It may sound controversial, but I assure you that it concerns a serious engineering issue :)
OPTIMIZATION OF VENTILATION UNIT PARAMETERS SELECTION BASED ON ENERGY CONSUMPTION ANALYSIS.
If we were to think deeper, throughout our entire lives, we primarily pay for energy in various forms. Electricity, heat, cooling, food, and fuel, it's all energy. Essentially, the development of our entire civilization is determined by the amount of energy and its sources. Energy prices are currently on everyone's lips due to various global fluctuations. Most people wonder how to reduce their energy bills and consumption. However, achieving energy efficiency requires the ability to calculate energy consumption and identify its main consumers. For the majority of society, energy remains a rather abstract concept. Everyone knows they have to pay for the values indicated on the meters, but not everyone knows how to optimize energy consumption and what it truly depends on. This is where, dear reader, there is a huge field of action for HVAC specialists who can help investors make the right decisions. While investors may not be able to calculate energy needs themselves, they are often skilled at managing finances.
In this article, I will focus on ventilation units, which are essential equipment for almost every building and can be the main source of energy consumption. I will present how, during the design stage, one can optimize the selection of ventilation units in such a way that it is most beneficial for both the investor and the environment. I must immediately emphasize that the analysis presented is not universal due to different energy prices that different entities may have, as well as varying operating schedules, etc. However, it shows the path to follow in making wise choices.
Selecting the right ventilation unit, contrary to appearances, is not an easy task. This is due to the multitude of parameters that must be considered in the selection process, as well as the typically tight time pressure that accompanies the preparation of project documentation. As the heart of the entire ventilation/air conditioning system, the operation of the ventilation unit not only determines whether the required parameters in a given room are met but also influences the economics of the entire ventilation/air conditioning system. Such systems usually operate for many years and are, in most cases, one of the most energy-consuming elements of building equipment. Therefore, it is extremely important to take a moment longer when selecting the ventilation unit and optimize its operating parameters properly. This is particularly crucial in current times when energy carrier prices have skyrocketed, and there is a high likelihood that these price increases will continue in the coming years. Unfortunately, my experience indicates that this stage is almost entirely dependent on the manufacturer of the ventilation unit. Manufacturers aim to sell their products and remain competitive compared to other manufacturers, so they often push the unit to its limits. There's nothing inherently wrong with this – it's how a free market operates. However, it reflects the skill level of the designer in determining the final equipment and whether they can work with the manufacturer to select the appropriate components, guaranteeing not only proper operation but also optimal energy consumption. To better illustrate this, I will use an analysis of a specific building case and the selection of ventilation units made by the renowned Polish company Frapol, known for its excellent price-to-quality ratio. For almost 30 years, Frapol has been providing air conditioning and ventilation systems to both domestic and international markets.
Let's consider the case of ventilation for office spaces, the purpose of which is to provide hygienic air supply and maintain a relative humidity of not less than 40%. The ventilation system is designed with the following parameters:
Ventilation unit with a capacity of 10,000 m3/h and required available pressure of 300 Pa.
Combined intake and exhaust unit.
Air filter of class G4 + G7.
Heat recovery exchanger.
Water air heater supplied from a gas boiler room.
Design parameters: outside air temperature (te) = -20°C, room temperature (tn) = +20°C, water temperature (tw) = +20°C, relative humidity (RH) = 40%.
Steam air humidifier to maintain 40% RH during winter.
Noise attenuator on the supply and exhaust lines.
Actual selections of ventilation units were made for different air flow velocities through the ventilation unit. The face velocity determines the size of the unit, its internal resistance, and ultimately the investment cost. A lower face velocity means a larger unit. All units are capable of fulfilling their tasks and comply with the applicable regulations. Selections were made for the following face velocities:
w1 = 1.2 m/s
w2 = 1.7 m/s
w3 = 2.2 m/s
For each face velocity variant, three different types of heat recovery exchangers were selected, namely:
Counter-flow heat exchanger
Non-hygroscopic rotary heat exchanger
Hygroscopic rotary heat exchanger
In total, 9 different selections of ventilation units were made. Below is a fragment of the first selection with data relevant for conducting the analysis. To avoid cluttering this article with unit selections, they are available on the website www.cad-instal.pl/download/centrale.pdf for those interested. Below, the main parameters of the units relevant for energy consumption calculations are presented in tabular form.
Table No. 1 - Summary of technical data of selected air handling units.
* The seasonal efficiency for the counter-flow heat exchanger is lower due to the defrosting process. However, for the purpose of this analysis, the maximum efficiency was adopted according to the selection chart.
Interestingly, it should be noted that even a "regular" non-hygroscopic rotary heat exchanger can recover moisture with an efficiency of up to 40% due to its design.
Energy consumption calculations were performed using the computer program IX-CHART (www.ix-chart.com), specifically one of the built-in modules called the Energy Consumption Calculator (Kalkulator Zużycia Energii - KZE). This module allows for energy consumption calculations throughout the year, based on climate data from multiple years for 60 cities in Poland. The climate data is sourced from the Ministry of Infrastructure and is intended for use by programs simulating seasonal energy consumption.
Assumptions made in the KZE module of the IX-CHART program:
• Climate data for the city of Warsaw
• End of heating season temperature (the air heater operates only when the outside temperature drops below tko = +12°C)
• Calculations were made for the ventilation unit's operating schedule as follows: operation for 6 days a week for 12 hours + night ventilation in hourly intervals.
Fig. 01 - Assumed work schedule of the air handling unit
In order to know the annual energy costs, the prices of energy carriers are presented below in two variants (net prices):
Variant 1 - optimistic:
Electricity - ce=0.85 PLN/kWh
Heating energy from gas - cg = PLN 0.21/kWh
Variant 2 - pessimistic (contracts for an indefinite period according to Internet sources):
Electricity - ce=PLN 2.3/kWh
Heating energy from gas - cg=0.35 PLN/kWh
In this way, 18 calculation results were obtained.
Sample calculation results obtained in the IX-CHART program for position LP 1 acc. Table No. 1 is presented below:
Fig. 02 - Results of energy consumption calculations obtained from the IX-CHART program
Analogous calculations were performed for all cases, obtaining the results presented in the table below:
Table No. 2 - Summary of energy consumption by air handling units
To better illustrate the obtained results, the graphs are presented below:
Chart No. 1 - Annual energy consumption by air handling units
From the point of view of operating costs, not only the total value of energy consumption is important, but also the structure of individual energy carriers due to different prices of energy carriers.
The charts below show the results of operating costs for individual head velocities of flow through the AHU for variants 1 and 2 of energy prices.
Diagram no. 2 – Annual cost of AHU operation for a frontal speed of 1.2m/s, cg=PLN 0.21, ce=PLN 0.85
Diagram no. 3 – Annual cost of AHU operation for frontal speed of 1.7m/s, cg=PLN 0.21, ce=PLN 0.85
Diagram No. 4 – Annual cost of AHU operation for frontal speed of 2.2m/s, cg=PLN 0.21, ce=PLN 0.85
Diagram no. 5 – Annual cost of AHU operation for frontal speed of 1.2m/s, cg=0.35 PLN, ce=2.3 PLN
Diagram no. 6 – Annual cost of AHU operation for a frontal speed of 1.7 m/s, cg=0.35 PLN, ce=2.3 PLN
Diagram No. 7 – Annual cost of AHU operation for frontal speed of 2.2m/s, cg=0.35 PLN, ce=2.3 PLN
Poniżej przedstawiono zbiorczo wykresy rocznych kosztów eksploatacyjnych poszczególnych central wentylacyjnych z uwzględnieniem struktury zużycia energii.
Diagram no. 8 - Annual cost of energy used for cg=0.21 PLN, ce=0.85 PLN
Chart no. 9 - Annual cost of energy used for cg=0.35 PLN, ce=2.3 PLN
Based on the combined charts 8 and 9, it is easy to deduce that the most cost-effective option for the given conditions is to use a ventilation unit with a hygroscopic rotary heat exchanger at a face velocity of 1.2 m/s. In general, the larger ventilation units among the selected ones have the lowest energy consumption. Why is that? The answer to this question can be observed from charts 2 to 7, where the energy cost structure is presented. By analyzing charts 2, 3, and 4, it is evident that as the face velocity increases, the energy consumption cost for fan operation also increases, as a consequence of the rising internal resistance of individual units, with relatively constant costs for other components of the unit. Moreover, from the charts, it is clear how significant it is to combine humidification with a hygroscopic rotary heat exchanger, which allows for a considerable reduction in humidification costs. Furthermore, the humidification costs can be further reduced by using a gas-fired steam humidifier, provided that the cost of 1 kWh of gas energy is lower than the cost of 1 kWh of electricity. However, this is subject to price fluctuations.
Alternatively, adiabatic humidification is an option, but it is more complex and requires additional investment in a water treatment station, which needs to be taken into account. Additionally, adiabatic humidification will only be economically viable if secondary heating is done using a heater with a cost of 1 kWh energy lower than that of electricity.
The ventilation unit with a counter-flow heat exchanger, despite having the highest thermal efficiency for heat recovery, turned out to be the most costly in operation due to the lack of humidity recovery. If we were to consider only the heating energy and fan operation energy costs, this unit would have the lowest operating cost. However, we are also considering humidification in this analysis.
The above conclusions do not yet determine the most critical aspect, which is the cost-effectiveness of a given solution. To assess this, we still need the prices of the individual units, which are presented in the following chart:
Chart no. 10 - Net list prices of selected air handling units.
Comparing the energy consumption dependency with the catalog prices, it is evident that there is an inverse correlation – the higher the price of the ventilation unit due to its larger size, the lower the energy consumption. However, most investors primarily base their decisions on CAPEX (Capital Expenditure), so they would likely choose the cheapest ventilation unit with a non-hygroscopic rotary heat exchanger and a face velocity of 2.2 m/s. But is this the right approach? Let's find out by utilizing the Simple Pay Back Time (SPBT) indicator. The SPBT indicator will allow us to calculate, in a simplified manner, how many years it will take for the additional capital invested in a particular solution to pay off in terms of savings.
In our case, the SPBT will be calculated in relation to the most expensive variant of the ventilation unit within each group, which generates savings compared to the others.
(Note: The specific SPBT calculation and its results are not provided in the text, but it is indicated that it will be used to determine the payback period for the more expensive ventilation units compared to the others.)
By considering the SPBT indicator, we can determine if choosing the more expensive and energy-efficient ventilation units will result in cost savings over time, even though they have a higher initial investment cost. This approach allows investors to make more informed decisions and consider the long-term benefits of investing in energy-efficient solutions.
Where:
ΔCinw – Difference in purchase cost, PLN
ΔCen – Difference in operating cost, PLN/year
It is generally accepted that if the payback time for an additional capital investment for an energy-saving solution is less than 10 years, then the solution makes economic sense.
The results of SPBT calculations are presented in tables 3 and 4 below for different energy price variants
Table No. 3 - Simple payback time for cg=0.21 PLN, ce=0.85 PLN
Table No. 4 - Simple payback time for cg=0.35 PLN, ce=2.3 PLN
As observed from analyzing tables 3 and 4, in almost every case, enlarging the ventilation unit, except for the unit with a counter-flow heat exchanger and face velocity of 1.7 m/s, cg=0.21 PLN, ce=0.85 PLN, resulted in a payback period of less than 10 years for the additional capital investment. It is important to note that an increase in energy prices significantly affects the speed of the payback period, as shown in table 4. However, if such a sharp price increase were to occur, it is likely that the prices of the ventilation units themselves would also rise. Nevertheless, units with lower face velocities that are already operating efficiently would further reduce their own SPBT.
The aspect of ecology remains, which is determined by CO2 emissions as presented in the chart below:
Chart no. 11 - CO2 emissions of individual AHU variants:
Conclusions:
1. Focusing on CAPEX when choosing a device can often lead to poor investment decisions.
2. Proper selection of AHU components so that it is actually energy-saving is not a simple task without the right tools. This process can be greatly simplified by using the IX-CHART program.
3. The high efficiency of the heat recovery exchanger does not determine the energy efficiency of the ventilation/air-conditioning unit, and the issue should be approached more generally, taking into account all the components of the unit that consume energy and its prices.
4. The most financially advantageous, taking into account the purchase price and the benefits resulting from the reduction of operating costs, turned out to be the variant of the unit for the frontal speed of 1.2 m/s and with a hygroscopic rotary heat exchanger.
5. Also, from the point of view of ecology and CO2 emissions, the above variant is the most advantageous.
Summary:
The size of the ventilation unit does indeed matter, and it matters significantly. The analysis showed that for almost every case, the payback period (SPBT) for the additional investment in larger, but more energy-efficient units, was below 10 years. This implies that, considering the current high energy prices, it is worth investing in larger and more energy-efficient devices. With energy prices not expected to significantly decrease and a potential for further increases, such analyses can be a valuable part of a project, for which investors may be willing to pay extra.
However, it is crucial to consider that each project has its own specific parameters, from the climate zone to heat recovery efficiencies, work schedules, and many others. These parameters can influence the choice of a more favorable solution, so the conclusions from this analysis should not be generalized. Instead, they serve as an encouragement for independent exploration and analysis, for example, using the IX-CHART program. In the coming years, there will be significant opportunities for HVAC designers to excel in their field.
References: [1] - Technical materials from Frapol company
This article was published in the April 2023 issue of the magazine "Chłodnictwo&Klimatyzacja."
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