erv projec report - university of texas at austin ·...

SPRING 2010

CRISTINA SAFIE -‐ DAVID ATTIA

HVAC DESIGN

SPRING 2010

ERV ECONOMICAL ANALYSIS

HVAC DESIGN SEMESTER PROJECT| ERV ECONOMICAL ANALYSIS

1

PROJECT STATEMENT

Our project is intended to find if the installation of an Energy Recovery Ventilators (ERV) in

TEXOMA surgical center is an economically viable option. An ERV works by reusing the energy

of the air being exhausted from the building transmitting energy to the outdoor air coming in to

the building. This system will reduce the amount of work performed by the Air Handling Unit

(AHU). The analysis consists in finding the cost of the equipment as well as installation and

operations and find out if the savings meet the required interest rate to see if it will be a viable

investment.

The analysis starts by calculating the energy saved by the difference in temperature after the air

comes out from the ERV. We will calculate the energy saved by the ERV using the data that

includes temperatures for every hour of the year in Austin, Texas. After getting the energy

saved we will account for the amount of electric energy used by the exhaust and supply fans to

overcome the pressure drops inside the ERV, as well as the motor to operate the energy wheel.

With this information we can make our economic analysis. The required rate of return for the

net savings of the ERV needs to be 8%, during the expected 15 years of life of the system, to

make the investment attractive.


2

BUILDING DESCRIPTION

The TEXOMA surgical center is an ambulatory surgical center located in Austin, Texas. The

building has high ventilation and humidification requirements based on Texas Administrative

Code (TAC). The figure below shows the floor plan of the building with the extra duct layout

that will need to be provided to install the ERV.

Figure 1. Building plan showing ERV duct layout


3

METHODOLOGY

The approach used for the project to make the initial investment analysis starts by setting our

assumptions on set points of our buildings, as well as the equipment’s efficiency. We will also

assume the expected life of the equipment and the mass flow rate of air coming in to the

system, as well as the amount of air being exhausted. After we have all the data we can

proceed to make the calculations to obtain the net energy savings of our system. The analysis

will be as followed:

• Obtain temperature difference from ERV using the assumed efficiency of 50%

• Calculate mass flow rate coming in to the building

• Obtain annual energy savings from ERV by multiplying temperature change; mass flow

rate and specific heat.

• Calculate energy used by supply fan

• Calculate energy used by exhaust fan

• Calculate energy used by energy wheel

• Calculate net energy savings by subtracting energy used by fans and wheel from energy

saved by the change in temperature.

• Convert power to dollars assuming the cost of $0.87/kwh

• Determine the required rate of return for economic analysis.

• Calculate the net present value

• Interpret the results


4

RESULTS

With the help of Microsoft Excel we calculated the energy saved and consumed by each of the

ERV components, we can see a summary of these results in Table 2 of this section. Table 2

shows that the energy saved in one year form the temperature difference of outdoor air before

moved by the ERV is 31,475.74KW, the energy used by exhaust and supply air fans required to

move air to and from the ERV is 2,177.44kw, as well as the energy used to move the energy

wheel. So the net energy savings from the system is 27,121.85kw. Converting the energy saved

during the year by the ERV to dollars, by using a cost of $0.087/kwh, gives us a total saving of

$2,359.6/year. By using this results we can now calculate what the net present value for our

investment is. If the cost for the ERV is $7,500, the required rate of return for the annual

savings is 8%, and the expected life of operation for the ERV 15 is years, we obtain a value of

$12, 696.95 in savings, from the decision of installing an energy recovery ventilator in the

building.

For the purposes of this analysis we simplified the scenario to where there is constant

operation at all times of the year. We have assumed the set point of the building to be 75 oF

and 50 % relative humidity, and 50% efficiency for the equipment. But in reality the mass flow

rates of air traveling through the ERV vary at all times, thus changing the results obtain in the

current report. There is also another factor we are leaving out from our calculations, that will

increase benefits to the owner for installing an ERV, a rebate program awarded by the city of

Austin provided that the equipment meets some requirements set by the city of Austin.


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Table 1. Assumptions, Equipment and design information

Table 2. Energy Calculations


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Figure 1. Timeline for economical analysis


7

CONCLUSIONS

From the results obtain from our analysis we can concluded that investing in a ERV for the

TEXOMA surgical center is a good investment. Our results show that the net savings per year

will be just below $2360, which means that the equipment will pay for itself after 3 years and 2

months. After that, all the obtained savings will benefit the owner by reducing electricity costs.

The total savings for the owner, using the required 8% interest rates, and a life expectancy of 15

years, will yield to a present value of $12,700. We have proven that the ERV will bring great

economical benefits, as well as increasing the sustainability of the building.

As engineers we recommend this system for this particular building, considering its size, use,

and location. We are assuming that the system will run 24 hours a day and a constant mass

flow rate. These assumptions will provide us with results that are not as accurate as what

happens in reality, but for the purpose of this analysis we created an ideal situation to simplify

calculations. The city of Austin also provides a rebate for buildings that use this kind of system.

erv projec report - university of texas at austin ·...

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