You have decided it's time to add an on-site Compressed Natural Gas (CNG) fueling station. Good choice. The United States is awash in abundant natural gas. The affordable, domestic and low carbon fuel is quickly replacing diesel and gasoline in fleet operations around the country. The choices and information available can be overwhelming. I have been involved in more than 30 CNG installations and here is my advice -- keep it simple. You can quickly prepare yourself to meet with potential vendors and partners by using the following questions to develop a CNG station development checklist.
1. Who is going to use the station and how much fuel do they need?
This is the first question to consider when beginning a CNG evaluation. What types of vehicles are going to use the fueling station? How much fuel (approximately) will each vehicle need?
Most stations sell fuel either in Gasoline Gallon Equivalents (GGEs) or Diesel Gallon Equivalents (DGE). Each of these units is equal in energy content to their liquid petroleum counterparts. You do not need to go beyond these metrics. Gas compressors are rated in Standard Cubic Feet per Minute (SCFM), and SCFM can be translated into GGE or DGE production capacity. You can simplify the picture by keeping all capacity ratings in GGEs or DGEs. Any station designer will be well versed on either one of these metrics. I recommend using the one that is most common to your customer base. If the users are primarily pumping gasoline today, use GGEs. If they are a heavy duty fleet using diesel every day, use DGEs. A few examples have been included below.
25 CNG pickup trucks x 8 GGE / Day = 200 GGE / Day
20 CNG Trash Trucks x 35 DGE / Day = 700 DGE / Day
50 Heavy tractors x 80 DGE / Day = 4,000 DGE / Day
Answering this question is at the heart of a CNG station's size. There are numerous compressor options on the market, each with a different capacity. Knowing the fuel requirement will allow you to craft your station specifications for vendor proposals. You want to size the station for what you anticipate the demand will be in year five of the program. Stations can always be upgraded, but obviously there is a cost in doing so. You will want to strike a balance between the station capacity and capital costs.
2. When will the vehicles fuel?
The next step in your evaluation is to determine WHEN the vehicles will fuel. CNG stations can be designed in one of three ways: fast-fill, time-fill or a combination of the two. Fast-fill stations dispense fuel between 5-20 gallons per minute, similar to gasoline and diesel stations. Most public access CNG stations are designed to fast-fill vehicles. If a fast-fill design makes sense, take a look at the peak fueling hours and determine how quickly each vehicle will need to fuel to keep your customers happy. In the pickup truck example above, you will need a system that produces 200 GGE / day. If all of these vehicles will show up after work between 5pm - 7pm, then the station needs to produce at least 1.6 GGEs per minute to ensure this will work (200 GGEs / 120 minutes = 1.6 GGEs / minute). Faster fueling times are certainly an option, but this gives you an idea of the minimum requirement.
The second design is an overnight time-fill station. These stations are typically used when a fleet returns to a central base each night and the vehicles are parked overnight. Time-fill stations are attractive due to lower equipment costs. You can spread the fueling capacity over a larger window of time to reduce the size (and therefore cost) of the compression system. With a time-fill system you will look at the number of hours the vehicles have to fuel vs. minutes. In our trash truck example above, we need 700 DGEs of fuel each day. If the vehicles are parked and idle for eight hours, we can simply divide 700 DGEs by eight hours to get 87.5 DGEs per hour as our station specification.
The final design is a combination station. Combo-stations typically have a secured private time-fill area and a separate fast-fill dispenser as well. Combination stations are a good fit if you have your own fleet of vehicles parked overnight, but you also want to retail fuel to third parties. You can do both while keeping your own fueling operations separated and secured. Once you evaluate when the fueling will occur and what design type is most appropriate, you can move to the next question.
3. What is the gas pressure at the location?
A CNG station is powered by a gas compressor. That compressor will tap into the local utility gas line in the street, which is (typically) going to be at a pressure between 5 - 60 pounds per square inch (psi). The compressor will take that pressure all the way up to 3,600 psi, which is the pressure rating of most CNG vehicles. One of the most important factors the station designers will need is the inlet gas pressure. To obtain this, you can call the local natural gas utility, ask for the builders' call line, give them the address, and ask them what the maximum gas inlet pressure would be for a CNG station.
4. Do you need redundancy?
CNG stations with multiple compressors that can supply fuel even when a single unit is offline are considered redundant. This means if a compressor is taken offline for maintenance or has a failure of some form, the station can still supply fuel to vehicles. The primary consideration for redundancy is what will happen to your customers if the station is down for a few hours? If there are other stations nearby, it may be advisable to install a single compressor.
If the station is a sole fueling outpost for mission critical fleet vehicles, redundancy may be a better choice. It is important to look at what each of the compressors can handle (e.g. how many GGEs per minute or hour they can produce). Station redundancy is summarized as a percentage. A 100 percent redundant station can still meet the exact fueling requirements even with an entire compressor offline. A 50 percent redundant station can still provide fuel, but only at half the capacity...and so on. If a station design has two small compressors that need to run 23 hours a day to meet your fuel requirement, this is not a redundant station (or a good solution). One unit cannot handle the job alone, and if both units are running all day, they will fail regularly. A 100 percent redundant design also has the added benefit of a longer life cycle because the compressors can be alternated. This means each machine racks up fewer running hours each day and will therefore last longer.
5. How will you maintain the station?
One of the greatest misconceptions in the CNG fueling industry is that stations are easy and simple to maintain. This is simply not true. A station needs love and care just like any major mechanical investment. Properly maintained, a CNG station can easily see 25 years of life or more. Improperly maintained machinery will quickly become a nightmare. A vendor who tells you it is easy and you can do it yourself with minimal training is looking for a quick buck. These are "turn and burn" station installers. Avoid them. A CNG station has a variety of systems and parts that will require both regular and preventative maintenance, as well as scheduled overhauls and intermittent emergency repairs. Most compressors come with a warranty to cover the system for at least a year, but it is prudent to have a plan in place by the time you open the station.
There are typically three options you can consider to maintain the station. The first is to get trained and certified to do it yourself. If you or members of your staff are experienced with high pressure gas systems, this may be an option. Most manufacturers offer training programs to get certified with specific compressors. However, this is typically expensive and will require the overhead (salary, benefits, insurance, etc.) necessary to keep someone on staff to handle preventative maintenance and repairs on a regular basis. Many oil and gas companies maintain their own stations because they have extensive expertise with compression systems.
The second option is a traditional time and materials program. Several vendors offer programs that include monthly inspections and preventative maintenance. They will also be available for emergency repairs. You will typically pay a monthly fee for the regular inspections, and then sign off on each purchase order for time and materials as needed. If you keep capital on-hand for ongoing facility and site repairs, then this type of pay-as-you-go maintenance plan may be a good fit.
The final option is referred to as an Operations & Maintenance (O&M) plan. This is typically a no-risk maintenance option where you pay an all-inclusive fee for a vendor to take on financial risks and costs associated with the station. The vendor will install remote monitoring (typically a broadband connection) at the station. This will give them access to do remote diagnostics and restarts. It will also allow them to respond to station outages in real time. Remote monitoring eliminates the need for a phone call or email from a customer telling them the site is down. They will handle all preventative maintenance, repairs and even full replacements if necessary. O&M programs are typically charged by the gallon. Industry costs range from $0.15 to $0.50 per gallon for most O&M programs and will vary based on the station and the fuel throughput. In return, most vendors will ask for minimum volume guarantee to ensure they have an opportunity to recoup their costs and make a profit. This option is attractive as it can be built directly into your fuel price and is easily forecasted. It also removes the burden of unforeseen repair costs and late night repair calls.
With this information in hand, you can easily draft a solicitation for equipment and installation of your CNG station. Answering these questions will prepare you to easily evaluate different equipment options, construction costs, and maintenance offerings. If you need a list of station vendors to get you started, I recommend the NGV America business directory, which can be found here. Good luck!