*5.4. Calculation Results, Interpretation*

Results of the calculations of the annual demands for primary energy for the representative HVAC systems shown in Table 4 are presented in Table 5.




A percentage comparison of the unit annual demand for the primary energy of variants of the HVAC system for various external climates and types of heat recovery, compared to variants without heat recovery, are shown in Figures 11–14. The subject of the assessment is the impact of the type of heat recovery and the external climate—as decision variables—for the selection of the energy-optimal variant.

**Figure 12.** Annual relative demand for the primary energy of the variants of HVAC system ISO8 x2pq (external recirculation) for various external climates and types of heat recovery compared to the variants without heat recovery.

Based on the analysis of the calculation results, the following can be stated:

1. For HVAC systems without air recirculation x1 the optimal device for heat recovery is a rotary sorption regenerator (p = 2) and, then, an energy regenerator (p = 1) and a crossflow exchanger (p = 3 or p = 4). The obtained energy savings are here a function of climate—Figure 11 and Table 5. Using the rotary sorption regenerator in the analyzed HVAC system ISO8 x1 makes it possible to decrease the annual primary energy demand by 63%, 64% and 24% in relation to the system without heat recovery, respectively, for continental (q = 1), subarctic (q = 2) and subtropical (q = 3) climates. For the rotary energy regenerator, the values are lower and equal 33%, 35% and 5%, respectively. For the crossflow exchanger, the savings are significantly lower and equal 19 ÷ 27% for the continental climate, 5 ÷ 7% for the subarctic climate and 4% for the subtropical climate. Therefore, in the subtropical climate, the only rational device for heat recovery is the rotary sorption regenerator, and the savings effect is mainly achieved by drying air.

The representative percentages of the annual primary energy demand for thermodynamic air treatment for individual components and optimal variant ISO8 x12 (with a rotary sorption regenerator) are shown in Figure 15.

**Figure 15.** Percentages of the annual primary energy demand for thermodynamic air treatment for individual components and optimal variant ISO8 x12 (without recirculation, rotary sorption regenerator): (**a**) continental climate (q = 1), (**b**) subarctic climate (q = 2) and (**c**) subtropical climate (q = 3).

For the continental climate (q = 1) and subarctic climate (q = 2), the dominant is the percentage of the demand for air humidification—56.5% and 62.0%, respectively; then, for heating air—27.5% and 32%, respectively, and cooling—16.0% and 6.0%, respectively. While, for the subtropical climate (q = 3), the dominant is the percentage of cooling—63.8%, then heating at 35.2%, including 35% of reheating after drying and, marginally, humidification— 1%. The conclusions resulting from the results of the calculations of representative shares of the annual primary energy demand for thermodynamic air treatment correlate directly with the conclusions concerning the optimal type of heat recovery.


for heat recovery is, similar to the system without recirculation, a rotary sorption regenerator and, then, an energy regenerator and a crossflow exchanger. For the considered system ISO5 x3 energy savings related to a system without heat recovery, primary energy and using the sorption regenerator equal 63%, 66% and 23%, respectively, for the continental, subarctic and subtropical climates—Figure 13. Lower savings are obtained by using an energy regenerator: 33%, 35% and 5% or a crossflow exchanger: 19 ÷ 27%, 5 ÷ 7% and 4%, respectively, for the continental, subarctic and subtropical climates. The percentages of the annual primary energy demand for thermodynamic air treatment for individual components (heaters, cooler and steam humidifier) of optimal variant ISO5 x32 (with a sorption regenerator) and external climates are practically identical as for HVAC system x12 (Figure 15).

4. For HVAC systems with external and internal recirculation x4 (optimal for cleanrooms with high cooling loads qj and relatively low percentages of outdoor air αo), additionally using heat recovery is energetically justified only for the subarctic climate and concerns only the rotary sorption regenerator—Figure 14 and Table 5. Savings in the primary energy demand for the analyzed HVAC system ISO7 x42 (with a sorption regenerator) and the subarctic climate equal 11% related to a system without heat recovery.

It should be noted that, in the other analyzed use cases of devices for heat recovery, especially the crossflow exchanger, the energy effect was opposite to what was expected; the primary energy demand increased 1 ÷ 5%, because the heat or cold recovery was lower than the inputs for forcing through by heat recovery exchangers.

#### *5.5. Validation of the Calculation Results*

Validation of the calculation results with the existing energy simulation tools is possible under the following conditions:


In this article, a simulation model was developed for each HVAC system structure. In these models, for each hour of the comparative year TRY (est. Reference Year), the optimal course of thermodynamic air treatment was determined, and on this basis, the energy consumption was obtained—after summing (8760 h), the annual energy consumption. The available energy simulation programs are universal, but also limited, among others:


The validation of the calculation results in this article was carried out by taking into account the above-mentioned limitations and the available other tool for energy simulation the HAP (Hourly Analysis Program) program developed by the CARRIER company. It is a closed-source program.

The possible scope of the simulation included CAV systems (constant air volume) with heat recovery (excluding the option of a recuperator with an electric preheater before the recuperator—x14 and x24) with or without external recirculation (x1 and x2 in the article). The calculation results are presented in Table 6.


**Table 6.** Primary energy demand for the representative HVAC systems x∗ <sup>n</sup><sup>g</sup> calculation results according to the HAP program (Hourly Analysis Program).

\*/ Related to own simulation.

Taking into account the above-mentioned conditions and limitations, it can be concluded that the obtained results of the calculations are satisfactory, and the differences in the annual energy demand according to our own calculations and the HAP program, related to the values obtained in our own calculations, are acceptable. These differences range from −9.2% to +8.2% (minimal differences: −0.2% to +0.75%). The mean absolute percentage of the differences in the results of these calculations is 5.1%. Taking into account that the simulation models of the other systems included in the article (x3—with internal recirculation and x4—with internal and external recirculation) are a modification of the models for the validated systems x1 and x2, it can be assumed that the obtained calculation results are also acceptable.

#### **6. Conclusions**

This article presents the original results of research on the optimization of HVAC systems for cleanrooms. The HVAC systems were described by vectors with coordinates defined by constant parameters and decision variables. Then, the authors defined, based on limitations, a set of acceptable variants covering the following structures of HVAC system: x1—without recirculation, x2—with external recirculation, x3—with internal recirculation and x4—with external and internal recirculation.

In the next stage, based on the optimization algorithm, the authors defined a set of energy-optimal structures of the HVAC system for cleanrooms as a function of key constant parameters and wide representative variability ranges of these parameters: cleanliness classes Cs—ISO5, ISO7 and ISO8; u nit cooling loads qj = (100 ÷ 500) W/m2 and percentage of outdoor air α<sup>o</sup> = (5 ÷ 100)%.

The original achievement of the research, which constitutes a new cognitive quality, is the development of relations approximating x∗ <sup>n</sup><sup>g</sup> = f(CS, αo, qj) defining the zones of energyoptimal structures of cleanroom HVAC systems; the equations derived the boundary lines separating these zones.

It was proven that HVAC systems with external recirculation (x2) are optimal structures for rooms with high cooling loads qj and low requirements concerning keeping the cleanliness class, HVAC systems with internal recirculation (x3) are optimal for rooms with low cooling loads qj and relatively high percentages of outdoor air αo, while HVAC systems with external and internal recirculation (x4) are optimal structures for rooms with high cooling loads qj and relatively low percentages of outdoor air αo.

The obtained results, due to the used wide ranges of variability of key constant parameters, are general in nature and have great application value.

An important result of the research was defining energy-optimal control algorithms and the type of heat recovery as an element of optimal structures of the HVAC system. At this stage, the equations of the boundary lines between the zones of optimal thermodynamic air treatment were determined, which is of great application importance.

In the optimization procedure based on simulation models, the objective function was defined as the minimum unit annual primary energy demand for thermodynamic air treatment of the HVAC system (Ep(x∗ ng = min). The algorithms take into account the energy demand for forcing through by heat recovery exchangers.

Summarizing the results of the analyses and calculations concerning the energetic profitability of using heat recovery in optimal structures of HVAC systems for cleanrooms, it can be stated that:


**Author Contributions:** Conceptualization, M.P.; methodology, M.P.; software, M.J.; validation, M.P. and M.J.; formal analysis, M.P. and M.J.; investigation, M.P. and M.J.; resources, M.P. and M.J.; data curation, M.P.; writing—original draft preparation, M.P.; writing—review and editing, M.J.; visualization, M.P.; supervision, M.P.; project administration, M.P. and funding acquisition, M.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Polish Ministry of Education and Science, grant number 504101/0713/SBAD/0948.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**


