a matter of both environmental and design problems. Operating the water cleaning system at night when the wind is faster brings dust-laden air to combine with the water drift you experience. This creates particles of dampened dust—some fall to the ground, some stick on the panel surfaces, and some are drawn into the inverter cooling fan air intakes. This phenomenon is more prevalent in the valley, as the water drift cools the air, making it denser and more likely to flow naturally toward lower elevations.”

Marnie jumped in to add, “The dampened dust might have passed through your air intakes without sticking, but tortuous paths and turbulent flow from your redesign increased drop-out of the wet dust, slowly strangling, and then killing, the cooling flow. This in turn increased your inverter temperatures, causing more power loss for longer portions of the day, as well as much more frequent inverter failures. In fact, your own maintenance crew noted trying to clean out the intake system was much more difficult than changing an air filter, so they couldn’t devote as much labor toward keeping the air flowing. Maya confirmed this as well, performing energy balance calculations, where she noted the cooling fan amperage was much less than the original specification due to the fans stalling from the blocked inlets. The final step to test the theory was confirmation by inspection.”

A Creative Solution

Returned to the cool of the meeting room, Mr. Mosweu sighed. “So, your recommendation is to disable each inverter in turn until we confirm they have proper cooling, and discontinuing water cleaning until we find a better way.”

“I also recommend, sir, that you return to using standard cartridge filters, which are simple to inspect and replace,” said Maya. “On the longer term, I recommend methods we have employed for dust reduction in my home country—windbreaks, laying of stone in heavily eroded areas, and finding a species of plant that can grow, even thrive, under the panel fields with minimal water use.”

“Maybe you could genetically engineer a hybrid of rooibos and my family’s special coffee, and adapt it to thrive along with your power station!” Marnie exclaimed hopefully. “Then, we could open a branch office here. Think of it, Maya! We could create The No. 1 Ladies’ Engineering Consultancy, and …”

Mr. Mosweu clapped his hands and laughed, “If that is required to continue such inspired solutions, I am certain anything is possible!”

“We will stay a couple of days to visit, Uncle, and send you a preliminary report by the end of the week,” replied Marnie.

“And, ma’am, do not forget to retrieve your ‘Triple-Platinum’ luggage tags when we arrive at the airport,” Maya added slyly. Already flush from the heat, Marnie’s blush of embarrassment was well-hidden.

Una Nowling, PE is an adjunct professor of mechanical engineering at the University of Missouri-Kansas City.

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