# Cost Optimal Renewable Electricity-Based HVAC System: Application of Air to Water or Water to Water Heat Pump

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Method

#### 2.1. Energy

_{s}= E

_{hp}+ E

_{aux}

_{hp}= E

_{hp,h}+ E

_{hp,c}

_{hp,h}= Q

_{hp,h}/COP

_{hp,c}= Q

_{hp,c/}EER

_{100%}

_{100%}

_{s,s2}/E

_{s,s1}

_{nren}), which is evaluated using the structure of electric energy use for Croatia [21] and Eurostat data [22], the non-renewable part of consumed electricity linked to carbon dioxide emissions can be estimated at approximately 50% (f

_{nren}= 0.5) and can be calculated as:

_{s,nren}= E

_{s}∙ f

_{nren}

_{PV}> E

_{s,nren}

_{s,PV}= E

_{s}−E

_{PV}

_{s,}

_{PV}/A

#### 2.2. Costs

_{I}is the initial investment cost for HVAC and PV system, C

_{a}is the annual operating cost multiplied by R

_{d}which is the average discount factor calculated for each year of the evaluation period, and V

_{f,τ}is the average residual value at the end of the evaluation period and it can be included into calculation if it exists. It is not included in present analysis, as the lifetime of HVAC and PV system is assumed to be equal for the considered period. This calculation procedure is repeated for each year of calculation period. The discount factor is calculated as:

_{d}(p) = (1/(1 + r/100))

^{p}

_{g}/A

## 3. Case Study

#### 3.1. Building

^{2}. This three-story building is used as a nursery. The attic is unheated and ventilated. The walls are massive, built from 75 cm thick stone. Carpentry is wooden with a single float glass. The main characteristics of the building are presented in Table 1.

^{2}) and for cooling the building, 31,301 kWh (45.4 kWh/m

^{2}).

^{2}K. The new carpentry will be double-winged with wooden frame and thermal insulated glass (U

_{w}1.4 W/m

^{2}K).

^{2}), and the energy for cooling is 31,792 kWh (46.1 kWh/m

^{2}). Design loads for heating and cooling are determined for the building in the state after implementation of energy efficiency measures. Design load for building heating of 85 kW is calculated according to the methodology proposed in EN 12831 [28]. Design load for building cooling of 65 kW is acquired by performing the calculation procedure from VDI 2078 [29]. From Figure 3 it is obvious that partial loads prevail.

#### 3.2. Building Energy Systems

#### 3.2.1. HVAC System with WWHP

^{3}. Variable speed pumps are provided for circulation of water in heat source and sink circuits of heat pumps. Heat pump WWHP-1 is provided with two circulation pumps, each with maximal electrical power 750 W. The maximal electrical power of two circulation pumps connected to heat pump HP-1 is 75 W each. The pumps speed is controlled to maintain design temperature difference during operation: 4 K towards heat exchanger HX-1 or HX-2, and 5 K towards the distribution system.

#### 3.2.2. HVAC System with AWHP

#### 3.2.3. Heat Pump Units

#### 3.2.4. PV System

^{2}which results in 0.6 kW of the nominal power. The panels are monocrystalline with a declared efficiency of 18%. The PV system was simulated using the Trnsys model Type562d. The electricity from PV is used to replace the non-renewable consumption of heating and cooling energy production systems, while the consumption for other systems in the building is not considered (e.g., lighting, fan coils, appliances…), so the surplus is considered to be sold to the grid. Given the current conditions for connecting a PV power plant to the grid in Croatia (so-called “net metering”), according to which electricity delivered to the grid is recognized to the prosumer at the same price as purchased if its production is less than or equal to the prosumer’s consumption, it follows that it is not necessary to install the accumulation of electricity, because the entire electric power system serves as an accumulation under the stated conditions.

#### 3.3. Costs

## 4. Results

^{2}.

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

A | area (m^{2}) |

C_{g} | global cost (€) |

CI | cost indicator (€/m^{2}) |

E | energy (kWh) |

EI | energy indicator (kWh/m^{2}) |

f | primary energy factor (-) |

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

X | energy ratio (-) |

aux | auxiliary |

AWHP | air to water heat pump |

c | cooling |

COP | coefficient of performance |

DHW | domestic hot water |

EEM | energy efficiency measure |

EER | energy efficiency ratio |

GWP | global warming potential |

h | heating |

H/C | heating and cooling |

hp | heat pump |

HVAC | heating, ventilation and air conditioning |

HX | heat exchanger |

nren | nonrenewable |

PLF | partial load factor |

PLR | partial load ratio |

PV | photovoltaic |

s | system |

SCOP | seasonal coefficient of performance |

SEER | seasonal energy efficiency ratio |

TRY | test referent year |

WWHP | water to water heat pump |

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**Figure 9.**Change of partial load factor PLF with partial load ratio PLR for operation of heat pump in heating.

**Figure 10.**Results of global cost and grid electricity consumption from simulations for AWHP and WWHP systems with PV collectors covering nonrenewable part of electricity for the case with neglected PLF (scenario 1).

**Figure 11.**Results of global cost and grid electricity consumption from simulations for AWHP and WWHP systems with PV collectors covering the non-renewable part of the electricity for cases with neglected PLF (scenario 1) and the manufacturer-provided PLF (scenario 2).

**Figure 12.**Seasonal efficiency indicators SCOP and SEER for AWHP and WWHP systems, calculated from simulation results for scenarios 1 and 2.

**Figure 13.**Comparison of seasonal efficiency indicators SCOP and SEER for AWHP and WWHP systems calculated from simulation results to values determined from Ref. [11].

Climate data | City | Poreč (Croatia) |

Longitude | 17°36′ E | |

Latitude | 45°13′ N | |

Dry bulb temperature-annual daily mean | 13.9 °C | |

Dry bulb temperature-daily mean minimum | −6.5 °C | |

Dry bulb temperature-daily mean maximum | 29.6 °C | |

Mean relative humidity | 74% | |

Global irradiation | 1428 kWh/m^{2} | |

Physics | Dimensions (length × width × height) | 24.4 m × 15.5 m × 20.5 m |

Conditioned area | 690 m^{2} | |

Conditioned volume | 5045 m^{3} | |

Envelope | External wall U-value | 1.4 W/m^{2}K |

Internal wall U-value | 1.5 W/m^{2}K | |

Floor on the ground U-value | 1.7 W/m^{2}K | |

Roof U-value | 3.1 W/m^{2}K | |

Ceiling towards the attic U-value | 1 W/m^{2}K | |

Window/door U-value | 3.2 W/m^{2}K | |

Ventilation | Infiltration/required ventilation rate | 0.48 h^{−1}/1.32 h^{−1} |

Mechanical ventilation | Not existing | |

Occupancy and operation | Occupancy | 5 days in week 6 A.M.–5 P.M. |

Number of persons | 120 | |

Internal heat gains | 6 W/m^{2} | |

Heating and cooling operation | Interrupted | |

Heating temperature set point | 22 °C | |

Cooling temperature set point | 24 °C | |

DHW set point | 45 °C |

Unit | Operation | Groundwater Temperature Regime | Water Temperature Regime | WWHP Temperature Regime |
---|---|---|---|---|

WWHP-1 | Heating | 15/11 °C (HX-1) | 13/9 °C (evaporator) | 40/45 °C (condenser) |

WWHP-1 | Cooling | 19/23 °C (HX-1) | 21/25 °C (condenser) | 12/7 °C (evaporator) |

WWHP-2 | Heating | 15/11 °C (HX-2) | evaporator: 13/9 °C | 45/50 °C (condenser) |

Regime | WWHP-1 | WWHP-2 | Flow Rate | Required Pressure Drop | Efficiency | Power Consumption |
---|---|---|---|---|---|---|

A | On | On | 6 L/s | 180 kPa | 50.5% | 2.137 kW |

B | On | Off | 4.5 L/s | 160 kPa | 45.9% | 1.568 kW |

C | Off | On | 1.5 L/s | 130 kPa | 22.2% | 0.879 kW |

**Table 4.**Investment cost, possibility for capacity control and provided PLF data for heat pump units considered in the analysis.

Manufacturer | Heat Pump Type | Refrigerant | Capacity Modulation | PLF Data Provided | Price for 2 Heat Pumps (H/C and DHW) |
---|---|---|---|---|---|

A | AWHP | R32 | Yes (12–100%, stepless) | Yes | 55.500 € |

B | AWHP | R410A | Yes (50, 100%) | No | 34.500 € |

C | AWHP | R410A | Yes (25, 50, 75, 100%) | Yes | 32.600 € |

A | WWHP | R410A | No (on-off) | No | 23.800 € |

B | WWHP | R410A | Yes (50, 100%) | No | 21.700 € |

C | WWHP | R410A | No (on-off) | No | 13.000 € |

D | WWHP | R410A | Yes, (20–100%, stepless) | Yes | 37.000 € |

System | Description | Price |
---|---|---|

WWHP | Groundwater heat source system (mechanical and geotechnical works) Auxiliary HVAC equipment and related installation works | 53.000 € |

AWHP | Auxiliary HVAC equipment and related installation works | 13.000 € |

PV | Procurement and construction of PV power plant | 800 €/kW |

Energy Source | Cost | Price | |
---|---|---|---|

Electricity | Day tariff | 0.083 | €/kWh |

Night tariff | 0.052 | €/kWh | |

Groundwater | 0.013 | €/m^{3} |

**Table 7.**Energy, cost and efficiency indicators and PV plant size (PV collectors covering nonrenewable electricity consumption).

System | Electricity from Grid,kWh/m ^{2} | Global Cost, €/m ^{2} | Investment, €/m ^{2} | Operating Cost, €/m ^{2} | Maintenance Cost, €/m ^{2} | PV Plant Size, kW |
---|---|---|---|---|---|---|

AWHP A (S1) | 27.0 | 213.5 | 139.0 | 27.8 | 46.7 | 20.4 |

AWHP B (S1) | 29.2 | 166.3 | 103.9 | 29.9 | 32.5 | 21.6 |

AWHP C (S1) | 30.7 | 165.2 | 102.7 | 31.3 | 31.2 | 23.4 |

WWHP A (S1) | 26.0 | 225.9 | 141.2 | 37.0 | 47.8 | 19.8 |

WWHP B (S1) | 24.0 | 218.4 | 136.1 | 35.9 | 46.3 | 18.6 |

WWHP C (S1) | 25.9 | 201.6 | 122.4 | 38.7 | 40.5 | 19.8 |

WWHP D (S1) | 24.0 | 256.2 | 162.7 | 36.8 | 56.7 | 18.6 |

AWHP A (S2) | 22.9 | 204.2 | 135.5 | 22.0 | 46.7 | 17.4 |

AWHP C (S2) | 28.8 | 159.3 | 100.6 | 27.5 | 31.2 | 21.6 |

WWHP D (S2) | 23.4 | 252.4 | 161.3 | 34.4 | 56.7 | 17.4 |

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**MDPI and ACS Style**

Delač, B.; Pavković, B.; Grozdek, M.; Bezić, L. Cost Optimal Renewable Electricity-Based HVAC System: Application of Air to Water or Water to Water Heat Pump. *Energies* **2022**, *15*, 1658.
https://doi.org/10.3390/en15051658

**AMA Style**

Delač B, Pavković B, Grozdek M, Bezić L. Cost Optimal Renewable Electricity-Based HVAC System: Application of Air to Water or Water to Water Heat Pump. *Energies*. 2022; 15(5):1658.
https://doi.org/10.3390/en15051658

**Chicago/Turabian Style**

Delač, Boris, Branimir Pavković, Marino Grozdek, and Luka Bezić. 2022. "Cost Optimal Renewable Electricity-Based HVAC System: Application of Air to Water or Water to Water Heat Pump" *Energies* 15, no. 5: 1658.
https://doi.org/10.3390/en15051658