Calculate cost and count on savings
The target reductions in CO2 emissions in developing countries will inevitably lead to increased pressure on consumers to reduce fuel consumption. This may well lead to increases in energy costs over the normal rate of inflation. Other methods will be also be used by governments to encourage the installation of more efficient capital equipment in order to reduce the consumption of energy.
Most organisations want to see a return on their investments, but it can be difficult to calculate the value of investing in high efficiency ventilation systems.
In this article we advocate the concept of LCC – Life Cycle Cost – as a method of putting figures to these aspects and achieve a relevant calculation as a base for investment decisions regarding ventilation.
What is LCC?
Life Cycle Cost, LCC, stands for the estimate of the total cost of purchasing, installing and running an item of equipment for a designated period of time; normally 20 years. The environmental cost of raw material extraction and final disposal may also be included in this sum.
Life Cycle Cost provides a method to allow us to compare the overall profitability of different solutions. It can also help us to choose the solution that will do the least harm to the environment.
LCC = Investment cost +Energy cost
+Taxes (if applicable)
However, as you can see in figure 1, energy and plant investment dominate in the overall cost estimate and since the environmental cost can be considered to be proportional to the energy consumption, it is normal to concentrate on estimating the energy and investment costs in the LCC calculation and ignore the other parts in order to make it easier to compare different air handling unit solutions.
To differentiate this type of analysis, we use the term Life Cycle Energy Cost (LCCE) to describe the simplified calculation involving the total cost of energy and the investment cost.
The LCCE method is typically used to compare different sizes of air handling unit selected for the same duty.
Life Cycle Energy Cost (LCCE)
The energy cost (LCCE) is the sum of the energy costs for each year of the designated period expressed in terms of a present value taking interest and inflation into account.
LCCE is a relative estimate and is intended for use in the evaluation of tenders. The true life cycle cost will, naturally, differ from the calculated LCCE depending on actual economic and climatic conditions as well as building usage during the economic period considered.
Investment and LCC
The energy cost is made up of two main parts: The electrical energy required to operate the fans and the thermal energy needed to maintain environmental conditions within the building. Heating is generally the larger part of the energy requirement in the UK but that energy is normally reduced by the use of heat recovery equipment as illustrated in Figure 2.
The blue bars in the diagram represent the investment and running cost of an air handling unit without any heat recovery and using 100% outdoor air while the green bars represent the costs for a unit with a rotary heat exchanger designed for the same duty.
In figure 3, the blue columns represent a typical unit without heat recovery while the green columns represent a unit selected for the same duty but with a rotary heat exchanger. We can see that, in this typical case, the heating energy requirement is the largest part but that cost can be more or less eliminated by the use of an efficient heat recovery system for a small increase in electrical power. Clearly, for a low LCCE cost it is important to employ heat recovery. The cost of providing cooling is also a significant factor and may need to be considered in many cases.
Specific Fan Power (SFP)
If we now assume that the air-handling unit is selected with heat recovery then the biggest energy consumers are the fans. The useful power output provided by the fan is the airflow rate and pressure multiplied together. Unfortunately, fans do not operate at 100% efficiency and more electrical power must be provided to get the airflow and pressure needed and overcome the losses in the fan and drive system.
The recent updates to Part L specify that SFP’s in new build should be no more than two in systems without energy recovery and no more than 2.5 with energy recovery. But there seems to be some confusion as how to calculate it.
The SFP is the sum of the power taken from the grid for both the supply and exhaust fans divided by the higher of the two airflows (which is the ventialtion rate of the building).
SFP = PSF + PEF
PSF = Grid power supply fan
PEF = Grid power extract fan
qb = higher of either supply airflow or extract airflow
Fan energy consumption
The fan energy cost depends on the airflow rate, the total pressure and the efficiency of the fan, the fan installation and the fan drive.
The electrical power absorbed by a fan installation is given by:
Power = q.DP (kW.)
hfan htrans hmotor hcontrol .1000
DP is the total pressure rise over the fan
h fan is the efficiency of the fan
h trans is the efficiency of the transmission between motor and fan
h motor is the efficiency of the motor
h control is the efficiency of the fan control system and the effect of power factor
The airflow rate, q, is assumed to be that required for adequate indoor air quality and to cover the needs of heating and cooling.
If we can assume that the airflow rate is fixed then the pressure and the efficiency are the quantities we need to aim at improving when trying to reduce LCCE. The fan pressure rise must be designed as low as possible while keeping the efficiencies high. Both the air handling unit internal pressure drop and the duct system pressure drop are extremely significant.
Of course the fan efficiency is important but it is equally important to consider the efficiencies of the motor and the drive system. Variable speed drives are very often installed today, and can aid in reducing energy consumption.
Energy recovery and LCC
Generally, the supply air temperature is more or less constant. The outdoor air temperature varies, of course and we use statistical data in the form of degree-hour tables to simulate that.
The LCC diagram (figure 4) shows a degree hour chart for an AHU with thermal wheel in London. As you can see the thermal wheel recovers the vast majority (95%) of the heating demand throughout the year.
The thermal wheel can also reduce the cooling requirement. As you can see, using energy recovery devices are vital in reducing the LCC of any air handling system.
Air handling systems are installed in order to create good Indoor Air Quality and proper temperature control so that optimum conditions are created for people and processes. This is still the goal. However, these systems must be installed as energy efficiently as possible.
Clearly in these energy conscious days, Life Cycle Costing will come to the fore when deliberating over the best value and, ultimately, most profitable system for a given project.