Cost / Benefit Analysis: LPG Engine to Power Generator

 

 

The following article is a cost / benefit analysis we prepared recently for a MarineDiesel customer showing a financial case for using LPG instead of diesel to power a medium  sized generator. The customer name has been removed, and costs are converted to Euros at local rates (prices will vary, based on your region, but this is a good, general comparison). The engine p

roposed is based on the 5.7L GM Vortec engine block.

Initial benefits of the MarineDiesel product line include:

  1. The ability to use LPG instead of gasoline or diesel for power.
  2. Customer payback time in a matter of months through fuel cost savings.
  3. Multiple markets for the engines, ranging in use from small generators to irrigation, mining, marine, pumps, or small vehicle power.
  4. Factory conversion to LPG, as opposed to cheap converter kits commonly found in the market. A primary benefit of this procedure is seamless integration of the LPG kit into the engine’s ECU. Aftermarket conversion kits often do not integrate fully, voiding the warranty or rendering engine fault warnings incorrect. For this reason, aftermarket conversion kits, such as are currently being sold cannot be recommended for proper installation and life cycle. Furthermore, MarineDiesel engines have been designed specifically with alternate fuels taken into account. Though there are some immediate cost savings associated with aftermarket conversions, they simply are not an accurate substitution for a proper OEM design.
  5. Full MarineDiesel warranty terms apply.
  6. Government financing may be available. 

 

Life Cycle

General Motors has been manufacturing the Vortec block configured to LPG for nearly thirty years.

Thousands of units have been sold and the engine has proved to be extremely reliable.

The MTBO is at 10,000 hours of use, however, it is highly dependent on climate and continuous RPM.

 

Cost to Operate

The 5.7L LPG consumes fuel based on engine load.

100% Load:         9.3 m3 / hour

75% Load:            7.1 m3 / hour

50% Load:            5.4 m3 / hour

25% Load:            3.8 m3 / hour

 

<<COMPARE VS EQUIVALENT DIESEL>>

100% Load:         27 L / hour

75% Load:            20 L / hour

50% Load:            12 L / hour

25% Load:            8 L  / hour

 

Current prices of fuel (local converted to Euros):

LPG:   0.30 / litre

Diesel: 0.79 / litre

Average annual use of 4,400 hours per year, assuming 75% load at 1,800 RPM.

LPG consumption 3.85 Liters / m3

27.335 Liters LPG / Hour

LPG annual cost of fuel consumption: 33,075

Diesel annual fuel consumption: 65,978

Cost of ownership, year one

Diesel:

Cost of Engine:                                                  5,000

Operating Cost:                                                                65,978

Service and spares (oil filters, etc):          2,900

Total Cost Year One:                73,878

LPG:

Cost of Engine:                                                  7,250

Operating Cost:                                                                33,075

Service and Spares:                                         2,500

Total Cost Year One:                42,825

 

Payback Time: 3 months

 

Amount of Power Produced

This is a difficult question to answer, since it will vary greatly based on the alternator chosen. In order to keep everything consistent, the same Kohler alternator 4P10X, will be used. This same alternator is used by numerous other manufacturers to produce the same amount of electricity, using LPG, Diesel, or CNG.

This alternator will produce 52 kW of power at 110 / 220 50 hz, using the 5.7L engine.

To put this amount in context, 52 kW is roughly enough to power 20 medium to large window unit air conditioners, or a medium-sized restaurant.

Therefore, given the usage described above, using LPG,

One Hour: 4.37

One Day: 91.88

One Month: 2756

One Year: 33,075

Cost per kWH: 0.147

 

A diesel genset produced by Baifa, using a Deutz engine made in China, producing 48kW using the same alternator would cost to run continuously:

One Hour: 10.35

One Day: 124.15

One Month: 3,725

One Year: 44,696

Cost per kWH: 0.216

 

A similar diesel genset, produced by John Deere using a four cylinder, John Deere diesel engine and using, again, the same alternator,

One Hour: 26.93

One Day: 323

One Month: 9,690

One Year: 116,276

Cost per kWH: 0.518

 

Tank Capacities

For small home / commercial use, portable LPG tanks come in the following sizes: 4 kg, 7 kg, 15 kg, and 48 kg.

The standard size for tanks installed in vehicles (permanently mounted, even on trailers, for safety reasons) is 58L (30.39 kg).

For commercial or industrial use, particularly when the engines will be used to power continuously, rather than as standby, typically large, above ground or underground tanks are used. The most common sizes of above ground tank are 1T (1,908 L) and 4.3 T (8,204 L).

Which tank that should be used and how long the generator will provide power on a single tank are really highly dependent on how the genset will be used. The smaller LPG “cooking gas” cylinders will occasionally “freeze” due to the atmospheric pressure difference if too much is drawn from the tank. Therefore, the smaller 4kg and 7kg tanks should not be used with the 5.7L. Remember additionally, 100% of the tank volume will never be able to be removed, again due to pressure. Therefore, a factor of 15% is reduced for simple numbers on LPG.

 

5.7 L Vortec with 4P10X, 12 hours per day continuous RPM use (1,800 RPM)

Tank Size Diesel Equivalent Cubic Meters LPG Reduce 15% below Full on LPG (Usable Amount in tank) Time between Fill-ups LPG Time between Fill-ups Diesel
15kg (Cylinder) 28.6L 8.085 7.03 m3 59 minutes 1 hour, 20 minutes
48 kg (Cylinder) 91.6 L 25.872 22.5 m3 3 hours, 10 minutes 4 hours, 40 minutes
30.39 kg (Taxi standard) 58 L 16.38 14.24 m3 2 hours 2 hours, 50 minutes
73.36 kg (Trailer max) 140 L 43.5 37.83 m3 5 hours, 30 minutes 7 hours
1,000 kg (above ground) 1,908 L 539 468.7 m3 5.5 days 8 days
4,300 kg (above ground) 8,204 L 2,317.7 2,015.4 m3 23.5 days 34 days