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View Subscription Details: Letter ( 08/28/2008 ) - HTML Format

Real energy savings

To estimate return on your investment in energy saving equipment, potential energy savings should be calculated.

Very often you can hear from sales persons that their equipment will save $50,000, $100,000 and etc. Is it real numbers?

To verify the estimation of potential energy savings, two steps should be done.

Step 1.  Ask a sales person to provide the calculation of potential energy savings.

Step 2. Verify all numbers used in this calculation.

Example. A refrigeration plant has compressor power of 2000 HP. Condenser power is 200 HP (120 HP of condenser fans and 80 Hp of water pumps). Condenser fan VFDs can be installed at costs of $52,000 . Energy cost is $0.1/KWh. Minimum allowable condensing pressure is 110 psig.

At 50% of VFD speed some energy can be saved. This energy savings will be 60 HP (fan cycling) - 20 HP (fan VFDs) = 40 HP

Manufacturer sale representative estimated a potential energy savings of condenser fan VFDs as follow:

40HP x 0.75KW/HP x $0.1/KWh x 8760Hrs = $26280

Payback is about 2 years.

It looks like a good investment. Closer look at real life operation of this refrigeration plant will give us a different picture.

During periods of hot weather (2000Hrs) and high refrigeration loads, condenser fans will operate at 100% speed. Condenser fan VFDs will not save energy during summer operation.

During winter operation (50% of operating time or 4380Hrs), required refrigeration capacity will be significantly lower than 50%. Average required condenser capacity will be 20%. Energy savings at this capacity will be 18 HP. Total winter energy savings from condenser fan VFDs will be

18HP x 0.75KW/HP x $0.1/KWh x 4380Hrs = $5913

Fall and spring total energy savings from condenser fan VFDs will be:

40HP x 0.75KW/HP x $0.1/KWh x 2380Hrs = $7140

Total real energy savings will be $5913 + $7140 = $13053

This saving is half of saving presented by manufacturer sales representative. Real payback is around 4 years.

Minimum allowable condensing pressure for this plant is 110 psig. Hot gas defrosting is the reason for this minimum. I think that every evaporator coil can be defrosted at 100 psig of condensing pressure or lower. Defrosting at low condensing pressure required precise adjustment of this process. Lowering minimum condensing pressure to 100 psig will save more energy than condensing fan VFDs described above. You will be able to save $52,000 of capital investment.

I know that estimation of real energy savings is not a simple task and sometimes can be tricky. Therefore, this month I start free service of assisting in estimation of potential energy savings.

How does it work?

You have got proposal from contractor (manufacturer) about investment in energy saving equipment. Ask him about calculation of potential energy savings. I will help you to check this calculation to make sure that this is a real potential energy savings.

I would like to remind you, that I continue another free service that I started last year. This service is an initial tuning up of refrigeration plants. Potential energy savings is $10,000 - $40,000. Your cost is $0.

If you have any questions or concerns about free services, do not hesitate to contact me.


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