Hybrid Power Systems - Issues & Answers
There are key elements to consider in evaluating an application for hybrid power. The table below allows you quick access to these sections.
Site Access Environmental Issues Load Profiles Renewable Resource Economics System Application Questions Sample Systems
Site Access
Environmental Issues
Load Profiles
Renewable Resource
Economics
System Application
Questions
Sample Systems
Introduction
You need power at a remote site. What's the first thing you do? Call the local electric power company (if there is one) and request service to the site. They will give you an estimate of the cost and time it will take to provide the power. Be prepared for a surprise. In these days of unregulated utilities, the local power company will charge you the actual cost to extend the utility grid to your site. Figure $8,000 to $35,000 per mile depending on terrain. Now you know the meaning of a remote site! What options do you have? Until recently, your only option was to buy an engine generator and put it at the site. This has worked for decades and it is still the most often used solution to the problem. However, with the engine generator come problems of hauling fuel and storing it at the site, figuring how to start and stop the engine, getting spare parts and skilled technicians to the site, and making sure the noise and pollution created by the generator will not cause environmental damage. Although the initial cost of the generator may be affordable, these factors can add significantly to the cost of the power system over time.
Since the mid-80's the renewable energy generator, driven by wind, sun (photovoltaic), or water (micro-hydro), has become a viable alternative to the engine generator for remote power production. The initial cost of a stand-alone renewable generator is higher than a comparable sized engine generator but the operating and maintenance costs are almost always lower than the engine generator. Of course, the fuel is free. As the advantages of the renewable generators became more widely known, system designers started looking for ways to combine both generators into one system and get the best of both worlds. The hybrid system was born.
This booklet was written because of the increasing interest in hybrid power systems (HPS). When should they be used? What are their benefits? Their drawbacks? Where can information about them be found? These questions and others will be addressed. We'll present and discuss both sides of the issues so that you can decide whether or not an HPS is right for your application. We'll list some things you'll need to know before you talk to a potential supplier. We'll give you a list of experienced systems designers, and we'll look at some applications around the world today where hybrid systems are the smartest choice--both technically and financially. We hope this information will help you make the best choice for your application.
What Is a Hybrid Power System?
The term hybrid power system is used to describe any power system with more than one type of generator--usually a conventional generator powered by a diesel or gas engine, and a renewable energy source such as a photovoltaic (PV), wind, or hydroelectric power system. There are thousands of these systems in use today. They range in size from a few tens of watts to tens of kilowatts. The smaller systems are mostly on remote residences where homeowners add a few PV modules to their existing Genset to reduce the noise and inconvenience of having the generator running all the time. Convenience may be more important than cost for these homeowner-maintained systems that are installed near their homes. We are going to focus on hybrid systems that are used mainly in remote areas. These systems are often the most cost-effective way to produce power reliably at an unattended site.
What else does a hybrid system include? Most have a way to store electrical energy to meet the peak load demand or for times when the renewable resource is low. Battery storage for wind or photovoltaic systems and pumped water storage for micro-hydro systems are commonly used. Using storage also increases the flexibility of system control and adds to overall system availability.
Anytime you have multiple generators, you need a smart controller. The ultimate smart controller is a person, but many hybrids are installed in remote locations. There you must rely on electronic controllers. Based on the parameters such as load demand, generator status, and battery state of charge, the controllers must act to keep power flowing to the load and protect the equipment. Doing this efficiently is not a small task and one consequence is that most hybrid controllers are custom designed.
What else does a hybrid include? An inverter (often called a power conditioning unit or PCU) is necessary to drive ac loads from a battery or a photovoltaic array. A battery charger/rectifier is required if batteries are to be charged from an engine generator or wind generator. Then there is the balance of systems (BOS). This is all the switches, fuses, junction boxes, grounding circuits, wires, connectors and safety equipment that make the system a system. These are the building blocks. Now, let's discuss why we should use an HPS anyway.
Why Use a Hybrid Power System?
Use a hybrid power system only if it will save you money. After all, choosing a power system should be a matter of dollars and sense. As we've said, you can buy a conventional generator cheaper than a renewable system but it is expensive to run a fueled generator and regularly deliver fuel, spare parts, and maintenance personnel. On the other hand, many renewable generators operate reliably and economically at remote unattended sites and require little attention. The beauty of a well designed hybrid system is that you can exploit the strong points of both to provide a flexible, reliable power system.
The advantages of fueled generators are:
Their shortcomings are:
Some of the benefits of renewable energy generators are:
Their disadvantages are:
Fine, but do you need a hybrid for your application or would a single generator, conventional or renewable, be better? How can you tell? There are three key questions to answer.
Will you need power at the site for the foreseeable future? (You have ruled out extending the utility grid as an option.) Is access to the site difficult or impossible for some periods of the year, yet you must have reliable power all the time? Is the site located in an environmentally sensitive area?
Will you need power at the site for the foreseeable future? (You have ruled out extending the utility grid as an option.)
Is access to the site difficult or impossible for some periods of the year, yet you must have reliable power all the time?
Is the site located in an environmentally sensitive area?
If you answered "Yes" to any of these questions, keep reading.
Consider the hybrid systems as the broad gray area between renewable and conventional generators. You can pick any point on the continuum. In general, the closer you pick to conventional, the lower the initial cost
but the higher the operation and maintenance (O&M) costs. O&M costs soar when regular fuel delivery and storage costs are factored in over a 20+ year system life. (Be fair--if it really costs $2.50 per gallon to get diesel fuel to the site and store it, don't use the cost of diesel fuel at the local pump for your baseline cost.) Also, consider all the environmental costs--real, like cleaning up a possible fuel spill--to less defined ones like adding toxic gases to the air we breathe. If future costs of environmental damage are included, it may be far cheaper to incorporate a renewable generator at first. These are the major drivers of the hybrid system decision. There are other considerations included in the list below but in general, these terms, taken together won't outweigh the site access or environmental factors:
We'll discuss these issues in turn without getting too deeply into system design. There are some exotic prediction programs available to help you design a hybrid system. However, at this point you probably don't care whether they project a 70:30 load split between generators or a 73:27 split. Right now, you just want to know if it makes sense to spend additional up-front money to include a renewable generator in your system. We've devised a scoring system that you can use to rate the different factors and determine if you should consider an HPS.
Top of page
The cost of getting the engine generator set up at a remote site can be significant. Roads may have to be built and maintained for regular fuel deliveries and maintenance. Sea or air lift may be required. Provisions for fuel storage must be made and adequate fuel storage tanks may not be available in some areas. If metal tanks are used in the United States, they must be protected from corrosion. [1] A cathodic protection system will add to the power requirements at the site. Getting spare parts for the generator is a problem in many remote areas or undeveloped countries. The cost of getting skilled technicians to install the spare parts and keep the generator running will depend on the access to the site. The more remote the site, the more reliable you will want the system hardware. In other words you will want high system availability. This term is defined as the number of hours that the system will be able to provide power to the load over a year. Many single generator systems struggle to attain an availability of 95 percent. To increase the availability, the generator (and/or battery storage) must be oversized to meet all contingencies--the worst case load under the worst weather conditions. This increases the cost exponentially and most of the time the generator operates at a non-optimum load factor. A hybrid system with two or more generators can achieve high availability easier and cheaper than a single generator system. How do you know what system availability you need? It depends on your load. If human life would be at risk during a power outage you want the highest system availability you can get. However, if a power outage would cause only minor irritation, then a lower availability, say 95 percent, might save you money. If the site is difficult to get to, particularly during parts of the year, then you'll want more than one generator at the site to increase the system availability. If you have a fueled generator you can increase availability by adding another one. However, this will only increase the need for fuel and spare parts at the site. Adding a renewable generator with batteries will increase availability and decrease the need for fuel and maintenance at the remote site. As system owner, you should decide on the minimum availability you can accept. If you need availability of greater than 95 percent, and your site is inaccessible, you should consider an HPS.
Here's a scorecard that you can use to calculate a site access score. Rate each item on its potential to cause you problems. The greater the chance of problems, the higher the score and the more likely a hybrid system would be advantageous for your application. The WF numbers are weighting factors showing the relative importance of each factor. For instance, the fact that you have to airlift fuel to a site carries more weight than whether you will have problems obtaining storage tanks. Enter the total as the Site Access score.
Site Access Factor Item Weight (WF) No (0) Some (3) Yes (5) Results Will extensive road work be required to transport fuel safely? 8 Will sea or air lift be required? 8 Can fuel be stored safely on site? 6 Are there problems obtaining storage tanks? 3 Are there months when the site is inaccessible? 4 Is system availability higher than 95%? 6 Total Score
Site Access Factor
Item
Weight (WF)
No (0)
Some (3)
Yes (5)
Results
Will extensive road work be required to transport fuel safely?
8
Will sea or air lift be required?
Can fuel be stored safely on site?
6
Are there problems obtaining storage tanks?
3
Are there months when the site is inaccessible?
4
Is system availability higher than 95%?
Total Score
Environmental Concerns - Generator
A conventional fueled generator poses a much higher threat to the environment than a renewable generator. Most of the problems stem from the risks (and subsequent costs) associated with storing fuel. Also, engines are noisy and smokestack emissions contain gases that harm the environment.
Use the scoresheet for engine generators to determine their environmental factor. The higher the numbers, the more likely a hybrid system will be a good option for your application.
Environmental Factor - Engine Generator Item Weight (WF) Low (1) Average (3) High (5) Results Rate the possibility of fuel spillage at the site. 8 What is the potential damage to humans, animals, plants, or water if fuel spills? 8 Rate the fire risk from stored fuel. 6 Rate the possibility of damage from toxic emissions. 4 Rate the problems that might be caused by operating noise. 4 Total Score
Environmental Factor - Engine Generator
Low (1)
Average (3)
High (5)
Rate the possibility of fuel spillage at the site.
What is the potential damage to humans, animals, plants, or water if fuel spills?
Rate the fire risk from stored fuel.
Rate the possibility of damage from toxic emissions.
Rate the problems that might be caused by operating noise.
Environmental Concern - Batteries
Although renewable energy systems are more benign to the environment, the use of electrochemical batteries is a drawback. Batteries are potentially hazardous to the environment because most contain toxic materials like lead, lithium, cadmium, and various acids. Installations must be in a well ventilated area and include methods to contain any electrolyte leakage from potential cell ruptures. These ruptures may be
caused by an explosion of hydrogen gas that is generated during charging of common lead-acid batteries. Finally, any evaluation of system costs must include the cost of disposing of the batteries in an environmentally responsible way. Use the scorecard to find the environmental rating for batteries. The weighting factors are negative because the need for batteries with a renewable systems makes an HPS less attractive for the environment.
Environmental Factor - Batteries Item Weight (WF) Low (1) Average (3) High (5) Results Rate the possibility of environmental damage from batteries -5 Estimate the cost of disposing of used batteries. -3 Total Score
Environmental Factor - Batteries
Rate the possibility of environmental damage from batteries
-5
Estimate the cost of disposing of used batteries.
-3
Load Demand Profile
Load demand profiles, both daily and yearly, are important for the design of any power system. The more variation in demand, the more reason to buy the flexibility offered by an HPS. For load demands that vary rapidly throughout the day a battery storage subsystem is required for a renewable generator and strongly recommended for a fueled generator. The peak demand spikes can by met from the batteries and the engine generator started and operated at a steady load when the battery state-of-charge drops below a preset level. Many smaller applications like telecommunications relay sites have hourly load demands that vary rapidly. A hybrid power system with the renewable energy generator plus storage designed to handle the load 80-95 percent of the time is a smart option for these applications.
There are some applications where the load demand is fairly predictable but it may increase 2-10 times during certain times of the year. For instance some applications will have large variations in power demand based on the seasons. Hybrid systems can be effective for these applications also--the renewable generator is sized to provide much of the baseload while the engine generator is used during the months with high demand. Fuel will be saved and the generator O&M cost reduced.
An HPS is a flexible power supply. There are many applications where the load demand increases over the years. An example would be a village power system where people buy more appliances over time. A well-designed HPS can meet these gradual increases in demand without significant changes in operation.
Use the score sheet to estimate the load profile factor. The more variable the load, the higher the number that should be assigned. (A load that varies ±25 percent has low variability; one that varies more than 300 percent has high variability.)
Load Profile Factor Item Weight (WF) Low (1) Average (3) High (5) Results Rate the variability of the daily load demand. 4 Rate the variability of the annual load demand. 4 Total Score
Load Profile Factor
Rate the variability of the daily load demand.
Rate the variability of the annual load demand.
Renewable Resource Quality
A renewable resource is judged by its magnitude and consistency. If there is plenty of sun or wind year round at your location, it may be a great site for a renewable or hybrid system. Any site that receives more than 1800 kilowatt-hours per year of solar insolation is considered a good site for a PV system. The southwestern desert region of the United States receives over 2500 kilowatt-hours per year, a daily average of 6.8 kWh, and the variation between winter and summer is not large. Such a location is a great site for a PV system and a kilowatt of installed PV modules will produce about 2000 kWh of electrical energy annually. An average wind speed of 13 miles per hour is considered a good wind resource. Some available wind machines will generate about 1400 kWh per year with a 13 mph resource. However, little energy will be produced if the wind speed drops below 10 mph.
The lack of abundant renewable resources does not rule out the use of a hybrid system. Some PV/diesel systems are installed in locations like Alaska where the sun may not rise above the horizon for a few months in the winter. Still the PV production from the long days of summer may give enough benefit to make the hybrid system the preferred choice in these environmentally sensitive remote areas.
Another meaningful factor is the daily match between load demand and energy production from the renewable generators. The better the match the more attractive the renewable generator becomes because the power can be supplied directly to the load and losses associated with storing the energy can be avoided. Building ventilation and heat exchange is a good example of a match between load demand and production from a PV system. The more solar insolation received, the hotter the building, the larger the load and the greater the PV production.
The monthly or seasonal match between renewable resource and load should also be considered. A good solar site like those in the southwestern USA will have a best-to-worst monthly ratio of 2:1 or less. A photovoltaic system will be a consistent producer with such uniform solar conditions and some are used to meet base load demand. In other areas the ratio for summer to winter insolation will range from 3:1 to infinity. For these areas the photovoltaic systems are more often used to meet the peak demand which usually increases also during the better solar seasons. How do you evaluate the renewable resource at your site? Call the local airport or university and ask who keeps long term weather records. If you can find local data, use it. If not, call a solar system installer. They usually have regional weather data that can be used to give an estimate for your site. Use the suggested score sheet on the next page to find the renewable resource factor.
Renewable Resource Factor Item Weight (WF) Low (1) Average (3) High (5) Results Daily solar insolation in kWh/m2 4 <3 3-5 >5 Average daily wind speed in miles/hour 4 <10 10 - 15 >15 Seasonal ratio of solar insolation 2 >4:1 3:1 <2:1 Rage the match between the load demand and the daily renewable resource. 2 Total Score
Renewable Resource Factor
Daily solar insolation in kWh/m2
<3
3-5
>5
Average daily wind speed in miles/hour
<10
10 - 15
>15
Seasonal ratio of solar insolation
2
>4:1
3:1
<2:1
Rage the match between the load demand and the daily renewable resource.
Renewable Resource Score
Access to Funds
Many power system decisions are made strictly on initial cost. If funds are limited it may not be possible to include a renewable generator even if all factors point to a hybrid as the best long-term power supply. (For loads with high daily variability, adding battery storage and an inverter should be considered even if a renewable power generator is not currently affordable. This should save on O&M expense and increase the life of the generator.) If you want to compare the cost of a hybrid system with a fueled generator alone you must perform a life cycle cost (LCC) analysis. [2] This allows you to reconcile the higher initial cost of the renewable generator and the higher operating and maintenance costs of the conventional generator by converting all costs to their present value. To calculate present value you have to make assumptions about the system's expected lifetime, inflation rate, future fuel use, maintenance, replacement, transportation costs, overhaul expense, and salvage value. Many people are suspicious of LCC results because of these required assumptions about future economic conditions. Despite this difficulty, LCC analysis is the only way to compare the total cost of different systems. We urge you to do an LCC analysis before you actually invest in a system. However, for now, you know what your budget is and you don't need an LCC analysis to estimate the difficulty of obtaining future O&M funds. Use the scorecard below to figure the Funds Access score.
Funds Access Factor Item Weight (WF) Low (1) Average (3) High (5) Results Rate the difficulty of obtaining funds for operation and maintenance of the generators. 5 Rate the difficulty of obtaining adequate funds for repair, overhaul, and replacement of engines. 3 Rate the difficulty of obtaining adequate funds for replacement of batteries used in a renewable energ;y system. -3 Estimate future inflation. Assume the average is 6%. 2 Total Score
Funds Access Factor
Rate the difficulty of obtaining funds for operation and maintenance of the generators.
5
Rate the difficulty of obtaining adequate funds for repair, overhaul, and replacement of engines.
Rate the difficulty of obtaining adequate funds for replacement of batteries used in a renewable energ;y system.
Estimate future inflation. Assume the average is 6%.
Should You Install a Hybrid Power System?
We have included a final scorecard that you can use to rate the desirability of an HPS for your application. There is a column for site specific weighting factors that you may wish to use. For instance, if protecting the environment is very important to you, you might want to use an additional weighting factor for this item.
Hybrid Systems Factor Item Score Weight (WF) Total Site Access Score Environmental - Generator Score Environmental - Battery Score Load Profile Score Renewable Resource Score Funds Access Score Total Score
Hybrid Systems Factor
Score
Total
Site Access Score
Environmental - Generator Score
Environmental - Battery Score
Load Profile Score
Funds Access Score
If your score is less than 120, buy a conventional generator. If your score is greater than 320 you should install a stand-alone renewable system. If it's between 121 and 319, a hybrid power system should be considered.
One more word of caution. What we have just presented is meant to highlight some of the factors you should consider and their relative importance to the HPS decision. The numbers are relative. In other words, a score of 300 is a stronger indicator of the desirability of an HPS than a score of 200. However, it should not be construed to imply that you should use a larger PV system, for instance. Leave these decisions to the designers. Any- time you condense a complex decision to a bunch of numbers you run a risk of overlooking something or not giving proper weight to an important factor. Making a high dollar decision that you must live with for 20+ years is not something that can be easily "cook-booked." Each application is different and you should not make your decision solely based on the scores achieved above. However, the important thing to understand is that hybrid power systems are technically feasible and economically competitive for a large number of applications. They should be considered. Let's look at an example and use the evaluation scorecards to rate a potential hybrid application.
A telecommunications system, currently powered with diesel engine generators, is located on a remote peak in the mountains of Southern California. Because of the strategic location, several State, Federal, and Public Service agencies have installed transponders on the peak to provide essential communications over the region. Two diesel generators are on the site&emdash;one for backup&emdash;because of the critical nature of the loads. The site is in rugged territory and inaccessible 2-3 months of the year because of heavy snow. The road is poor and requires major conditioning each time the tanker trucks go to the site. When all delivery costs are included, the diesel fuel costs over $3.00 per gallon. The cost of electricity generated at the site is estimated to cost about $0.40 per kWh.
There are 12 independent transponders at the site. They operate at 24 volts and the current required depends on whether they are in standby, transmit, or receive modes. This variability causes the generators to operate inefficiently and air quality around the site is degraded by an estimated 1000 tons of carbon dioxide that is generated each year. Also, the cost of diesel maintenance has been high. When major repairs are required, the engine must be removed from the mountain. This incurs considerable expense plus it increases the risk of complete communications shutdown if the other generator fails.
Fuel is stored in above-ground tanks at the site. These are old and must be replaced. There has been leakage and many fuel spills at the site over the years. These have caused damage to the trees and plants around the site and, in some places, traces of diesel fuel can be found ten feet below the surface. In 1992 the Environmental Protection Agency told the operators to "clean up or shut down". The need for an alternative to the diesel generators was clear!
The agencies contacted the PV Systems Assistance Center at Sandia National Laboratories and inquired about the feasibility of using PV power for the site. They used the score sheets to evaluate this application.
#1 - Site Access. The fuel must be transported to the site by tanker truck. Enough fuel must be stored at the site to keep the generator running 24 hours per day during the snow season. The cost of fuel delivered to the site is $3/gallon.
Site Access Factor Item Weight (WF) No (0) Some (3) Yes (5) Results Will extensive road work be required to transport fuel safely? 8 X 40 Will sea or air lift be required? 8 X 0 Can fuel be stored safely on site? 6 X 30 Are there problems obtaining storage tanks? 3 X 0 Are there months when the site is inaccessible? 4 X 20 Is system availability higher than 95%? 6 X 30 Total Score 120
X
40
0
30
20
120
#2 - Engine Environmental Factor. The fuel leakage and spills that have occurred have caused extensive damage to the trees and plants around the site. Some soil will have to be removed and the storage tanks replaced. Minimizing the amount of fuel required at the site is of prime importance. Also, the amount of air pollutants can be reduced by minimizing generator operation.
Environmental Factor - Engine Generator Item Weight (WF) Low (1) Average (3) High (5) Results Rate the possibility of fuel spillage at the site. 8 X 24 What is the potential damage to humans, animals, plants, or water if fuel spills? 8 X 40 Rate the fire risk from stored fuel. 6 X 6 Rate the possibility of damage from toxic emissions. 4 X 12 Rate the problems that might be caused by operating noise. 4 X 4 Total Score 86
24
12
86
#3 - Battery Environmental Factor. The PV system envisioned for the site will include enough commercial grade lead-acid batteries to carry the load for 2-3 days. The batteries will be installed in a protected area that will contain any possible leakage from ruptured cells. The heavy-duty batteries are expected to last 6-9 years with this kind of operation. They will be removed from the mountain for proper disposal.
Environmental Factor - Batteries Item Weight (WF) Low (1) Average (3) High (5) Results Rate the possibility of environmental damage from batteries -5 X -5 Estimate the cost of disposing of used batteries. -3 X -15 Total Score -20
-15
-20
#4 - Load Profile Factor. There are 12 independent transponders at the site. They operate at 24 volts dc with a current demand of 10, 4, and 0.5 Amperes when they transmit, receive, or are in standby respectively. Although the instantaneous load of one transponder is highly variable, the total load demand can be assumed constant with little daily or seasonal variation.
Load Profile Factor Item Weight (WF) Low (1) Average (3) High (5) Results Rate the variability of the daily load demand. 4 X 12 Rate the variability of the annual load demand. 4 X 4 Total Score 16
16
#5 - The Renewable Resource Factor. The solar resource at the site is rated at 5.6 kWh/m2/day annual average with a minimum of 3.3 and a maximum of 7.7 kWh/m2/day. The wind resource has not been evaluated.
Renewable Resource Factor Item Weight (WF) Low (1) Average (3) High (5) Results Daily solar insolation in kWh/m2 4 <3 3-5 >5 X 20 Average daily wind speed in miles/hour 4 <10 10 - 15 >15 N/A Seasonal ratio of solar insolation 2 >4:1 3:1 X <2:1 6 Rage the match between the load demand and the daily renewable resource. 2 X 10 Total Score 36
N/A
10
36
#6 - Funds Access. Obtaining funds for operation and maintenance at the site is problematic because of the many agencies involved. It is not expected to get any better in the future. In this case it may be easier to get "up-front cash" from each participant to install a PV system than to get O&M funds.
Funds Access Factor Item Weight (WF) Low (1) Average (3) High (5) Results Rate the difficulty of obtaining funds for operation and maintenance of the generators. 5 X 25 Rate the difficulty of obtaining adequate funds for repair, overhaul, and replacement of engines. 3 X 9 Rate the difficulty of obtaining adequate funds for replacement of batteries used in a renewable energ;y system. -3 X -15 Estimate future inflation. Assume the average is 6%. 2 X 6 Total Score 25
25
9
And, now the grand total. For this application we have added a 50% penalty to the weighting factor for the generator environmental score because of prior damage and the need to prevent further contamination of the site.
Hybrid Systems Factor Item Score Weight (WF) Total Site Access Score 120 1 120 Environmental - Generator Score 86 1.5 129 Environmental - Battery Score -20 1 -20 Load Profile Score 16 1 16 Renewable Resource Score 36 1 36 Funds Access Score 25 1 25 Total Score 306
1
1.5
129
306
This score of 306 gives a strong indication that a hybrid power system with a large renewable component should be considered for this application. The site operators contacted several firms that specialize in large hybrid systems and asked them about the cost and feasibility of such a system. All the firms encouraged them to look further into the issues Their system designers worked with the operators to get the best system for the money. We'll look at some of the issues that were discussed.
Getting Your Hybrid System Designed
What do you do now? Hopefully, we have convinced you that a hybrid power system using at least one renewable power generator should be considered for many applications. You're interested but you have some questions.
1. Can you get an HPS as easily as you can get a fueled generator?
2. Who designs and installs these systems?
3. How much will it cost?
4. How long will it take to get one?
Let's try to answer them in order. Will it be as easy as buying an engine generator? No. You can probably buy a generator with a few phone calls. To obtain a hybrid power system for your specific application will require a partnership approach between you and the hybrid system designer. However, your reward for the extra time you spend will be a reliable low-maintenance power system that will last many years. Also, since you've already considered most of the factors pertinent to hybrid systems, you'll find buying a hybrid system almost as easy as buying a generator.
Who designs and installs these systems? Call the Solar Energy Industries Association (SEIA) at 703-524-6100 for companies in your area that provide solar systems. Regardless of who you call, they will need information from you to do their job well. They will do the calculations and specify the hardware but here are some of the things they will want to ask you.
Do these questions look familiar? They're the same ones you answered to determine if you should use an HPS. Since you have already thought about these issues, you'll be ready to work with the system designer. The more you get involved the higher will be your satisfaction with the system.
How much do they cost? How long will it take? These are questions that we cannot answer here because there are too many site specific issues that affect the system design. Your system designer can give you an estimate. The good news is that most hybrid systems can be operating only a few months after they are ordered.
Hybrid Systems in Use Today
Four operating hybrid power systems were selected to illustrate the different applications where hybrid power systems are reliable, economical producers.
Etisalat/Salabik
Five hybrid power systems were installed recently by Etisalat, the national telecommunications company of the United Arab Emirates to power a portion of their cellular telephone network. Located in remote and environmentally fragile desert areas around Salabik, these PV-diesel hybrid systems were chosen because the desert temperatures and high air and soil salinity make a diesel only system vulnerable. The systems were designed and installed by Integrated Power Corp. of Rockville, Maryland, (IPC) in 1990. The load is continuous and includes passive and active cooling systems. High system availability is required. The PV system was sized to carry 50 percent of the annual load. This kept initial cost reasonable, yet promised fuel savings and lower O&M costs for the diesel.
The HPS includes a 9.3 kW PV array, a 14.4 kW diesel generator, 228 kWh of storage battery, and IPC's microprocessor-based system controller. The load is 69 amperes at 48 volts dc.
The PV arrays consist of 156 Solarex MSX-60 PV modules. The generator is a Lister TR3 diesel engine, combined with a Newage alternator and a rectifier. The diesel is expected to operate about 1200 hours per year.
The system controller automatically starts the diesel engine whenever the battery state-of-charge drops below 50 percent. It then charges the batteries from 50 percent state-of-charge to 95 percent state-of-charge. It is expected that the engine will use approximately 5000 liters of fuel per year, about one-fourth the amount of a diesel generator running full time. Also, the presence of a PV array allows the diesel engine to be run only at night. This reduces the risk of problems caused by the high desert temperatures in the daytime. The diesel engine is run at least once every two weeks to keep the engine lubricated. The IPC microprocessor based system controller operates the cooling system by sensing indoor and outdoor temperatures and opening louvres and starting fans.
Tern Island
A PV power system with diesel backup was installed on Tern Island because of environmental considerations. Tern Island, 550 miles west of Oahu, Hawaii, is a sanctuary for marine birds, the endangered Hawaiian monk seal, and the threatened green sea turtle. The island is managed by the U.S. Fish and Wildlife Service and used by small groups of scientists to study the wildlife, their habitat, and habits. From 2-12 persons are on the island year round. The island power for communications, lighting, tools, dehumidifier, water pump and refrigerators had been provided by diesel generators. The scientists were worried that the noise and pollution of the generators was causing the birds to alter their life-style. Also, fuel was expensive--in 1986, the quote for scheduled delivery of 10,000 gallons of diesel fuel was $40,000. An alternative had to be found.
The Service turned to the Design Assistance Center at Sandia National Laboratories for help. Within 6 months, a PV system, installed by Solar Engineering Services of Lacey, Washington, was operating on the island.
The 2.8 kW PV system with 28,800 kWh of battery storage provides 95 percent of the annual power required for the 9,000 kWh average daily load.
Two diesel generators, a 3.8 kW Yanmar and a 15 kW Onan, would be used only for back-up. In the five years since the PV system has been installed, performance has exceeded expectations. There have been no power problems and the diesels have been used infrequently. The need for fuel delivery has been almost eliminated. The scientists are happy with the noiseless power system. The birds and seals aren't talking but appear content. The payback to the U. S. Fish and Wildlife Service for the hybrid power system was approximately two years.
Charleston TACTS
A PV-wind-diesel hybrid power system is being used by the U.S. Navy to provide power for Tactical Aircrew Combat Training Systems (TACTS) off the coast of South Carolina. The need for reliable power and the potential cost of delivering and storing fuel on off-shore platforms were the key factors in deciding to use an HPS. The power systems were designed and installed by Northern Power Systems (NPS) of Moretown, Vermont. There are eight systems, three of which have loads consisting of the Navy's tracking information subsystem, microwave communication links, a helicopter-landing lighting system, and support equipment such as strobes, beacons, and fire protection systems. The loads vary from 2.9 to 3.6 kW average at 120 volts dc and the power demand is 24 hours per day (~80 kWh/day). Five of the platforms are slave stations with about 8 kWh/day loads operating at 24 volts dc. A stand-alone PV system with batteries is used on the slave stations.
The hybrid system at the three main platforms consists of two 12 kW North Wind MR12 wind machines, a 6.5 kW PV array using 120 Solarex modules, a 302 kWh battery bank, and a 25 kW diesel generator made by Lister Petter.
The renewable generators are designed to power the load most of the year. The PV array will produce about 30 kWh daily and each wind machine will produce about 65 kWh on a typical day.
Since the system was installed in 1984 the diesel generators have been used only to operate the crane used to unload supply boats. Fuel deliveries have almost been eliminated. The wind machines have required frequent maintenance and some of them will be replaced. The PV arrays have been steady producers and there have been no problems.
Santa Maria Magdalena
In 1991, Integrated Power Corporation, Rockville, Maryland installed a photovoltaic/wind/diesel hybrid system to provide utility-grade power to this village located in Hidalgo, Mexico. The electric utility company that serves the state funded the hybrid system as the most economical method of providing electricity to the remote village. The 45 kWh per day system will provide electric power for 43 homes, 3 schools, 3 stores, a church and an auditorium.
The hybrid system consists of a 4.3 kW PV array, a 5 kW wind turbine, an 18.4 kW diesel generator, a 1100 ampere-hour, a 120 volt battery, plus inverter and controls.
The power will be used for lighting and radio/TV in the residences and schools. Refrigeration is an added load in the three stores. The HPS is located in the center of the village with an overhead power distribution grid serving the homes, schools, and stores. The villagers hope to add a clinic in the near future. This will increase the load demand, a common occurrence with village power systems as the availability of electricity causes more use. However, the flexibility of the HPS will allow this additional load to be met without modification of the system.
Summary
Over the last decade, hybrids have become viable alternatives for power production because they allow the designer to capitalize on the strengths of both conventional and renewable energy sources. Hybrid systems cover a broad spectrum of applications. Some are used for smaller loads such as telecommunications sites and water pumping systems in remote areas. They can provide near 100 percent system availability at reasonable cost. The renewable generator can be sized to meet 90-95 percent of the load during the year. The storage batteries can supply the peak load demand, and the fueled generator will be used only to recharge the batteries. This minimizes generator run-time and fuel use. At the other end of the spectrum are large hybrid power systems where all power generators are run concurrently every day. Typical applications are rural village electrification and island grid power. The conventional generators run at their most effective load point with power going directly to the load and to the batteries. The energy in the batteries can meet spikes in the power demand and the renewable energy generator will reduce fuel consumption and engine generator maintenance.
Many issues must be considered when deciding whether or not to install a hybrid. These have been presented and discussed in this booklet. We have devised and presented a method for grading your application to determine if an HPS should be considered. We've discussed how to estimate the size needed and get a rough indication of how much it will cost. Finally, we looked at some examples of hybrid power systems that are in use today.
Hopefully, you have decided to continue your investigation into hybrid power systems. We have provided a list of references and recommended readings. Appendix A contains a list of companies (reviewers of this booklet) that are known to have experience with commercial hybrid systems. Call them or a solar company in your area that provides hybrid systems. They will be happy to assist you with a cost-effective hybrid system design for your application.
Cited References
1) Protection of Buried Metal Storage Tanks. United States Code of Federal Regulations 40 CFR 280, pp. 20-70.
2) Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices, Sandia National Laboratories, Report No. SAND87-7023, November 1991.
3) C. T. McCloskey, et al, "PV-Diesel Hybrid Power for Etisalat," Modern Power Systems, pp. 25-28, September 1991.
4) Oser, R., "A Photovoltaic Application at an Island Wildlife Refuge," Phoenix, Arizona, SAND88-0146, pp. 60-61, January 1988.
5) Barlowe, G., et al, "Hybrid Renewable Energy Systems for Off-Shore Naval Installations," Proceedings of the 20th Photovoltaic Specialists Conference, pp. 1179 -1181, 1988.
6) Integrated Power Corporation News Releases; April 1991
Additional Reading
Best, D., "Electrifying the World," Solar Age, Vol. 10 No.7, pp 65-67, July 1985.
Jones, G., and Chapman, R., "Photovoltaic/Diesel Hybrid Systems: The Design Process," Proceedings of the 19th Photovoltaic Specialists Conference, pp. 1024-1030, 1987.
Neil, D. R., et al, "Hybrid Energy Systems are Ready," Solar Today, Vol. 3, No. 5, pp. 15-16, September-October 1989.
Wardrop, N., "The French Island Lodge Commercial Hybrid Power Supply System," Solar and Wind Technology, Vol. 7, No. 1, pp. 37-42, 1990.
Acknowledgment and Disclaimer
