# A Systematic Heat Recovery Approach for Designing Integrated Heating, Cooling, and Ventilation Systems for Greenhouses

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Case Study Description

^{2}four-span commercial greenhouse located in Saskatoon is selected as a case study [19]. In the original design, which is referred to as the reference case hereafter, the greenhouse heating is supplied by a natural gas (NG) boiler connected to a heating distribution circuit including a set of fan coil units (FCU loop). Furthermore, the cooling and dehumidification loads are met through air exchange by natural ventilation. In the present study, the feasibility of enhancing the reference climate control system’s performance by integrating ventilation waste heat is examined. The basic idea is to integrate the natural ventilation system with a mechanical ventilation system equipped with a heat recovery heat pump and heat storage. So, the proposed system is composed of (1) an air handling unit (AHU) to accommodate an efficient heat exchanger network to collect the ventilation waste heat, (2) borehole thermal energy storage (BTES) that acts as seasonal energy storage, allowing the waste heat to be stored during the warmer months and then used during the colder months, and finally, (3) a heat pump. Figure 1 shows the streams representing the suggested system annotated from 1 to 10 and the potential heat integration schemes in greenhouse heating and cooling modes. A detailed description of the streams can be found in our previous work [18] and is not reiterated here. Table 1 briefly presents the specifications of all streams in the system, including the source and target temperatures, as well as the heat capacity (CP = $\dot{m}$·c

_{p}) for each stream. Note that the streams associated with the water flow passing through the FCUs and the air passing over the coils (streams 9 and 10) are excluded from analysis since they are already matched via fan coil heat exchangers and cannot be integrated in any other way (see Figure 1). Similarly, the water/glycol mixture flow supplied to the boreholes (stream 8) is disregarded (this stream can only exchange heat with the ground through borehole heat exchangers). The returning flow from boreholes are represented by two non-coexisting streams depending on whether the boreholes are charging (stream 5) or discharging (stream 6) since the working temperature of the BTES is different in heating and cooling modes (see Table 1). Furthermore, “hot and cold utilities” are supplied by a natural gas boiler and natural ventilation, respectively.

## 3. Methodology

#### 3.1. HEN Design Strategy

#### 3.2. Equipment Sizing

#### 3.3. BTES Sizing

_{b}, R

_{ga}, and R

_{gst}are the ground thermal resistances per unit length. PLF denotes the part-load factor during the heating month; T

_{g}is the undisturbed ground temperature, which is 5 °C for the greenhouse location; Q

_{a}and Q

_{eva}represent the net annual average heat transfer to the ground and evaporator capacity, respectively. ${T}_{wi},{T}_{wo}$ are the inlet and outlet borehole temperatures at design conditions. Finally, T

_{p}represents the long-term ground temperature penalty caused by ground heat transfer imbalances. According to this equation, the heat imbalance during charging/discharging of the BTES can affect Q

_{a}and ${T}_{p},$ which result in a larger BTES. Therefore, direct charging of the BTES through ventilation heat may improve the thermal balance and allow a smaller BTES to be selected. It also substantially reduces the electricity consumption required for active charging of the BTES using the HP.

_{a}. Hence, given the HENs’ layouts and the size of all heat exchangers, dynamic modeling of the HEN is performed for all scenarios using the Simulink platform. Yearly analysis of HEN yields the total heat recovered to charge the BTES, allowing for the calculation of Q

_{a}. Then, the Q

_{evap}and ${PLF}_{m}$ are set based on the corresponding HP size and the BTES length is calculated. Note that Q

_{evap}is the averaged heat extracted from the ground, which is calculated by deducting the contribution of the air evaporator from the nominal evaporator capacity.

#### 3.4. Technoeconomic Analysis

#### 3.4.1. Economic Model

#### 3.4.2. Economic Performance Criteria

#### 3.4.3. Energy Saving Potential

## 4. Results

#### 4.1. Configuration of HEN Design Alternatives

#### 4.2. Specifications of the Designed HENs

#### 4.3. Yearly Performance Analysis of the Designed HENs

#### 4.4. Economic Analysis

#### 4.5. Selection of Optimal HEN Design

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

Abbreviations | |

AHU | air handling unit |

AHX | air-to-air heat exchanger |

BTES | borehole thermal energy storage |

CAD | Canadian dollar |

CCs | composite curves |

COP | coefficient of performance of heat pump |

EAHP | exhaust air heat pump |

FCU | fan coil unit |

FTHX | finned tube heat exchanger |

GHI | global horizontal irradiance (W/m^{2}) |

GSHP | ground source heat pump |

HEN | heat exchanger network |

HP | heat pump |

HX | heat exchanger |

HVAC | heating, cooling, and air conditioning |

NG | natural gas |

PA | pinch analysis |

PBP | payback period |

PESR | primary energy saving ratio |

PHX | plate heat exchanger |

TAC | total annualized cost |

TD | typical day |

Subscripts | |

air | airflow passing through AHU |

annul | annualized |

ce | cold stream outlet |

ci | cold stream inlet |

cond | condenser of heat pump |

comp | compressor of heat pump |

eva | evaporator of heat pump |

g | gas/or ground |

he | hot stream outlet |

hi | hot stream inlet |

l | liquid |

R | refrigerant |

ref | reference greenhouse |

w | water |

Superscripts | |

Design | designed capacity |

elec | electricity |

inv | investment |

Load | heating demand |

H | heating |

OP | operational |

Variables | |

A | total heat transfer area (m^{2}) |

${A}_{min}$ | min flow area (m^{2}) |

C | cost (USD) |

CP | heat capacity of streams (kW/K) |

c_{p} | specific heat capacity (kJ/kg·K) |

d | tube inside diameter (m) |

D | tube outside diameter (m) |

${D}_{h}$ | hydraulic diameter (m) |

E | electricity consumption (kWh) |

f | friction coefficient |

G | mass flux (kg/m^{2} s) |

h | convection heat transfer coefficient (W/m^{2} K) |

k | conduction heat transfer coefficient (W/m K) |

L | length (m) |

$\dot{m}$ | mass flow rate (kg/s) |

Nu | Nusselt number |

Pr | Prandtl number |

$\mathrm{Q}$ | thermal energy (kWh) |

$\dot{\mathrm{Q}}$ | thermal energy rate (kW) |

Re | Reynolds number |

t | plate thickness (m) |

T | temperature (°C) |

U | overall heat transfer coefficient (W/m^{2} K) |

$\dot{\mathrm{V}}$ | volumetric flow rate (m^{3}/s) |

$\dot{\mathrm{W}}$ | power consumption (kW) |

x | saturated vapor quality |

Greek letters | |

$\beta $ | chevron angle, degrees |

$\rho $ | density (kg/m^{3}) |

${\eta}_{is}$ | isentropic efficiency |

## Appendix A. Heat Exchanger Models

#### Appendix A.1. Plate Heat Exchangers (PHXs)

#### Appendix A.1.1. Single Phase

_{h}is the hydraulic diameter and k is the thermal conductivity.

#### Appendix A.1.2. Two-Phase Region

_{i}is the quality of refrigerant for the ith region [32].

#### Appendix A.2. Air-to-Air Heat Exchangers (AHXs)

#### Appendix A.3. Finned-Tube Heat Exchanger (FTHXs)

HX Type | Correlation | Ref. |
---|---|---|

PHX | $\u2206P=f\frac{l}{D}\frac{\rho {V}_{m}^{2}}{2}$ $f=[\frac{0.5\mathrm{cos}\left(\beta \right)}{(0.18\mathrm{tan}\left(\beta \right)+0.36\mathrm{sin}\left(\beta \right)+{f}_{0}/\mathrm{c}\mathrm{o}\mathrm{s}(\beta ){)}^{0.5}}+\frac{1-\mathrm{cos}\left(\beta \right)}{\sqrt{15.2{f}_{1}}}{]}^{-2}$ ${f}_{0}=64/Re$ ${f}_{1}=597/Re$ | [36] |

AHX | $\u2206P=f\frac{l}{D}\frac{\rho {V}_{m}^{2}}{2}$ $f=6.536R{e}^{-0.421}$ | [37] |

FTHX (over the tube bank) | $\u2206P=\frac{{G}^{2}}{2{\rho}_{g}}[f\frac{A}{{A}_{min}}\frac{{\rho}_{g}}{{\rho}_{l}}+(1+{\sigma}^{2}\left)\right(\frac{{\rho}_{g}}{{\rho}_{l}}-1\left)\right]$ $\sigma =\frac{minimum\text{}free\text{}flow}{frontal\text{}area}$ | [38] |

FTHX (inside the tubes) | $\u2206P=f\frac{l}{D}\frac{\rho {V}_{m}^{2}}{2}$ $f=\frac{0.064}{{Re}^{0.2}}$ |

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**Figure 1.**Schematic of heat integration schemes of the proposed climate conditioning systems for the greenhouse (red and blue colors are used for heating and cooling and gray arrows represent the streams).

**Figure 2.**Design methodology structure (the non-colored sections were discussed in our previous paper [18]).

**Figure 3.**Selected typical days, their associated weights, and corresponding typical climates; blue color shows the typical winter days, light blue shows the typical mild-cold climate (transition days), and red color denotes the typical warm days [18].

**Figure 4.**Composite curves and corresponding grid diagrams for different subnetworks during different times of the day (TD5); from top to bottom: t = 0 h, t = 13 h, and t = 16 h.

**Figure 5.**Variations of weather parameters and the required heating and ventilation rate during TD5.

No. | Stream Name | Description | Hot/Cold | Fluid | T_{s} (°C) | T_{t} (°C) | CP (kW/°C) |
---|---|---|---|---|---|---|---|

1 | HP_Cond | refrigerant flow passing through the condenser of HP | H | R152a | 78.7 | 45 | constant |

2 | HP_Eva | refrigerant flow passing through the evaporator of HP | C | R152a | −10 | −5 | constant |

3 | Supply air | airflow delivered to the greenhouse using AHU | H/C | air | T_{out} | T_{AHU} | dynamic |

4 | Exhaust air | airflow rejected from the greenhouse using AHU | H | air | T_{in} | T_{out} | dynamic |

5 | BTES_{ch} | BTES return flow during BTES charging (cooling mode) | C | water/glycol | 10 | 15 | dynamic |

6 | BTES_{d} | BTES return flow during BTES discharging (heating mode) | H | water/glycol | 0 | −5 | dynamic |

7 | Heating loop | FCU loop return flow | C | water | 30 | 40 | dynamic |

Equipment | Component | Correlation | Ref |
---|---|---|---|

HP | PHX | ${C}_{PHX}=805{A}^{0.74}$ | [23] |

Compressor | ${C}_{Comp}=\frac{71.7\dot{m}}{0.92-{\eta}_{is}}PrLn\left(Pr\right)$ | [24] | |

Expansion valve | ${C}_{valve}=114\dot{m}$ | [24] | |

AHU | FTHX | ${C}_{FTHX}=100{A}^{0.85}$ | [25] |

Fan | ${C}_{Fan}=1500{\left(\frac{\dot{m}}{10}\right)}^{0.36}$ | [26] | |

AHX | ${C}_{AHX}=231{A}^{0.639}$ | [23] | |

BTES | Borehole heat exchanger (drilling, installation, and pipe costs) | 43 CAD/m | [27] |

NG boiler | NG boiler | ${C}_{Boiler}=205$ ${Q}_{Boiler}^{0.87}$ | [28] |

**Table 3.**Specifications of the HENs designed for different typical days (Ref denotes the reference greenhouse).

No. of HEN | Ref | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|

HP heating capacity (kW) | - | 167.5 | 141.7 | 99.2 | 30.8 | 71.5 | 54 | 35.6 | 52.1 | 62.5 | 38.5 | 34 | 33.1 |

Boiler capacity (kW) | 167.5 | - | 25.8 | 68.3 | 136.7 | 96 | 113.5 | 131.9 | 115.4 | 105 | 129 | 133.5 | 134.4 |

BTES total length (m) | - | 4092 | 3671 | 2879 | 895 | 806 | 611 | 309 | 499 | 634 | 345 | 283 | 275 |

PHX (m^{2}) | - | 13.9 | 12.7 | 10.5 | 5.6 | 8.7 | 7.5 | 6 | 7.4 | 8.2 | 6.2 | 5.8 | 5.7 |

AHX (m^{2}) | - | - | - | - | - | 37 | 354 | 247 | 384 | 264 | 239 | 153 | 165 |

FTHX (m^{2}) | - | - | - | - | - | 54 | 110 | 274 | 107 | 132 | 410 | 569 | 588 |

Fans capacity (m^{3}/s) | - | - | - | - | - | 3 | 11.4 | 19.6 | 16 | 12.6 | 42.6 | 43 | 50 |

No. of HEN | Ref | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Total heating (MWh) | 481.4 | 481.4 | 481.4 | 481.4 | 481.4 | 475.9 | 469.6 | 472.4 | 468.6 | 469.1 | 460.8 | 457.7 | 457.6 |

Ventilation loss (kwh) | 37,834 | 37,834 | 37,834 | 37,834 | 37,834 | 32,298 | 25,988 | 28,814 | 25,009 | 25,538 | 17,231 | 14,121 | 14,000 |

Forced/natural ventilation ratio | 0 | 0 | 0 | 0 | 0 | 0.17 | 0.53 | 0.72 | 0.65 | 0.57 | 0.92 | 0.92 | 0.95 |

HP heating ratio | 0 | 1 | 0.99 | 0.94 | 0.5 | 0.82 | 0.72 | 0.57 | 0.71 | 0.78 | 0.56 | 0.54 | 0.54 |

No. of HEN | 4 | 5 | 6 | 8 | 9 | 10 |
---|---|---|---|---|---|---|

${PESR}^{HEN}$ | 0.326 | 0.573 | 0.48 | 0.466 | 0.509 | 0.41 |

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## Share and Cite

**MDPI and ACS Style**

Ghaderi, M.; Reddick, C.; Sorin, M.
A Systematic Heat Recovery Approach for Designing Integrated Heating, Cooling, and Ventilation Systems for Greenhouses. *Energies* **2023**, *16*, 5493.
https://doi.org/10.3390/en16145493

**AMA Style**

Ghaderi M, Reddick C, Sorin M.
A Systematic Heat Recovery Approach for Designing Integrated Heating, Cooling, and Ventilation Systems for Greenhouses. *Energies*. 2023; 16(14):5493.
https://doi.org/10.3390/en16145493

**Chicago/Turabian Style**

Ghaderi, Mohsen, Christopher Reddick, and Mikhail Sorin.
2023. "A Systematic Heat Recovery Approach for Designing Integrated Heating, Cooling, and Ventilation Systems for Greenhouses" *Energies* 16, no. 14: 5493.
https://doi.org/10.3390/en16145493