COMPUTER SIMULATION OF VARIOUS HEAT RECOVERY SYSTEMS IN A ROOFTOP UNIT AND ANALYSIS OF THEIR PERFORMANCE
Belén Llopis-Mengual, Lucas Álvarez-Piñeiro, Antonio Cazorla-Marín and Emilio Navarro Peris
Instituto Universitario de Investigación de Ingeniería Energética (IUIIE), Universitat Politècnica de València (UPV), Camino de Vera s/n, 46022, Valencia, Spain
Belén Llopis-Mengual [email protected]
Abstract: Heat recovery in rooftop units can generate significant energy savings, leading to an improvement in the overall performance of the system. These systems are mainly based on heat recovery from the exhaust air and incorporate other elements to utilize its energy. In this paper, we model with Modelica and IMST-ART several recovery systems to simulate their operation integrated in a rooftop unit with a reversible heat pump operating in both cooling and heating mode. With the generated model, we are able to analyze how the system operates. We compare the performance of the rooftop unit with each of the modeled heat recovery systems with the Seasonal Energy Efficiency Ratio (SEER) and the Seasonal Coefficient of Performance (SCOP) calculated according to the standard EN 14825. In conclusion, the heat recovery system that presents the best performance is a combination of a rotary wheel heat exchanger together with thermodynamic heat recovery.
Keywords: rooftop unit, heat recovery, seasonal energy efficiency ratio, heat pump
1. INTRODUCTION
Currently, in Europe, 36 % of CO2 emissions, including indirect emissions, come from buil- dings [1]. Therefore, reducing HVAC energy consumption in buildings is a good strategy to reduce them. To achieve this, it can be helpful to incorporate heat recovery systems in air handling units to make them more efficient. The most widely used and established heat re- covery systems on the market are rotary wheel and fixed-plate [2]. However, other recovery systems can be used in this type of unit with less cost, based on the use of exhaust air.
For this purpose, it is important to have models of these units to develop new heat recovery systems and evaluate and compare them. Modelica [3] is an object-oriented modeling lan- guage for systems helpful for this work. There are libraries dedicated to simulating building energy systems with this modeling language. For example, IDEAS [4] is an open-source li- brary with models of buildings and some HVAC systems. AixLib [5] is another open-source library that models HVAC systems such as air handling units or heat pumps.
These Modelica libraries do not incorporate detailed models of heat exchangers from their geometry, nor many recovery systems that can be adopted in an air handling unit.
In this work, we develop a rooftop air handling unit model that combines Modelica with the complete heat pump model obtained with IMST-ART [6]. In addition, in this model, various heat recovery systems are added to compare the performance of the rooftop in the same model.
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2. METHODOLOGY
This work is based on the modeling of a rooftop unit (packaged air handling unit designed for outdoor use), which includes a reversible heat pump for both cooling and heating.
We have designed the heat pump in IMST-ART software, which is used to model and simulate vapor compression cycles [6]. We have developed each of the submodels of the heat pump components with this software. This heat pump has two uneven tandem scroll compressors, which modulates the capacity according to the demand. These compressor submodels are incorporated in IMST-ART with the AHRI polynomials of mass flow rate and compressor input power [7]. We have modeled the evaporator and condenser with the corresponding geome- trical data of the tube and fin heat exchangers.
Once we have the heat pump model, we incorporate it into the Modelica-based static model of the rooftop unit in OpenModelica, an open-source environment [8].
We have modeled the different air flows in the rooftop unit, as shown in Figure 1. In a normal operating mode without heat recovery, Outside Air (OA) passes through the outside coil (con- denser in cooling and evaporator in heating) and is exhausted (Exhaust Air, EA). The Return Air (RA) from the zone is recirculated mixed with part of the Outside Air to renew it, and part is exhausted. This mixture is the one that passes through the internal coil (evaporator in coo- ling and condenser in heating) and constitutes the Supply Air (SA).
Figure 1. Schematic diagram of the rooftop model studied.
In addition to the heat pump’s performance and the air flows, this rooftop unit model inte- grates into Modelica the characteristic curves of pressure and power input for the supply, return, and external fans, and pressure drop in filters and sections of the unit. With the entire model, we can then incorporate any modifications that allow us to simulate heat recovery systems with the rooftop operating at any conditions.
In this work, we have considered a rooftop unit with a nominal cooling capacity of 50 kW in all our models. The airflow rates are a supply and return air of 8000 m3/h and an outside and exhaust air of 20000 m3/h.
2.1. Thermodynamic heat recovery
The thermodynamic heat recovery uses the heat contained in the return air to mix with the outside air before passing through the external coil (see Figure 2). Thus, it improves the heat pump’s performance by decreasing the condensing temperature in cooling mode and increa- sing the evaporating temperature in heating mode. We have considered 70% of the return air recirculation in this work, and the remaining 30% is mixed with the outside air before passing through the external coil.
Figure 2. Thermodynamic heat recovery in the rooftop unit
2.2. Heat recovery wheel/fixed plate heat exchanger
These recovery systems are already established in the rooftop market and are generally used. The most commons are the heat recovery wheel and the fixed plate heat exchanger.
We also include them in our model to compare their performance with other heat recovery systems. For this purpose, we define them based on their efficiency (ε):
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In addition to the heat pump's performance and the air flows, this rooftop unit model integrates into Modelica the characteristic curves of pressure and power input for the supply, return, and external fans, and pressure drop in filters and sections of the unit. With the entire model, we can then incorporate any modifications that allow us to simulate heat recovery systems with the rooftop operating at any conditions.
In this work, we have considered a rooftop unit with a nominal cooling capacity of 50 kW in all our models.
The airflow rates are a supply and return air of 8000 m3/h and an outside and exhaust air of 20000 m3/h.
2.1. Thermodynamic heat recovery
The thermodynamic heat recovery uses the heat contained in the return air to mix with the outside air before passing through the external coil (see Figure 2). Thus, it improves the heat pump's performance by decreasing the condensing temperature in cooling mode and increasing the evaporating temperature in heating mode. We have considered 70% of the return air recirculation in this work, and the remaining 30% is mixed with the outside air before passing through the external coil.
Figure 2. Thermodynamic heat recovery in the rooftop unit
2.2. Heat recovery wheel/fixed plate heat exchanger
These recovery systems are already established in the rooftop market and are generally used. The most commons are the heat recovery wheel and the fixed plate heat exchanger. We also include them in our model to compare their performance with other heat recovery systems. For this purpose, we define them based on their efficiency (ε):
1
Where Q is the recovered heat, Qmax is the maximum heat that could be recovered, m the mass amount of air, Cp the specific heat capacity, Tr1, Tr2 the inlet and outlet from the return air and Te1, Te2 the inlet and outlet from the outside air. In this work, when we use this heat recovery system in our simulations, we consider that ε=0.45.
In addition, we have added to the model the possibility of incorporating thermodynamic recovery along with conventional. Hence, the return air can be used after passing through the recuperator, mixing it with the outside air, and improving the air conditions at the inlet of the outer coil. Figure 3 shows the air flows of the rooftop with this type of heat recovery.
(1)
Where Q is the recovered heat, Qmax is the maximum heat that could be recovered, m the mass amount of air, Cp the specific heat capacity, Tr1, Tr2 the inlet and outlet from the return air and Te1, Te2 the inlet and outlet from the outside air. In this work, when we use this heat recovery system in our simulations, we consider that ε=0.45. In addition, we have added to the model the possibility of incorporating thermodynamic recovery along with conventional. Hence, the return air can be used after passing through the recuperator, mixing it with the outside air, and improving the air conditions at the inlet of the outer coil. Figure 3 shows the air flows of the rooftop with this type of heat recovery.
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Figure 3. Schematic diagram of the heat recovery wheel or fixed plate model alone (left) and combined with thermodynamic heat recovery (right).
2.3. Heat recovery with secondary heat pump
Figure 4. Heat recovery with a secondary heat pump.
This type of recovery consists of incorporating another smaller secondary heat pump in the rooftop unit. We can add it to our Modelica model after designing the heat pump in IMST-ART.
The heat exchangers of the secondary heat pump can be seen in Figure 4. One heat exchan- ger is in series with the indoor coil (evaporator in cooling and condenser in heating). The other heat exchanger uses the energy from the exhaust air and operates as the condenser in cooling and evaporator in heating. The disadvantage of this type of recovery is that, apart from recovering heat, it also increases consumption as it incorporates another compressor.
2.4. Hybrid heat recovery
In this heat recovery, we incorporate an additional heat exchanger in series in the heat pump.
Thus, depending on the mode, this series condenser/evaporator can achieve higher cooling/
heating capacity with an additional subcooling/superheat. To provide stability, we add to
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the model a liquid receiver between the two heat exchangers, therefore ensuring saturation conditions between the two heat exchangers.
This extra coil is added in the return air and is not exhausted. Furthermore, after passing through this heat exchanger, it is recovered in the outer coil as thermodynamic heat recovery.
To model this type of recovery, IMST-ART does not allow the addition of two heat exchan- gers in series with an intermediate receiver. To incorporate it in Modelica, we use the expec- ted temperature difference (∆T) of refrigerant from the energy we estimate to be recovered.
With this, the specific heat capacity and the mass flow rate of refrigerant, we can compute the additional recovered capacity.
Figure 5 shows the two cases of hybrid recovery that we have considered, which are recove- ring before or after the recirculation of the return air.
Figure 5. Schematic of hybrid and thermodynamic heat recovery performed before (left) and after (right) recirculation.
2.5. Seasonal Energy Efficiency Ratio (SEER) and Seasonal Coefficient of Performance (SCOP)
Table 1. Partial load conditions for SEER and SCOP calculation [9]
Cooling Heating (warmer)
Point
Outside air dry bulb temperature
(ºC)
Inside air dry bulb (wet bulb) temperature
(ºC) Point
Outside air dry bulb (wet bulb) temperature
(ºC)
Inside air dry bulb (wet bulb) temperature (ºC)
A 35 27 (19)
B 30 27 (19) B 2 (1) 20 (15)
C 25 27 (19) C 7 (6) 20 (15)
D 20 27 (19) D 12 (11) 20 (15)
We use the Seasonal Energy Efficiency Ratio (SEER) and the Seasonal Coefficient of Perfor- mance (SCOP) according to the standard UNE-EN 14825 [9] to measure the heat pump’s per- formance. Using SEER and SCOP instead of EER and COP at a single point, respectively, gives us more information about its performance over an entire season. In the case of heating, we have considered the warmer climate zone for the SCOP calculation. Therefore, we simulate the model at the partial load conditions indicated by the standard (see Table 1) and calculate the EER or COP of each partial load condition as:
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an intermediate receiver. To incorporate it in Modelica, we use the expected temperature difference (∆T) of refrigerant from the energy we estimate to be recovered. With this, the specific heat capacity and the mass flow rate of refrigerant, we can compute the additional recovered capacity.
Figure 5 shows the two cases of hybrid recovery that we have considered, which are recovering before or after the recirculation of the return air.
Figure 5. Schematic of hybrid and thermodynamic heat recovery performed before (left) and after (right) recirculation.
2.5. Seasonal Energy Efficiency Ratio (SEER) and Seasonal Coefficient of Performance (SCOP) Table 1. Partial load conditions for SEER and SCOP calculation [9]
Cooling Heating (warmer)
Point
Outside air dry bulb temperature
(ºC)
Inside air dry bulb (wet bulb)
temperature (ºC) Point
Outside air dry bulb (wet bulb) temperature (ºC)
Inside air dry bulb (wet bulb) temperature (ºC)
A 35 27 (19)
B 30 27 (19) B 2 (1) 20 (15)
C 25 27 (19) C 7 (6) 20 (15)
D 20 27 (19) D 12 (11) 20 (15)
We use the Seasonal Energy Efficiency Ratio (SEER) and the Seasonal Coefficient of Performance (SCOP) according to the standard UNE-EN 14825 [9] to measure the heat pump's performance. Using SEER and SCOP instead of EER and COP at a single point, respectively, gives us more information about its performance over an entire season. In the case of heating, we have considered the warmer climate zone for the SCOP calculation.
Therefore, we simulate the model at the partial load conditions indicated by the standard (see Table 1) and calculate the EER or COP of each partial load condition as:
,
2
With Qcooling and Qheating the cooling and heating capacity of the rooftop unit, Wcomp the compressor power input and Wfans the total consumption of all the fans
(2)
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With Qcooling and Qheating the cooling and heating capacity of the rooftop unit, Wcomp the com- pressor power input and Wfans the total consumption of all the fans
3. RESULTS
We have simulated for each heat recovery system described all the points required by the standard to calculate the SEER and SCOP and compare the performance of each system. Fi- gure 6 shows the results for SEER and Figure 7 for SCOP.
Figure 6. Results of SEER of the considered heat recovery systems.
Figure 7. Results of SCOP of the considered recovery systems.
4. DISCUSSION
The results show that conventional recovery systems (recovery wheel or fixed plate) have higher SEER and SCOP, as expected, since we use a recovery system with an already defined efficiency. Furthermore, we observe that both SEER and SCOP increase when adding ther- modynamic recovery to the conventional one (rotary + thermodynamic). The disadvantage of using this system is the additional cost to the unit due to the recovery modules that must be integrated into it. Analyzing the results of the other recovery systems in our model, we observe that the thermodynamic heat recovery means an improvement in the SEER in com- parison to not having recovery and an even more significant improvement in the SCOP. It is an excellent alternative to conventional recovery, especially in the case of heating, since it does not require added elements, only airflow recirculation systems through the external coil. We can observe that hybrid recovery (which also includes the thermodynamic) has very similar results to only using thermodynamic recovery. Our model does not show that adding an exchanger in series can improve the heat pump’s performance. We should further inves- tigate this, since the heat exchanger model used in this recovery system is not modeled in detail. Regarding where to place the hybrid recovery, the results show that SEER improves before recirculation, but for the SCOP, it is better after recirculation. It should be considered which mode is more convenient to achieve the best performance. Finally, the recovery sys- tem using a secondary heat pump does not practically improve SEER and SCOP, as the heat that is being recovered does not compensate for the consumption of another compressor.
We can use the model that we have generated to optimize heat recovery systems that do not use additional components that involve a high cost as the rotating wheel. Thus, as in this study we have used the same airflows and heat pump design in all cases, we can compute with our model the optimal combination of airflows and heat pump design that can improve the performance of each heat recovery system.
5. CONCLUSIONS
We have developed in this work a model combining Modelica and IMST-ART that simulates the operation of a rooftop unit with all its elements: the complete design of the heat pump, fans, filters, and airflows. With this tool, we have introduced modifications to model various heat recovery systems and calculate the SEER and SCOP achieved. The conventional recovery system with rotary wheel or fixed plates generates better performance results than other recovery systems, but with other methods such as thermodynamic heat recovery, the perfor- mance is further improved. Our model allows future research such as analyzing which heat pump design or airflow rates optimize the SEER and SCOP of each heat recovery system, as well as enhanced models of the studied systems and adding new types of recovery.
ACKNOWLEDGMENTS
Belén Llopis-Mengual acknowledges the Spanish “Ministerio de Universidades” through the
“Formación de Profesorado Universitario” program ref. FPU 19/04012. This publication has been carried out in the framework of the project “DECARBONIZACIÓN DE EDIFICIOS E INDUS- TRIAS CON SISTEMAS HÍBRIDOS DE BOMBA DE CALOR”, funded by the Spanish “Ministerio de Ciencia e Innovación (MCIIN)” with code number PID2020-115665RB-I00.
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