# Techno-Economic Analysis of a Heat Pump Cycle Including a Three-Media Refrigerant/Phase Change Material/Water Heat Exchanger in the Hot Superheated Section for Efficient Domestic Hot Water Generation

^{1}

^{2}

^{3}

^{4}

^{5}

^{*}

## Abstract

**:**

## 1. Introduction

_{2}in our atmosphere. In the United States, for example, the share of HP sales for newly constructed buildings exceeds 40% for single-family dwellings, and is nearly 50% for new multi-family buildings. In addition, the EU market is expanding quickly, with 12% annual average growth since 2015. France, Italy and Spain are responsible for half of all sales in the European Union, while Sweden, Estonia, Finland and Norway have the highest penetration rates, with more than 25 HPs sold per 1000 households each year [1].

_{el}were found in this study. However, due to these simplified calculation methods used in [15], several aspects could not be addressed properly:

- the possibility to pre-heat the process water with the HP’s condenser during energy efficient DHW generation (compared with operating mode (c) in Section 2.1 and Appendix A);
- the limitations in the storage capacity of the RPW-HEX and the DHW storage devices;
- the heat losses to the surrounding of the RPW-HEX and the decentralized DHW storage devices;
- the solar radiation, ventilation rates and many other constraints of the building;
- the control strategy.

## 2. Case Studies and Operation Modes of the System

#### 2.1. Operation Modes of the HP with Integrated RPW-HEX

- (a)
- heating operation and charging the RPW-HEX (0 < SoC↑ ≤ 1)
- (b)
- cooling operation and charging the RPW-HEX (0 < SoC↑ ≤ 1)
- (c)
- energy efficient DHW generation by discharging the RPW-HEX and pre-heating via the condenser (0 ≤ SoC↓ ≤ 1)
- (d)
- conventional (inefficient) direct DHW generation (SoC = 0)
- (e)
- heating operation when the RPW-HEX is fully charged (SoC = 1)
- (f)
- cooling operation when the RPW-HEX is fully charged (SoC = 1)

#### 2.2. Control Strategy

#### 2.3. Case Studies

- Case #1: A passive house located in Helsinki with a “low-temperature heating” distribution system and a PCM with a phase transition at 64 °C
- Case #2: A low energy building located in Strasbourg with an “intermediate-temperature heating” distribution system and a PCM with a phase transition at 64 °C
- Case #3: A refurbished building located in Athens with an “intermediate-temperature heating” system and a PCM with a phase transition at 64 °C

- The ground floor of the standard apartment is a square with 75 m
^{2}and the room height is 3 m. - Two outer walls are considered which are oriented to the south and the west. The U-values for the walls are 0.09, 0.11, and 3 Wm
^{−2}K^{−1}for the buildings located in Helsinki, Strasbourg, and Athens, respectively - The wall (south and west) to window ratio is 20% and the U-values for the windows are 0.75, 0.9, and 5 Wm
^{−2}K^{−1}for the buildings located in Helsinki, Strasbourg, and Athens, respectively. - If cooling is needed, the windows are shaded with a solar radiation transmittance of 15%.
- The ventilation rate is 0.8 m
^{3}m^{−2}h^{−1} - The air heat recovery efficiency is 75% for the passive house located in Helsinki
- The heat gains from lights and equipment are 5 Wm
^{−2} - The DHW consumption of each full-scale apartment was 5.845 kWh, which is comparable to a medium water consumption as defined in [18].
- The apartments were scaled to multiples of 1/4 of the full-scale. The DHW storage devices were considered in full-scale for each apartment.
- Fresh water for the decentralized DHW storage devices is provided at 12 °C.

## 3. Methodology

#### 3.1. Simulation Models

_{h}, coefficient of performance related to the hot side of the HP for DHW: COP

_{DHW}and RPW-HEX utilization factor: ε

_{RPW}. The latter is the ratio of energy transferred to the RPW-HEX compared to the total energy on the hot side of the HP. The simulations were carried out in the Dymola/Modelica modelling environment using ThermoCycle library components [21]. Additionally, models for the RPW-HEX, the outdoor unit and the four-way valve were developed in-house [22,23]. Thermodynamic properties were taken from the CoolProp library [24]. A detailed description of the system model can also be found in [13,14]. The performance indicators were derived from the dynamic simulation when the system is in steady-state or when the SoC reached 50% for heating and cooling, respectively. Defrosting operation was neglected, and electric power consumption for COP calculations solely reflects the consumption of the compressor and the fan of the outdoor unit. All HP geometry/component design parameters, efficiencies and heat transfer coefficients were taken from design sheets or calculated from well-established equations and were later experimentally validated with measurements of the prototype air source HP used in the H2020 project HYBUILD [16] without RPW-HEX. Measured COPs of the HP without RPW-HEX for heating were 3.0, 4.3, 5.3 and 7.1 for ambient temperatures of −7, 2, 7, and 12 °C, inlet water temperatures to the heating system of 43, 37, 33, and 28 °C, and part load ratios of 88%, 54%, 35% and 15%, respectively. More information about the HP performance can be found in [15].

#### 3.2. Annual Energy Efficiency Calculations

#### 3.3. Parameter Variations

- eco-mode: charging starts if hot water (in a perfect thermocline) is below 40 L and stops at 90 L
- standard-mode: DHW generation is initiated if hot water is below 55 L and stops at 105 L
- comfort-mode: charging starts if hot water is below 70 L and stops at 120 L

#### 3.4. Economic Performance Indicators

_{RPW,max}is the maximum storage capacity of the RPW-HEX. The fixed and variable costs were estimated after manufacturing a first version of the RPW-HEX for [25]. The following costs and properties were taken into account for a series production: 5.00 EUR/kg for PCM, 4.55 EUR/kg for aluminum, 6.00 EUR/m

^{2}for mineral wool (10 cm thickness), a latent phase change energy of the PCM of 250 kJ/kg, a specific heat capacity of 2.00 kJkg

^{−1}K

^{−1}for the PCM and 0.900 kJkg

^{−1}K

^{−1}for aluminum, a PCM/aluminum ratio of 1:2.44 and a temperature difference of 10 K between the charging and discharging of the RPW-HEX during operation. The so-calculated fixed and variable costs for (1) are summarized in Table 2. Please note that the costs of the insulation do not correlate linearly with the storage capacity as the surface does not linear dependent on the volume. Nevertheless, for the sake of simplicity and the comparatively small share of the insulation material to the total costs, a linear dependency was also used for the costs of the insulation material.

_{payback}was calculated as the time when the net present value becomes equal to the investment costs:

_{el}${\tilde{c}}_{\mathrm{el}}$ was assumed to be 0.22 EUR/kWh at the time of the investment (average energy price per kWh for households in the Euro area in 2018) with an increase of 0.006 EUR/year [26]. The discount rate i per time period tp was correlated with a fixed rate of 2 %/year and N is the number of periods at the payback time. Please note that waste disposal costs and profits from recycling were not considered. Using a standard Newton-solver, the payback time t

_{payback}was calculated numerically from (2).

_{el}(${\tilde{c}}_{\mathrm{el}}$) in the year of operation t:

_{profit}after t

_{lifetime}years follows from:

## 4. Results and Discussion

_{el}/year and the minimum cost savings are 137 EUR/year (calculated with the electric energy price in the first year of operation). Generally, it can be observed that the annual energy saving per year increases with RPW-HEX storage size until 5–6 kWh.

_{el}/year or at least 122 EUR/year, respectively. For small DHW storage devices (140 L) operating in standard- or comfort-mode, the payback time is always more than 20 years.

_{DHWc}in Figure 6). This contribution from the condenser is highly appreciated from an energetic point of view. If the water return temperature increases and the condensing temperature becomes higher in operating mode (c) than it is during heating, the controller turns the HP off and solely the energy discharged from the RPW-HEX is used for DHW generation. This control rule was defined because it is better to discharge the RPW-HEX (which was previously charged with a “heating COP”) than to run the HP with a COP higher than the “heating COP”.

_{th}/day), whereas in Case #3, only 1.75 families (apartments) are supplied with domestic hot water (10.2 kWh

_{th}/day). Remember that the number of apartments was calculated considering that the HP (same design for all case studies and parameters) is able to provide the maximum heating or cooling demand of the building. Accordingly, the DHW consumption for each building results from the calculated number of apartments. In Case #3 (Figure 6b), the energy gained from heating operation (a) with charging the RPW-HEX is most of the time more than enough to cover the entire daily DHW demand during the winter season (which is depicted as a dotted line in Figure 6b). Therefore, the HP often switches to operating mode (e)—heating without charging the RPW-HEX—simply because the RPW-HEX is already fully charged and cannot be discharged to the likewise fully charged DHW storage devices.

## 5. Conclusions and Outlook

_{el}, and the highest profits after 20 years of 1083 EUR were found for Case #1, a passive house located in Helsinki with 7.75 standard apartments—operation in eco-mode with 8 large (280 L) DHW storage devices. It turns out that for buildings with medium and large DHW storage devices, the selection of the DHW charging mode (eco, standard or comfort) does not produce an as significant difference in the energetic and economic system performance as for buildings with small DHW storage devices. Thus, operation in the eco-(charging) mode is critical for small DHW storage devices. The results indicate further potential for significant improvements through the interaction with PV if feeding generated electric energy to the grid is less beneficial than storing the energy on site. Such systems rely on DHW storage with a comparatively high volume (high investment costs) in order to store PV generated thermal energy during day time in summer. The integration of an RPW-HEX would not only reduce the electric energy needed for DHW generation (and therefore PV size) but would also add an extra benefit to the installed DHW storage, making effective use of this storage in winter, i.e., in times of the year when the available storage capacity is only partially used by PV.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

Symbols | |

a,b,d,k_{1} | Constants of the moving boundary model for the DHW storage (W, -, J^{−1}, J) |

A_{DHW} | Ground area of the DHW storage (m^{2}) |

$\tilde{c}$ | Specific costs (EUR/kWh_{el}) |

c_{p,w} | Specific heat capacity of water (Jkg^{−1}K^{−1}) |

C | Costs (EUR) |

COP | Coefficient of Performance |

L | Height of the DHW storage (in the moving boundary model) (m) |

$\dot{m}$ | Mass flow rate (kg s^{−1}) |

Q | Thermal energy (J or kWh) |

$\dot{Q}$ | Heat flow rate (W) |

t | Time (years) |

w | Width of the heat transfer cross-section of the DHW storage |

W_{el} | Electric energy (kWh) |

ε_{RPW} | Ratio of thermal energy that can be transferred to the RPW-HEX |

ℓ | Length of the cold section in the moving boundary model of the DHW storage (m) |

ϑ | Temperature (°C) |

τ | Time span of the DHW charging process(s) |

Abbreviations | |

c | Cold |

con | Condenser |

DHW | Domestic Hot Water |

HP | Heat Pump |

msc | Minimum speed of compressor |

PCM | Phase Change Material |

RPW-HEX | Refrigerant-PCM-water heat exchanger |

RPW,P | Contribution of PCM in RPW-HEX |

RPW,R | Contribution of refrigerant in PCM |

REF | Reference system without RPW-HEX |

SoC | State of Charge |

sp | Set-point |

w | Water |

## Appendix A

_{sp}is the set-point temperature for heating which is also used for DHW generation in operating mode (c) to preheat the water at a high COP. The sensible energy of the hot gas transferred through the RPW-HEX can be calculated from the efficiency of the RPW-HEX

_{RPW}is the fraction of the total heat at the hot side of the HP that can be transferred to the RPW-HEX and has to be taken from experiments or simulations (cf. [15]). The heat transfer rate needed to be transferred from the PCM to the DHW ΔQ

_{RPW,P}is the heat that cannot be provided by the refrigerant of the HP. It can be calculated from (A1) and the energy extracted from the RPW-HEX after the time τ is:

_{sp}, the HP can operate at a high part load (since the volume flow rate is fixed) and therefore can provide a high share of thermal energy to the DHW charging process. If ${\vartheta}_{\mathrm{DHW}}^{\left(\mathrm{out}\right)}$ is close to the condensing temperature or even higher, the HP will turn off, and all the energy must be provided by the PCM. Hence, ${\vartheta}_{\mathrm{DHW}}^{\left(\mathrm{out}\right)}$ has a high impact to the RPW-HEX storage capacity needed for providing a certain amount of thermal energy for the DHW storages.

**Figure A1.**(

**a**) Scheme of the moving boundary approach. (

**b**) Fitting (A12) to experimental data of a charging process of a 140 L enerboxx

^{®®}storage (storage used in [28]) for different temperatures and volume flow rates. The solid lines in (

**b**) represent the moving boundary approach calculated from (A12), the dotted line represent a lumped parameter approach (not described) and the markers denote measured temperatures from the experiment.

_{h}), which is always heated to a temperature corresponding to the constant inlet temperature and a lower volume at cold temperature (ϑ

_{c}), whose temperature corresponds to the fresh water temperature. Both volumes are separated by a moving boundary ($\ell $) which depends directly on the stored energy:

_{DHW}is the ground area of the DHW storage, L is the height of the DHW storage, ρ

_{w}is the density and c

_{p,w}is the specific heat capacity of water. Note that with the aid of the three-way valve (component M in Figure 1), the inlet temperature to the DHW storage ${\vartheta}_{\mathrm{DHW}}^{\left(\mathrm{in}\right)}$ is held constant at a fixed temperature by mixing the process water leaving the RPW-HEX with the water leaving the condenser in the real machine. Hence it can be assumed as constant.

_{h}($\ell $ = 0) and the storage is empty if the entire storage has a temperature of ϑ

_{c}($\ell $ = L). Heating of the DHW storage only takes place in the cold region of the storage. Using an NTU description for a constant wall temperature (ϑ

_{c}) one finds for the heat transfer to the DHW storage:

_{1}can be determined by solving the initial value problem at t = 0:

_{nom}= 484 Wm

^{−2}K

^{−1}to measured data from a charging experiment with an enerboxx

^{®®}storage. Furthermore, the presented approach is compared to a lumped parameter approach, where a perfectly mixed water volume was assumed. The behavior of the outlet temperature in the region of interest (low temperatures indicated by the strong solid line of the moving boundary approach in Figure A1b) can be reproduced significantly better with the presented approach. Nevertheless, the moving boundary method will always underestimate the real temperature whereas the lumped parameter approach will always overestimate the real temperature.

_{sp}(outlet temperature of the water after the condenser) and secondly by the minimum heat transfer that is provided at the minimal rotational speed of the compressor ${\dot{Q}}_{\mathrm{con},\mathrm{msc}}$. The compressor turns off, once ${\vartheta}_{\mathrm{DHW},\mathrm{msc}}^{\left(\mathrm{out}\right)}$ at the minimum speed of the compressor is reached:

_{msc}from (A15) is negative as in the first case of (A16), the HP is always off because the outlet temperature of the DHW storage is already above ${\vartheta}_{\mathrm{DHW},\mathrm{msc}}^{\left(\mathrm{out}\right)}$ at the beginning (t = 0). Therefore no energy is transferred directly from the refrigerant to the process water (Q

_{con}= 0, Q

_{RPW,R}= 0) and all the energy for the storage has to be provided by the RPW-HEX. If the outlet temperature of the DHW storage is always below ${\vartheta}_{\mathrm{DHW},\mathrm{msc}}^{\left(\mathrm{out}\right)}$, 0 ≤ t < τ

_{msc}, as in the second case of (A16) the HP is turned on all the time and the compressor is controlled to provide always as much energy as needed to reach the set-point ϑ

_{sp}at the condenser water outlet. If the HP reaches its minimum power within the time of the operation 0 ≤ τ

_{msc}≤ t, the HP will be turned off at τ

_{msc}and the energy is then provided by the RPW-HEX, only.

_{DHW}needed to charge the DHW storages to a certain energy set-point ${Q}_{\mathrm{DHW}}^{\mathrm{sp}}$ (e.g., the upper limit) analytically:

_{DHW}from (A18) as t in (A17) gives finally the energy Q

_{RPW,P}that has to be extracted from the RPW-HEX to charge the DHW-storage to this certain set-point ${Q}_{\mathrm{DHW}}^{\mathrm{sp}}$.

_{sp}= 43 °C, ϑ

_{c}= 12 °C, ${\vartheta}_{\mathrm{DHW}}^{\left(\mathrm{in}\right)}$ = 60 °C, 210 L storage, standard-mode). Due to the increasing temperature of the water leaving the storage ${\vartheta}_{\mathrm{DHW},\mathrm{msc}}^{\left(\mathrm{out}\right)}$ (decreasing cf. Figure A1a), the heat transfer rate decreases with time (cf. Figure A2a). Additionally, due to the restriction of the fixed set-point for the HP (ϑ

_{sp}= 43 °C), the heat provided by the condenser has to decrease. Hence, the HP reaches the minimum operation speed of the compressor at τ

_{msc}and the compressor turns off. Until the DHW storage is fully charged at τ

_{DHW}, the remaining energy must be provided by the PCM. At the end of the charging process, about one third of the energy for providing DHW at 60 °C was taken from the HP operating with a heating COP and about two third of energy were taken from the energy stored in the RPW-HEX (which was also stored when the HP was operated in heating mode with a high COP (cf. Figure A2b).

**Figure A2.**(

**a**) Calculated heat transfer rate to the decentralized DHW storages during the energy efficient DHW charging operating mode (c) for a storage at the lower charging limit in standard operating mode at t = 0. The heat transfer rate consists of contributions from the refrigerant via the condenser (green) and the RPW-HEX (blue) and of the contribution from the PCM and the aluminum in the RPW-HEX (orange). (

**b**) shows the calculated total amount of energy transferred to the DHW storages during the charging process.

_{RPW}to extract a certain amount of energy from the RPW-HEX can be calculated numerically, e.g., with a Newton-solver, by inserting (A9) in (A17). Note that this is only possible if the energy stored in the RPW-HEX is lower than the maximum transferable energy from the RPW-HEX to the DHW storage which is limited by the DHW storage size or the defined maximum charging level of the DHW storage, respectively. If this is the case, the charging time can be calculated with (A18) and the remaining difference of energy remains in the RPW-HEX after the DHW-charging operation process.

## References

- IEA. Heat Pumps, IEA, Paris. 2020. Available online: https://www.iea.org/reports/heat-pumps (accessed on 8 September 2020).
- Pardiñas, A.A.; Alonso, M.J.; Diz, R.; Kvalsvik, K.H.; Fernández-Seara, J. State-of-the-art for the use of phase-change materials in tanks coupled with heat pumps. Energy Build.
**2017**, 140, 28–41. [Google Scholar] [CrossRef][Green Version] - Li, Y.; Nord, N.; Xiao, Q.; Tereshchenko, T. Building heating applications with phase change material: A comprehensive review. J. Energy Storage
**2020**, 31, 101634. [Google Scholar] [CrossRef] - Kapsalis, V.; Karamanis, D. Solar thermal energy storage and heat pumps with phase change materials. Appl. Therm. Eng.
**2016**, 99, 1212–1224. [Google Scholar] [CrossRef] - Zou, D.; Ma, X.; Liu, X.; Zheng, P.; Cai, B.; Huang, J.; Guo, J.; Liu, M. Experimental research of an air-source heat pump water heater using water-PCM for heat storage. Appl. Energy
**2017**, 206, 784–792. [Google Scholar] [CrossRef] - Song, M.; Deng, S.; Dang, C.; Mao, N.; Wang, Z. Review on improvement for air source heat pump units during frosting and defrosting. Appl. Energy
**2018**, 211, 1150–1170. [Google Scholar] [CrossRef] - Spitler, J.; Bernier, M. Ground-source heat pump systems: The first century and beyond. HVAC R Res.
**2011**, 17, 891–894. [Google Scholar] - Cabeza, L.F.; Castell, A.; Barreneche, C.; de Gracia, A.; Fernández, A.I. Materials used as PCM in thermal energy storage in buildings: A review. Renew. Sustain. Energy Rev.
**2011**, 15, 1675–1695. [Google Scholar] [CrossRef] - Ravotti, R.; Fellmann, O.; Lardon, N.; Fischer, L.J.; Stamatiou, A.; Worlitschek, J. Synthesis and Investigation of Thermal Properties of Highly Pure Carboxylic Fatty Esters to Be Used as PCM. Appl. Sci.
**2018**, 8, 1069. [Google Scholar] [CrossRef][Green Version] - Du, K.; Calautit, J.; Wang, Z.; Wu, Y.; Liu, H. A review of the applications of phase change materials in cooling, heating and power generation in different temperature ranges. Appl. Energy
**2018**, 220, 242–273. [Google Scholar] [CrossRef] - Baxter, V.D. Comparison of Field Performance of a High-Efficiency Heat Pump with and without a Desuperheater Water Heater. ASHRAE Trans.
**1984**, 90, 180. [Google Scholar] - Lee, A.H.W.; Jones, J.W. Thermal performance of a residential desuperheater/water heater system. Energy Convers. Manag.
**1996**, 37, 389–397. [Google Scholar] [CrossRef] - Heinz, A.; Lerch, W.; Heimrath, R. Heat pump condenser and desuperheater integrated into a storage tank: Model development and comparison with measurements. Appl. Therm. Eng.
**2016**, 102, 465–475. [Google Scholar] [CrossRef] - Shao, S.; Shi, W.; Li, X.; Ma, J. A new inverter heat pump operated all year round with domestic hot water. Energy Convers. Manag.
**2004**, 45, 2255–2268. [Google Scholar] [CrossRef] - Emhofer, J.; Barz, T.; Marx, K.; Hochwallner, F.; Cabeza, L.F.; Zsembinszki, G.; Strehlow, A.; Nitsch, B.; Weiss, M. Integration of a compact two fluid PCM heat exchanger into the hot superheated section of an air source heat pump cycle for optimized DHW generation. In Proceedings of the 25th IIR International Congress of Refrigeration, Montreal, Canada, 24−30 August 2019. [Google Scholar]
- EN 14825:2018. Air Conditioners, Liquid Chilling Packages and Heat Pumps, with Electrically Driven Compressors, for Space Heating and Cooling—Testing and Rating at Part Load Conditions and Calculation of Seasonal Performance; British Standards Institution: London, UK, 2018. [Google Scholar]
- Barz, T.; Seliger, D.; Marx, K.; Sommer, A.; Walter, S.F.; Bock, H.G.; Körkel, S. State and state of charge estimation for a latent heat storage. Control Eng. Pract.
**2018**, 72, 151–166. [Google Scholar] [CrossRef] - EN 16147:2017. Heat Pumps with Electrically Driven Compressors–Testing, Performance Rating and Requirements for Marking of Domestic Hot Water Units; British Standards Institution: London, UK, 2017. [Google Scholar]
- Hauer, S.; Judex, F.; Bres, A. BMG- Building Model Generator: Reducing the effort for thermal building simulation by automation. In Proceedings of the e-nova International Congress 2016, Pinkafeld, Austria, 24–25 November 2016; pp. 229–236. [Google Scholar]
- Bres, A.; Eder, K.; Hauer, S.; Judex, F. Case study of energy performance analyses on different scales. Energy Procedia
**2015**, 78, 1847–1852. [Google Scholar] [CrossRef][Green Version] - Quoilin, S.; Desideri, A.; Wronski, J.; Bell, I.; Lemort, V. ThermoCycle: A Modelica library for the simulation of thermodynamic systems. In Proceedings of the 10th International Modelica Conference 2014, Lund, Sweden, 10–12 March 2014. [Google Scholar]
- Emhofer, J.; Barz, T.; Palomba, V.; Frazzica, A.; Sergi, F.; Varvagiannis, S.; Karellas, S.; Oró, E.; Zsembinszki, G.; Cabeza, L.F. Deliverable D3.1 of the HYBUILD-Project: Modular Flow Sheet Simulation of the Hybrid (sub-)System. Available online: http://www.hybuild.eu/publications/deliverables/ (accessed on 8 September 2020).
- Frazzica, A.; Palomba, V.; Sergi, F.; Ferraro, M.; Cabeza, L.F.; Zsembinszki, G.; Oró, E.; Karellas, S.; Varvagiannis, S.; Emhofer, J.; et al. Dynamic Modelling of a Hybrid Solar Thermal/electric Energy Storage System for Application in Residential Buildings. In Proceedings of the 12th International Conference on Solar Energy for Buildings and Industry, Raperswil, Switzerland, 10–13 September 2018. [Google Scholar]
- Bell, I.H.; Wronski, J.; Quoilin, S.; Lemort, V. Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp. Ind. Eng. Chem.
**2014**, 53, 2498–2508. [Google Scholar] [CrossRef] [PubMed][Green Version] - Marx, K.; Emhofer, J.; Barz, T.; Krämer, J.; Cabeza, L.F.; Zsembinszki, G.; Strehlow, A.; Nitsch, B.; Wiesflecker, M.; Zitzenbacher, R.; et al. Dynamic Performance Tests of a Heat Pump Cycle Integrated Latent Heat Thermal Energy Storage for Optimized DHW Generation. Unpublished work. 2020. [Google Scholar]
- EUROSTAT, Electricity Price Statistics–Statistics Explained. Available online: https://ec.europa.eu/eurostat/statistics-explained/index.php/Electricity_price_statistics#Electricity_prices_for_household_consumers (accessed on 8 September 2020).
- Zsembinszki, G.; Fernández, A.G.; Cabeza, L.F. Selection of the Appropriate Phase Change Material for Two Innovative Compact Energy Storage Systems in Residential Buildings. Appl. Sci.
**2020**, 10, 2116. [Google Scholar] [CrossRef][Green Version] - HYBUILD Project, Innovative Compact Hybrid Storage Systems for Low Energy Buildings. Available online: http://www.hybuild.eu/ (accessed on 8 September 2020).

**Figure 1.**Concept of the proposed system for an example scenario with three apartments during: (

**a**) heating operation, (

**b**) cooling operation, (

**c**) energy efficient domestic hot water (DHW) generation by discharging the RPW-HEX and (

**d**) direct DHW generation. (S) Decentralized sensible DHW storage, (H) fan-coils suitable for heating and cooling, (C) condenser, (R) RPW-HEX, (W) four-way valve, (B) bypass expansion valve, (X) regular expansion valve, (F) fluid phase heat exchanger, (E) evaporator and fan, (P) compressor, (M) three-way valve. The temperatures indicated in (

**a**,

**b**,

**d**) were taken from steady-states at a state of charge (SoC) of 50% for heating and cooling, respectively, whereas they were taken shortly after switching from mode (a) to (c) in (

**c**). The ambient temperature $\left(\vartheta \right)$ was 0 °C in (

**a**,

**c**,

**d**) and 35 °C in (

**b**).

**Figure 2.**Payback time (

**a**) and savings per year (

**b**) for Case #1 (passive house located in Helsinki). Markers “▼”, “●” and “▲” denote small (140 L), medium (210 L) and large (280 L) decentralized DHW storage devices.

**Figure 3.**Payback time (

**a**) and savings per year (

**b**) for Case #2 (low-energy building located in Strasbourg). Markers “▼”, “●” and “▲” denote small (140 L), medium (210 L) and large (280 L) decentralized DHW storage devices. Please note that for the case with small decentralized storage devices operated in comfort mode, the payback time is higher than 35 years for all RPW-HEX storage sizes. Therefore, this case is not shown in (

**a**).

**Figure 4.**Profit after 20 years of operation for (

**a**) Case #1 (passive house located in Helsinki) and (

**b**) Case #2 (low energy building located in Strasbourg). Markers “▼”, “●” and “▲” denote small (140 L), medium (210 L) and large (280 L) decentralized DHW storage devices. Please note that for some cases, no profit can be made after 20 years of operation, so they are not shown in the figures.

**Figure 5.**Payback time (

**a**) and savings per year (

**b**) for Case #3 (refurbished building located in Athens). Markers “▼”, “●” and “▲” denote small (140 L), medium (210 L) and large (280 L) decentralized DHW storage devices.

**Figure 6.**Daily DHW consumption and provided energy for energy efficient DHW generation by operating mode (c) Q

_{DHW,c}for (

**a**) Case #1 and (

**b**) Case #3 and variations from Table 3. The different colors mark the contribution from the condenser (“con”, green), from the refrigerant transferred via the RPW-HEX (“RPW-ref”, blue) and from the energy stored in the PCM-material/aluminum (“RPW-PCM”, orange). The dotted line marks the average DHW consumption and the dashed line mark the average contribution of DHW to the DHW consumption by operating mode (c).

Constraint | Case #1 Passive House in Helsinki | Case #2 Low Energy Building in Strasbourg | Case #3 Refurbished Building in Athens |
---|---|---|---|

Apartments and total floor area per building | 7.75, 581 m^{2} | 5.25, 394 m^{2} | 1.75, 131 m^{2} |

Heating demand per year and m^{2} (kWh year^{−1} m^{−2}) | 14.8 | 31.5 | 61.7 |

Maximum heating demand (kW) | 6.97 | 9.89 | 7.15 |

Maximum cooling demand (kW) | 10.8 | 9.68 | 10.67 |

Daily DHW demand without losses (kWh) | 45.3 | 30.7 | 10.2 |

Annual heating demand (kWh) | 8598 | 12,413 | 8096 |

Annual cooling demand (kWh) | 6903 | 5360 | 9401 |

Annual DHW demand without losses (kWh) | 16,534 | 11,200 | 3733 |

HP design load heating P_{design,heating} (kW) | 7.22 (at −22 °C) | 9.66 (at −10 °C) | 13.0 (at 2 °C) |

HP design load cooling P_{design,cooling} (kW) | 10.843 (at 35 °C) | 10.843 (at 35 °C) | 10.843 (at 35 °C) |

ϑ_{water,heating,distribution,in} (°C) | 22–35 | 24–45 | 24–45 |

ϑ_{water,cooling,distribution,in} (°C) | 7–11.5 | 7–11.5 | 7–11.5 |

Main purpose | mostly heating | heating | mostly cooling |

${\mathit{C}}_{\mathit{i}\mathit{n}\mathit{v}\mathit{e}\mathit{s}\mathit{t},\mathit{f}\mathit{i}\mathit{x}}\text{}\left(\mathbf{EUR}\right)$ | ${\tilde{\mathit{c}}}_{\mathit{i}\mathit{n}\mathit{v}\mathit{e}\mathit{s}\mathit{t},\mathit{v}\mathit{a}\mathit{r}}\text{}(\mathbf{EUR}/\mathbf{kWh})$ | ||
---|---|---|---|

Manufacturing costs RPW-HEX | 500 | Aluminum | 137.0 |

Three-way valve and additional piping | 20 | PCM | 61.7 |

Insulation (mineral wool) | 2.6 | ||

Total | 520 | 201.3 |

**Table 3.**Performance of systems with 5 kWh RPW-HEX and medium DHW storage devices working in standard-operation mode.

Case #1 | Case #2 | Case #3 | |
---|---|---|---|

Energy demand for heat-ing per year (RPW/REF) | 2990 kWh_{el}/ | 3972 kWh_{el}/ | 2231 kWh_{el}/ |

2939 kWh_{el} | 3918 kWh_{el} | 2180 kWh_{el} | |

Energy demand for cool-ing per year (RPW/REF) | 2000 kWh_{el}/ | 1692 kWh_{el}/ | 3520 kWh_{el}/ |

2059 kWh_{el} | 1753 kWh_{el} | 3616 kWh_{el} | |

Energy demand for DHW per year (RPW/REF) | 6086 kWh_{el}/ | 3713 kWh_{el}/ | 900 kWh_{el}/ |

6634 kWh_{el} | 4150 kWh_{el} | 1191 kWh_{el} | |

Energy savings for heating per year | −51.2 kWh_{el} (−1.74%) | −55.5 kWh_{el} (−1.42%) | −50.5 kWh_{el} (−2.32%) |

Energy savings for cooling per year | 59.0 kWh_{el} (2.86%) | 61.6 kWh_{el} (3.51%) | 95.8 kWh_{el} (2.65%) |

Energy savings for DHW per year | 549 kWh_{el} (8.27%) | 436 kWh_{el} (10.5%) | 291 kWh_{el} (24.4%) |

Total energy savings per year | 557 kWh_{el} (4.78%) | 442 kWh_{el} (4.50%) | 336 kWh_{el} (4.81%) |

Investment costs | 1526 EUR | 1526 EUR | 1526 EUR |

Payback time | 14.1 years | 18.3 years | 25.6 years |

Minimum cost savings per year (based on first year) | 122 EUR | 97.3 EUR | 74.0 EUR |

Profit after 20 years | 760 EUR | 171 EUR | - |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Emhofer, J.; Marx, K.; Barz, T.; Hochwallner, F.; Cabeza, L.F.; Zsembinszki, G.; Strehlow, A.; Nitsch, B.; Wiesflecker, M.; Pink, W.
Techno-Economic Analysis of a Heat Pump Cycle Including a Three-Media Refrigerant/Phase Change Material/Water Heat Exchanger in the Hot Superheated Section for Efficient Domestic Hot Water Generation. *Appl. Sci.* **2020**, *10*, 7873.
https://doi.org/10.3390/app10217873

**AMA Style**

Emhofer J, Marx K, Barz T, Hochwallner F, Cabeza LF, Zsembinszki G, Strehlow A, Nitsch B, Wiesflecker M, Pink W.
Techno-Economic Analysis of a Heat Pump Cycle Including a Three-Media Refrigerant/Phase Change Material/Water Heat Exchanger in the Hot Superheated Section for Efficient Domestic Hot Water Generation. *Applied Sciences*. 2020; 10(21):7873.
https://doi.org/10.3390/app10217873

**Chicago/Turabian Style**

Emhofer, Johann, Klemens Marx, Tilman Barz, Felix Hochwallner, Luisa F. Cabeza, Gabriel Zsembinszki, Andreas Strehlow, Birgo Nitsch, Michael Wiesflecker, and Werner Pink.
2020. "Techno-Economic Analysis of a Heat Pump Cycle Including a Three-Media Refrigerant/Phase Change Material/Water Heat Exchanger in the Hot Superheated Section for Efficient Domestic Hot Water Generation" *Applied Sciences* 10, no. 21: 7873.
https://doi.org/10.3390/app10217873