As you may know, I’m a former electric power utility equipment engineer. Although no longer active, I continue to follow developments in my former industry. Hurricanes Irma and Maria significantly damaged Puerto Rico’s ailing state owned public utility. Because of economic conditions on the island and electric power pricing decisions, the utility has been unable to maintain its generation and distribution equipment. Apparently, the utility was also deferring tree maintenance long its rights of way. Because of the lack of care and the strength of the storms, most of the island’s retail distribution is down and some of its backbone transmission is damaged. Puerto Rico is in the position of rebuilding its transmission and distribution and cold starting its generation.
Electric Power Systems
Electric power systems are systems that produce, transmit, and distribute electric power to their clients. A system is an entity whose that exists and derives its behavior from the interconnection of its component parts. In general, an electric power system is composed of
- Generating stations
- Transmission substations
- Transmission lines
- Distribution substations
- Distribution lines
- Client transformer and service drop
Most systems have multiple types of generation including
- hydroelectric generators
- thermal electric generators (coal, gas, and oil)
- photovoltaic generation
- wind turbine generation
- nuclear thermal electric generation
Traditionally, generation is at a small number of large central stations. More recently, systems include small scale wind and photovoltaic generation as part of the distribution system. Larger entities may connect these in a micro-grid to provide local power.
Micro-grids are another recent innovation. A large customer has some generation and purchases some power. The on-site generation is designed and managed to work together with the utility generation. In the past, we called this cogeneration. In the newer micro-grid concept, multiple customers in a small area cooperate to supply some of their power needs, sell surplus power to the utility, and buy supplemental power from the utility. The equipment in the micro-grid is designed to be compatible with the utility’s voltage control, frequency control, and load sharing schemes.
Puerto Rico’s Circumstances
As a result of economic conditions on the island, Puerto Rico’s public electric power utility is bankrupt, behind on its system maintenance, and is in restructuring and redevelopment. Add on top of these structural problems the recent damage by hurricanes Irma and Maria that damaged 85% of the island’s transmission and distribution lines. The cash-strapped utility is in the position of replacing most of its pole and wiring with no money to do so. How might the utility go about recovery?
Motivation for this Article
The press covers complex stories like this poorly because reporters are journalism majors, not system engineers or power engineers having the subject matter knowledge needed to tell complex technical stories. The better stories appear in the technical press written by reporters that interview engineers involved in the story and others in the larger industries.
Tesla’s Overture in a Nutshell
Tesla has received a lot of superficial press from Elon Musk’s tweets that Tesla is donating power wall storage and engineering services to Puerto Rico. Recently, Tesla completed a PV & power storage installation at a Puerto Rican hospital that will tide the facility over until commercial power becomes available.
Tesla has significant experience in renewable power system for tropical islands. Its overtures to the Puerto Rico power authority represent its most ambitious undertaking to date because of the scale of the project. Puerto Rico’s population is about 3.5 million. As a rule of thumb provide 1 KW of capacity for each resident. For renewable assets, it takes about 3 KW of nameplate capacity to provide 1 KW of power as units are out of service, becalmed or clouded over, etc. By installing excess capacity in a geographically diverse fashion, the island can meet its power needs with limited reliance on off-island inputs (coal, oil, natural gas).
Hurricanes pose a number of challenges for various types of generation. Let’s take a minute to review these.
Central stations are the large scale generation that provides electric power to the system Puerto Rico has about 13 of these, most nearing the end of their service life and most operating with numerous maintenance jury rigs in place (temporary repairs). Because of its financial straights the utility has been unable to make many of its needed permanent repairs or to replace its obsolescent and tired boilers.
Central stations require off-site power to operate. The station’s generator supplies power to the transmission system which, in turn, supplies power to the retail distribution that services the plant. The station is unable to directly supply its hotel loads (lights, pumps, fuel handling equipment, etc) from the output of the main generator. When the off-site power source fails, the plant is forced to shut down. Emergency generators start to protect equipment from damage while it settles to rest but are insufficient to start or operate the station.
Nuclear central stations also require off-site power to operate. Current doctrine is to shut down and cool down nuclear units when loss of off site power is anticipated.
Hydroelectric stations can be started with on-site auxiliary power to operate the water turbine inlet valves. This makes them the first units to be recovered in a black start environment.
Distributed Renewable Generation
Recent images of Puerto Rico appearing in the Washington Post show wind and utility scale solar installations following the hurricane. The wind turbines in the path of the storm remained standing but the blades failed. In the images I saw, every turbine suffered catastrophic blade failure. If a blade wasn’t completely on the ground, it was twisted or the tip was missing. Irma and Maria exceeded the design wind load limits of the turbines.
Images of a central PV solar station show catastrophic damage also. The wind peeled panels off their support structures just like it peels sheathing off of wood frame structures. The site was littered with failed support structure and scattered panels.s The failed structure is scrap. From the drone images one could not tell if the panels could be reused.
The large pylon mounted 3 phase transmission lines are the backbone of the electric power system. These lines carry power from the generating station to the retail distribution subsystems. These lines must be repaired and energized to restore electric power in the parts of the system that they serve. These lines must be repaired and energized to restart most central stations. The hurricane severed most of Puerto Rico’s core transmission links. The Army Corps of Engineers is working with the utility and volunteers to rebuild or repair these lines.
The transformers and circuit breakers in the transmission substations are robust but subject to damage by prolonged exposure to wind driven spray. If de-energized, these must be tested to confirm insulation integrity before they are reenergized. If a transformer must be replaced, the circuits it supports can be out of service for a year while a replacement is manufactured. Nobody maintains an inventory of these because of their bespoke design, size, weight, and cost. There has been no discussion of the condition of the transformers and switch gear in the press. These usually survive without major damage. A fresh water wash down to remove salt spray deposits is generally all that is needed as these components are sealed to keep in cooling and insulating fluids.
Ninety percent of Puerto Rico’s retail distribution is compromised by wind or water damage. Broken poles must be repaired, broken lines mended, and the lines checked for freedom from faults. This work proceeds outward from the distribution substations. The network is a tree with large branches supporting smaller limbs and twigs. The linemen fix the limbs first, then the branches, and finally the twigs. The last thing they do is to repair the line serving an individual property. Army Corps of Engineers personnel, utility staff, and volunteers are undertaking this mission. The Corps of Engineers is funding most of this activity.
In Puerto Rico, all of its generation tripped and many of its transmission lines failed, and most of its retail distribution failed. Recovery from this sort of damage takes planning and working through the system in a methodical fashion.
- The generating stations, transmission lines. and transmission substations must be repaired.
- The hydro electric stations are started first because they need only limited or no off-site power to start. These supply power to the transmission system. It takes about 6 minutes to start a hydro-electric generator.
- As individual central stations become ready, hydro-electric power is brought through the transmission and retail distribution to the central station and used to start the station.
- When a station is ready for loading and there is sufficient reachable load, it can be synchronized and used to start additional stations or pick up some customer service.
This process continues until sufficient generation is available to serve the reachable customers. In the absence of damage, it can take 24-48 hours to fully recover the tripped units.
A tale of two islands
Recovery, of necessity, will repair and restart the existing system. Once recovered, how might the Puerto Rico power authority modernize its system? In this section we’ll examine the various structures to speculate on a way ahead.
Hawaii and Puerto Rico are remote islands so fuel delivery costs are high and fuel costs dominate electric energy pricing. Hawaii government, in response to customer demand, liberalized rules permitting renewable generation in the retail distribution. Customers eagerly purchased and commissioned residential PV systems. These changes were largely unmanaged. The local utility learned several things.
- The retail distribution was not designed to have generators out there. There was no way to coordinate the activity of numerous photovoltaic panel installations on residences.
- Home owners installed more PV generation on the homes connected to a transformer than it could handle. On a bright day, the transformer was overloaded and popped its fuses. The utility had to learn to coordinate PV installations at each transformer in the retail distribution.
- Home PV systems are designed to follow the commercial power. They cannot participate in voltage control or frequency control. This limits the amount of older generation that can be installed in a retail distribution network.
- Home PV systems are designed to use the commercial power to carry excess demand and to provide power during the overnight hours.
- Most home PV systems are not designed to operate off-grid. They must have a commercial power source to follow in voltage and frequency.
- Few home PV systems have the storage needed to be self-sufficient over night.
- Home PV systems are not designed to participate in system frequency and voltage control.
Introduction of PV generation in Hawaii has reduced resident’s power bills while the utility has had to learn to cope with and plan for introduction of renewables in the retail distribution. ABB, General Electric, and Siemens have made progress in developing PV components and small system coordination protocols that allow small systems to participate in voltage control and to respect the retail distribution transformer capacity. The utilities will have to redesign the retail distribution to use these technologies but there is emerging practice ready for trial deployments.
Puerto Rico Way Ahead
So what is Puerto Rico to do? Clearly, distributed wind and solar generation are subject to catastrophic wind storm damage. The scale of major hurricanes means that a single storm can affect the entire island and the island cannot rely on geographical diversity to have adequate working generation during or after a major hurricane. Some amount of thermal generation would be required to supplement the easily built renewables.
I believe Puerto Rico should, in the short term, use PV generation for most of its initial capacity replacement. Adding PV systems is relatively simple provided you have a place to put them. One of the nice things about solar is that it is easily added in house-sized increments. One of the bad things about solar is that you have to add a lot of house-sized increments to service your customer base. And the sun takes half the time off so storage or other base load generation is required to provide night time service. You can build it as you go.
In parallel with solar installation, Puerto Rico should install wind turbine generation to carry part of its base load. Wind turbines may be installed in 1-10 MW increments rather than in 600 MW increments as in a central station allowing them to be added as needed.
Wind turbines work when ever there is reasonable wind but can be feathered when their output is not needed. During storms, wind can be too variable (gusty) or too strong, requiring that the turbine be feathered for the duration of the storm. On the mainland, it is common to feather excess wind generation and to feather it during nor’easters or other severe storms. System design accommodates wind turbine variability by installing turbines in a geographically diverse manner. To serve 1 MW of load requires 3 MW of diversely sited turbines. When some are sidelined by too little or too much wind, the more remote units continue to operate.
It takes about a year to install a wind turbine. The utility has to order the machine. While it is being produced, the utility builds the foundation. This is a substantial chunk of concrete with specialized rebar mesh internal to the concrete. Construction and curing takes significant time. Get this wrong and a perfectly reasonable wind load blows your turbine over. So it is a bit longer to install than PV which uses standard components ordered from stock.
To date, there is little utility scale wind generation in hurricane alley. Conditions can be rough on the North Sea so there is design experience validated to 100 KPH exposure. The wind turbines on Puerto Rico were exposed to 200 to 250 KPH (120-150 MPH) seen in major hurricanes. The machines directly in the path of the storm core failed catastrophically and dramatically. Puerto Rico would need to over-build to allow for loss of the turbines at the landfall and in transit path.
Base Load Power
The catch is that hurricanes come in island size. During a hurricane, most PV and wind generation would be unavailable over the entire island. So Puerto Rico needs sufficient storage and central station generation to press on through and after a hurricane, Of the available forms, molten salt thorium fueled reactors are a good choice. Because molten salts have 1000 C difference between their melting point and boiling point and robustly retain fission products in the molten liquid fuel, it is easy to design these machines to experience a station blackout without damage or release of fission products to the environment. Although high temperatures are present, there are no mechanisms present that would release and disperse the fission products from the molten fuel salt.
Because the fuel is already molten and the plant is designed to be station blackout safe, without electrical power for cooling or equipment protection, these machines, unlike our current water cooled reactors, can continue to operate during a wind storm. When off site power is lost, the reactor is scrammed (loss of power releases the shutdown rods), and a freeze plug melts releasing the fuel to the fuel storage tank. This tank has a subcritical geometry (shape and absence of moderator) and is designed for natural circulation cooling to the atmosphere. Since the hot fuel is a liquid at atmospheric pressure, there is no mechanism trying to scatter the liquid fuel should it become depressurized. No complex systems are needed to condense spilled fuel vapors to protect the reactor building from over-pressurization. Because there is no water, there is no mechanism producing explosive hydrogen that must be removed from the reactor building atmosphere. The atmosphere serves as ultimate heat sink so there is no need to supply make up cooling water.
Molten salt reactors , appear to be reasonable to build provided they can be built in modular series production like super tankers. Because the fuel is liquid salt, it can remain in the reactor long enough to use up 99% of the fissile material in the fuel. The reactor burns long-lived actinide products and the associated storage problem are designed out of these machines. These reactors can also be designed to burn our existing spent fuel inventory.
Important rare earth elements for phosphors and magnets are a by-product of thorium mining, and reactor operation produces valuable fission products used in nuclear medicine, non-radioactive noble gases, and other non-nuclear materials of commercial value. The noble gases are easily recovered. The other materials can be recovered by those willing to take on the commercialization challenges. The fission products that are not of commercial value decay away to background in about 300 years.
Thorium Power Faces the First Mover Problem
Asian countries including China have active molten salt reactor development programs assisted by US nuclear engineering startup companies like FLiBe, TransAtomic Power, TerraCon, and ThorCon. It is likely that the first startups will be in China. Introduction of these machines in the US requires new licensing procedures and criteria. NRC is now self-funded from user fees. The first mover would be forced to bear the cost of the NRC learning to license LFTR reactors. So far, no US customer has been willing to assume those costs and Congress had not elected to donate them.
The first mover is also the unlucky owner who discovers the additional engineering that needs to be done. Although the materials and processes needed are pretty well understood, it remains unknown how to design the physical systems to operate for the 40 year life time expected of major energy equipment. One approach is to design some plant components for a 5 year life and replace these parts and the fuel load every 5 years. Experimentation is needed to learn the best commercial design choices.