# A Comparison of Different District Integration for a Distributed Generation System for Heating and Cooling in an Urban Area

^{1}

^{2}

^{*}

## Abstract

**:**

_{2}emissions as the environmental objective function. The energy system is optimized for different district integration option, in order to understand how they affect the optimal solutions compared with both the environmental and economic objects.

## 1. Introduction

- research focusing on the optimization of the operation of energy systems, ranging from the optimization of a single component, to the operation of the overall DG system;
- research dealing with the optimization of the system synthesis; and
- research focusing on synthesis, design and operation optimization.

- almost all models rely on linear programming or mixed integer linear programming (MILP). However, some approaches based on meta-heuristics (simulated annealing, genetic algorithms, etc.) have been proposed, but they present some difficulties concerning the determination of search parameters and the judgment about optimality [11,12,13].
- the research normally focus only on specific targets: operation or synthesis optimization, economic and/or environmental optimization, unit or district heating network (DHN) optimization, etc.

_{2}emissions. Carvalho [20] presented a model for the synthesis and operation optimization of residential units, considering environmental and economic aspects.

_{2}emissions as the environmental objective function.

## 2. MILP Model

#### 2.1. Decision Variables and Constraints

- binary variables: they represent the existence/absence of each component and the operation status (on/off) of each component in each time interval. There are other additional binary variables which do not represent any physical quantity, added to linearize some relations; and
- continuous variables: they represent the size of components, the size of pipelines, the load of components in each time interval, the energy content of the storages and the connection flows.

- components;
- district heating and cooling network;
- thermal storage; and
- energy balances.

#### 2.2. Components

**X**,

_{ice}**O**) is taken into account in each time interval and throughout the year: the component j can be installed only if the component j − 1 has been already adopted (Equation (1)), and the component j can never be in operation if it has not been adopted (Equation (2)):

_{ice}**X**(j,u) ≤

_{ice}**X**(j − 1,u)

_{ice}**O**

_{ice}(m,d,h,j,u) ≤

**X**

_{ice}(j,u)

_{ice}(m,d,h,j,u) = Kh

_{ice}(m,d,h,1)·

**E**(m,d,h,j,u) + Kh

_{ice}_{ice}(m,d,h,2)·

**O**(m,d,h,j,u)

_{ice}_{ice}(m,d,h,j,u) = Kf

_{ice}(m,d,h,1)·

**E**(m,d,h,j,u) + Kf

_{ice}_{ice}(m,d,h,2)·

**O**(m,d,h,j,u)

_{ice}_{ice}

_{,lim}(m,d,h,u,1)·

**O**(m,d,h,j,u) ≤

_{ice}**E**(m,d,h,j,u) ≤ E

_{ice}_{ice}

_{,lim}(m,d,h,u,2)·

**O**(m,d,h,j,u)

_{ice}_{ice}and Kf

_{ice}can be obtained through a linear regression of the load curves. The constraints which describe the MGT can be easily inferred by changing in each variable or coefficient the subscript “ice” with the subscript “mgt”. A variable size ICE can be installed in the central unit. The constraints which describe this component (Equations (6)–(12)) are different from the previous constraints, because both size and load are decision variables. Therefore, it is necessary to introduce additional constraints and decision variables in order to maintain the linearity of the problem.

_{ice}

_{,lim,c}(1)·

**X**

_{ice}**≤**

_{,c}**S**≤ S

_{ice}_{,c}_{ice}

_{,lim,c}(2)·

**X**

_{ice}

_{,c}**O**

_{ice}

_{,c}(m,d,h) ≤

**X**

_{ice}

_{,c}

**E**), the sub-product (H

_{ice,c}_{ice,c}) and the fuel flows (F

_{ice,c}):

_{ice}

_{,c}(m,d,h) = Kh

_{ice}

_{,c}(m,d,h,1)·

**E**

_{ice}**(m,d,h) + Kh**

_{,c}_{ice}

_{,c}(m,d,h,2)·

**O**

_{ice}**(m,d,h) + Kh**

_{,c}_{ice}

_{,c}(m,d,h,3)·

**ξ**

_{ice}

_{,c}(m,d,h)

_{ice}

_{,c}(m,d,h) = Kf

_{ice}

_{,c}(m,d,h,1)·

**E**

_{ice}**(m,d,h) + Kf**

_{,c}_{ice}

_{,c}(m,d,h,2)·

**O**

_{ice}**(m,d,h) + Kf**

_{,c}_{ice}

_{,c}(m,d,h,3)·

**ξ**

_{ice}

_{,c}(m,d,h)

**ξ**

_{ice,c}(m,d,h), which allow us to introduce a linear relation with two independent variables—size

**S**and load

_{ice,c}**E**(m, d, h)—avoiding inconsistent results when the engine is off:

_{ice,c}**S**

_{ice}**+ S**

_{,c}_{ice}

_{,lim,c}(2)·(

**O**(m,d,h) − 1) ≤

_{ice}_{,c}**ξ**

_{ice}

_{,c}(m,d,h) ≤

**S**

_{ice}_{,c}_{ice}

_{,lim,c}(1)·

**O**

_{ice}**(m,d,h) ≤**

_{,c}**ξ**

_{ice}

_{,c}(m,d,h) ≤ S

_{ice}

_{,lim,c}(2)·

**O**

_{ice}**(m,d,h)**

_{,c}**E**

_{ice}

_{,c}(m,d,h) ≤

**S**

_{ice}

_{,c}

_{boi,lim,c}= 0.1) has been taken into account and auxiliary variables

**ψ**(m, d, h) play a role analogous to variables

_{boi,c}**ξ**

_{ice,c}(m, d, h), previously introduced for the ICE:

_{boi}

_{,c}(m,d,h) =

**H**

_{boi}**(m,d,h)/η**

_{,c}_{boi}

_{,c}(m,d,h)

_{boi}

_{,lim,c}·

**ψ**

_{boi}**(m,d,h) ≤**

_{,c}**H**(m,d,h) ≤

_{boi}_{,c}**ψ**(m,d,h)

_{boi}_{,c}**S**

_{boi}**+ S**

_{,c}_{boi}

_{,lim,c}(2)·(

**O**(m,d,h) − 1) ≤

_{boi}_{,c}**ψ**(m,d,h) ≤

_{boi}_{,c}**S**

_{boi}_{,c}_{boi}

_{,lim,c}(1)·

**X**

_{boi}**≤**

_{,c}**S**≤ S

_{boi}_{,c}_{boi}

_{,lim,c}(2)·

**X**

_{boi}

_{,c}**C**(m,d,h,j,u) ≤ H

_{abs}_{ice}(m,d,h,j,u) + H

_{mgt}(m,d,h,j,u)

**X**(j,u) ≤

_{abs}**X**(j,u) +

_{ice}**X**(j,u)

_{mgt}**O**(m, d, h, j, u) = 1—or cold—

_{hp,h}**O**(m,d,h,j,u) = 1, but not simultaneously:

_{hp,c}_{hp}(j,u) ≤ X

_{hp}(j − 1,u)

**O**

_{hp}**(m,d,h,j,u) ≤**

_{,h}**X**(j,u)

_{hp}**O**

_{hp}**(m,d,h,j,u) ≤**

_{,c}**X**(j,u)

_{hp}**O**

_{hp}**(m,d,h,ju) +**

_{,h}**O**

_{hp}**(m,d,h,j,u) ≤ 1**

_{,c}_{hp}(m,d,h,j,u) = K

_{hp}(m,d,h,u,1)·

**E**

_{hp}**(m,d,h,j,u) + K**

_{,h}_{hp}(m,d,h,u,2)·

**O**

_{hp}**(m,d,h,j,u)**

_{,h}_{hp}(m,d,h,j,u) = K

_{hp}(m,d,h, u,3)·

**E**

_{hp}**(m,d,h,j,u) + K**

_{,c}_{hp}(m,d,h,u,4)·

**O**

_{hp}**(m,d,h,j,u)**

_{,c}_{hp}

_{,lim}(m,d,h,u,1)·

**O**

_{hp}**(m,d,h,j,u) ≤**

_{,h}**E**(m,d,h,j,u) ≤ S

_{hp}_{,h}_{hp}

_{,lim}(m,d,h,u,2)·

**O**

_{hp}**(m,d, h, j, u)**

_{,h}_{hp}

_{,lim}(m,d,h,u,1)·

**O**

_{hp}**(m,d,h,j,u) ≤**

_{,c}**E**(m,d,h,j,u) ≤ S

_{hp}_{,c}_{hp}

_{,lim}(m,d,h,u,2)·

**O**

_{hp}**(m,d,h,j,u)**

_{,c}_{hp}(m,d,h,j,u) =

**E**

_{hp}**(m,d,h,c,u) +**

_{,h}**E**

_{hp}**(m,d,h,j,u)**

_{,c}_{stp}(m, d, h, u) and K

_{pv}(m, d, h, u)—is evaluated a priori considering inclination, orientation angle of installation and hourly solar radiation of the site of the plant.

#### 2.3. District Heating and Cooling Network

_{p}is fixed, the transferred heat ${\dot{Q}}_{p}$ reported in Equation (28) depends on two variables, which are the cross section of the pipeline A

_{p}and the temperature difference between inlet and outlet pipelines ∆t [28]. Assuming a fixed temperature of the network and a fixed temperature difference between the inlet and outlet, the modelling introduces a constant ratio between the size of the pipeline and the maximum flow which can be transferred, and considers the size and the layout of the network as decision variables, constrained by the pipeline super-structure and the flow rate limits of each pipe. The ∆t adopted normally ranges between 15–25 °C depending on the application, while the medium velocity v

_{p}ranges between 1.5–2.5 m/s. The thermal losses are considered proportional to the length of each pipeline through the coefficient δ

_{t}:

_{t}(u,v) = δ

_{t}·l

_{p}(u,v)

**X**(u,v) +

_{tp}**X**(v,u) ≤ 1

_{tp}_{net}

_{,lim}(1)·

**X**(u,v) ≤

_{tp}**S**(u,v) ≤ H

_{H}_{,net}_{net}

_{,lim}(2)·

**X**(u,v)

_{tp}**H**(m,d,h,u,v) ≤

_{net}**S**(u,v)

_{H}_{,net}#### 2.4. Thermal Storage

**Q**=

_{ts}**V**·ρ

_{ts}_{p}·c

_{p}·∆t

**Q**(m,s,d,r,h,u) − K

_{ts}_{los}

_{,ts}(u)·

**Q**(m,s,d,r,h − 1,u) =

_{ts}**H**(m,d,h,u)

_{ts}**Q**(m,s,d,r,h,u) − K

_{ts}_{los}

_{,ts}(u)·

**Q**(m,s,d,r − 1,24,u) =

_{ts}**H**(m,d,h,u)

_{ts}**Q**, has to be lower than storage size

_{ts}**S**:

_{ts}**Q**(m,s,d,r,h,u) ≤

_{ts}**S**(u)

_{ts}#### 2.5. Energy Balances

**E**(m,d,h,u) +

_{ice}**E**(m,d,h,u) + E

_{mgt}_{pvp}(m,d,h,u) +

**E**(m,d,h,u) = E

_{bgt}_{cc}(m,d,h,u) + E

_{hp}(m,d,h,u) + E

_{dem}(m,d,h,u) +

**E**(m,d,h,u)

_{sol}_{mgt}(m,d,h,u) + H

_{ice}(m,d,h,u) + H

_{stp}(m,d,h,u) +

**H**(m,d,h,u) + H

_{boi}_{hp}(m,d,h,u) +

**H**(m,d,h,v,u)·(1 − p

_{net}_{t}(v,u)) =

**H**(m,d,h,u) + H

_{ts}_{abs}(m,d,h,u) + H

_{dem}(m,d,h,u) +

**H**(m,d,h,u,v)

_{net}_{mgt}(m,d,h,u) + H

_{ice}(m,d,h,u) + H

_{stp}(m,d,h,u) −

**H**(m,d,h,u) ≥ 0

_{ts}**C**(m,d,h,u) +

_{abs}**C**(m,d,h,u) + C

_{cc}_{hp}(m,d,h,u) = C

_{dem}(m,d,h,u) +

**C**(m,d,h,u)

_{ts}**C**(m,d,h,u) +

_{abs}**C**(m,d,h,u) + C

_{cc}_{hp}(m,d,h,u) −

**C**(m,d,h,u) ≥ 0

_{ts}_{ice}

_{,c}(m,d,h) +

**H**(m,d,h) + H

_{boi}_{,c}_{stp}

_{,c}(m,d,h) =

**H**(m,d,h) +

_{net}_{,c}**H**(m,d,h)

_{ts}_{,c}_{mgt}(m,d,h,u) + H

_{ice}(m,d,h,u) + H

_{stp}(m,d,h,u) +

**H**(m,d,h,u) + H

_{boi}_{hp}(m,d,h,u) +

**H**(m,d,h,v,u)·(1 − p

_{net}_{t}(v,u)) +

**H**(m,d,h)·(1 − p

_{net}_{,c}_{t}

_{,c}) =

**H**(m,d,h,u) + H

_{ts}_{abs}(m,d,h,u) + H

_{dem}(m,d,h,u) +

**H**(m,d,h,u,v)

_{net}**H**,

_{ts,c}**H**,

_{ts}**C**) which are free. Positive values represent input flows, while negative values mean an energy extraction from the thermal storage. The temperatures of the thermal flows are not taken into account because it would have compromised the linearity of the problem. However, this is not a strong approximation considering that the thermal energy required by the users is normally supplied at a temperature of 50–55 °C and all components are able to produce the thermal energy at higher temperatures. Some restrictions due to the coupling of components related to the operating temperatures (ICE, MGT together with ABS) have been considered through a particular conformation of the superstructure and with additional constraints which consider this matter (e.g., Equations (17) and (18)).

_{ts}#### 2.6. Objective Functions

_{tot}= c

_{inv}+ c

_{man}+ c

_{ope}

_{inv}) is the sum of the investment cost of the sites, of the central unit, and of the network. The investment cost of a site can be evaluated through:

_{inv}

_{,u}(u) = Σ

_{j}[f

_{mg}

_{t}·

**X**

_{mg}**(j,u)·c**

_{t}_{mg}

_{t}(j,u) + f

_{ice}·

**X**

**(j,u)·c**

_{ice}_{ice}(j,u) + f

_{hp}·

**X**(j,u)·c

_{hp}_{hp}(j,u) + f

_{abs}·

**X**(j,u)·c

_{abs}_{abs}(j,u)] + f

_{boi}·

**S**(u)·c

_{boi}_{boi}+ f

_{cc}·

**S**(u)·c

_{cc}_{cc}+ f

_{pvp}·S

_{pvp}(u)·c

_{pvp}+ f

_{stp}·

**S**(u)·c

_{stp}_{stp}+ f

_{ts}·

**S**(u)·c

_{ts}_{ts}+ f

_{ts}·

**S**(u)·c

_{cs}_{ts}

_{inv}

_{,c}= f

_{ice}(

**S**

**·c**

_{ice}_{,c}_{ice}

_{,v}+

**X**

**·c**

_{ice}_{,c}_{ice}

_{,f}) + f

_{boi}·(

**S**

_{boi,c}·cboi,v +

**X**

_{boi,c}·cboi,f) + fstp·

**S**

_{stp,c}·cstp,c + fts ·

**S**

_{ts,c}·cts,c + fnet ·(cnet,f,c ·

**X**

_{net,c}+ cnet,v,c ·

**S**

_{H,net,c})

_{net}= f

_{net}·Σ

_{u,v}[c

_{net}

_{,f,c}(1)·(

**X**(u,v) +

_{tp}**X**(u,v)) + c

_{cp}_{net}

_{,f,c}(1)·

**X**(u,v) + c

_{net}_{net}

_{,v}·(

**S**(u,v) +

_{H}_{,net,c}**S**(u,v))]

_{C}_{,net}_{ope}

_{,u}(u) = Σ

_{m,d,h}[c

_{fue}

_{,chp}(m)·(F

_{ice}(m,d,h,u) + F

_{mgt}(m,d,h,u)) + c

_{fue}

_{,boi}(m)·F

_{boi}(m,d,h,u) + c

_{el}

_{,bgt}(m,d,h)·

**E**(m,d,h,u) − c

_{bgt}_{el}

_{,inc}·E

_{pvp}(m,d,h,u) − c

_{el}

_{,sol}(m,d,h)·

**E**(m,d,h,u)] · wgt(m,d,h)

_{sol}_{u}[cfue,ice,c·Fice,c(m,d,h) + c

_{fue}

_{,boi}(m)·F

_{boi}

_{,c}(m,d,h)–c

_{el}

_{,sol}(m,d,h)·

**E**

_{ice}**(m,d,h)]·wgt(m,d,h)**

_{,c}_{2}emissions). The total annual emissions are related to the net electric energy received from the grid (bought minus sold) and to the fuel consumption by the DG energy system (boilers and/or CHP). Therefore, the total annual emissions can be evaluated through:

_{tot}= em

_{el}·Σ

_{m,d,h,u}[

**E**(m,d,h,u) −

_{bgt}**E**(m,d,h,u) −

_{sol}**E**(m,d,h)]·wgt(m,d,h) + emf,chp ·Σ

_{ice}_{,c}_{m,d,h,u}[F

_{ice}(m,d,h,u) + F

_{mgt}(m,d,h,u)]·wgt(m,d,h) + emf,boi ·Σ

_{m,d,h,u}[(F

_{boi}(m,d,h,u) + F

_{boi}

_{,c}(m,d,h)]·wgt(m,d,h) + em

_{f}

_{,cen}·Σ

_{m,d,h}F

_{ice}

_{,c}(m,d,h) · wgt(m,d,h)

## 3. Case Study

- layout of the roads which connect the buildings;
- position of the underground utilities (waterworks, sanitation, gas network, etc.); and
- location of the boiler rooms of the buildings.

_{2}emissions related to the consumption of electricity and natural gas have been assumed from literature [33]. The natural gas CO

_{2}emissions depend on the chemical composition of the gas and then on its provenience. However, the slight difference can be ignored considering the same value for the natural gas CO

_{2}emissions. The same approximation cannot be made for electricity. In fact, electricity carbon intensity depends heavily on the national electricity system. The reference case study has been optimized assuming the average electricity carbon intensity of the European Union in 2007–2009 (0.356 kgCO

_{2}/kWh), while a second set of optimizations has been performed assuming the average electricity carbon intensity of the OECD Americas (Canada, United States, Mexico, Chile) in 2007–2009 (0.485 kgCO

_{2}/kWh). The natural gas carbon intensive has been assumed equal to 0.202 kgCO

_{2}/kWh.

- natural gas detaxation for cogeneration use; and
- renewable energy production incentives.

## 4. Results of the Optimizations

_{2}emissions. Using the ϵ-constrained method the Pareto fronts have been obtained for different plant configurations.

- conventional solution;
- isolated solution;
- distributed generation solution without central unit and district cooling network;
- distributed generation solution with central unit but without cooling network; and
- complete distributed generation solution.

^{®}Optimization Suite. X-press

^{®}is a commercial software produced by FICO

^{®}(Fair Isaac Corporation, San Jose, CA, USA) for solving large optimization problems by means of the application of resident algorithms. The mathematical model has been implemented through Mosel, a modelling and programming language that allows users to formulate problems, to solve them by using the solver engines, and to analyse the solutions.

^{®}processor Core

^{TM}i7CPU [email protected] GHz, 6.00 GB RAM and a 64 bit operating system. An optimization of the overall problem, accepting a gap of 1%, takes about 100 h.

#### 4.1. Conventional Solution

#### 4.2. Isolated Solution

#### 4.3. Distributed Generation Solution

#### 4.4. Distributed Generation Solution Integrated with the Central Solar System

_{2}emissions (−22%), compared to that shown in Figure 9.

#### 4.5. Complete Distributed Generation Solution

## 5. Conclusions

_{2}emissions during operation. The aim of the work is to compare on a common basis, different district heating integration options. In more detail, five cases have been considered:

- conventional solution;
- isolated solution;
- distributed generation solution without central unit and district cooling network;
- distributed generation solution with central unit but without cooling network; and
- complete distributed generation solution.

_{2}emissions (16%). In this case, the adoption of local thermal storages is suggested from both the economic and environmental points of view). This performance is similar to the one obtained for the economic optimization of the complete distributed generation; the isolated solution also implies a much lower investment cost (Table 7). On the other hand, the complete distributed generation solution is cheaper in the long term view (over 20 years) and allows also a greater reduction of the annual emissions. In fact, important reductions of the annual emissions can be obtained only with the adoption of the solar field, of the seasonal storage and of the district heating network.

^{2}. The optimal operations identified in this paper show that the central thermal storage is operated with seasonal charging/discharging cycles only when the environmental objective function is considered in the optimizations (Figure 13). The heat produced by the solar field during warmer months is used during the first colder months. Meanwhile, pure economic optimizations provide a weekly operation of the central thermal storage: the heat produced by the solar field during week-end, when the energy demand is lower, is used during the following working days.

## Supplementary Materials

## Author Contributions

## Funding

## Conflicts of Interest

## Nomenclature

δ_{t} | Thermal losses percentage |

∆t | Difference between outlet and inlet temperatures (K) |

η_{boi,c} | Central BOI efficiency |

ψ_{boi,c} | Additional variable for the centralized BOI |

ρ_{p} | Medium density (Kg/m^{3}) |

ξ_{ice,c} | Additional variable for the centralized Internal Combustion Engine (ICE) |

A_{p} | Diameter of the pipeline (m^{2}) |

c | Central unit |

C_{abs} | Cold produced by the Absorption Chiller (ABS) (kWh) |

c_{abs} | ABS investment cost (e) |

c_{boi} | BOI investment cost (e) |

c_{boi,f} | BOI fixed investment cost (e) |

c_{boi,v} | BOI variable investment cost (e/kW) |

C_{cc} | Cold produced by the Compression Chiller (CC) (kWh) |

C_{dem} | User cooling demand (kWh) |

c_{el,bgt} | Electricity cost (e/kWh) |

c_{el,inc} | Photo-voltaic panels (PV panels) incentive (e/kWh) |

c_{el,sol} | Electricity income (e/kWh) |

c_{fue,boi} | BOI fuel cost (e/kWh) |

c_{fue,chp} | Combined Cooling Heat and Power (CHP) fuel cost (e/kWh) |

c_{fue,ice,c} | Central ICE fuel cost (e/kWh) |

c_{hp} | HP investment cost (e) |

C_{hp} | Cold produced by the HP (kWh) |

c_{ice} | ICE investment cost (e) |

c_{ice,f} | ICE fixed investment cost (e) |

c_{ice,v} | ICE variable investment cost (e/kW) |

c_{inv} | Investment annual cost (e/y) |

c_{inv,c} | Central unit annual investment cost (e/y) |

c_{inv,u} | Site annual investment cost (e/y) |

c_{man} | Maintenance annual cost (e/y) |

c_{mgt} | Micro Gas Turbine (MGT) investment cost (e) |

c_{net} | DHCN annual investment cost (e/y) |

c_{net,f,c} | Fixed cost of the DHCN pipeline (e/m) |

c_{net,v} | Variable cost of the DHCN pipeline (e/kW · m) |

c_{net,v,c} | Variable cost of the central DHN pipeline (e/kW · m) |

c_{ope} | Operating annual cost (e/y) |

c_{ope,c} | Central unit annual operation cost (e/y) |

c_{ope,u} | Unit annual operation cost (e/y) |

c_{p} | Specific heat(Kj/kg K) |

c_{pvp} | PV panels investment cost (e/m) |

c_{stp} | Solar thermal panels (ST panels) investment cost (e/m^{2}) |

c_{stp,c} | Central ST panels investment cost (e/m^{2}) |

c_{tot} | Total annual cost (e/y) |

C_{ts} | Cooling energy storage input (kWh) |

c_{ts} | Thermal Storage (TS) investment cost (e/kWh) |

c_{ts,c} | Central TS investment cost (e/kWh) |

d | Generic day |

E_{bgt} | Electricity bought from the network (kWh) |

E_{cc} | Electricity required by the CC (kWh) |

E_{hp,c} | Electricity required by the HP when producing cold (kWh) |

E_{dem} | User electricity demand (kWh) |

E_{hp,h} | Electricity required by the HP when producing heat (kWh) |

E_{hp} | Electricity required by the HP (kWh) |

E_{ice} | Electricity produced by the ICE (kWh) |

E_{ice,c} | Electricity produced by the centralized ICE (kWh) |

E_{ice,lim} | ICE operation limits (kW) |

em_{el} | Electricity carbon intensity (kgCO_{2}/kWh) |

em_{f,boi} | BOI fuel carbon intensity (kgCO_{2}/kWh) |

em_{f,cen} | Central CHP fuel carbon intensity (kgCO_{2}/kWh) |

em_{f,chp} | CHP fuel carbon intensity (kgCO_{2}/kWh) |

E_{mgt} | Electricity produced by the MGT (kWh) |

em_{lim} | Emission limit in the s-constrained optimization (kgCO_{2}/kWh) |

em_{tot} | Total annual CO_{2} emissions (kg) |

E_{pvp} | Electricity produced by the PV panels (kWh) |

E_{sol} | Electricity sold to the network (kWh) |

f_{abs} | ABS amortization factor (y^{−1}) |

F_{boi} | Fuel required by the BOI (kWh) |

f_{boi} | BOI amortization factor (y^{−1}) |

F_{boi,c} | Fuel required by the central BOI (kWh) |

f_{cc} | CC amortization factor (y^{−1}) |

f_{hp} | HP amortization factor (y^{−1}) |

F_{ice} | Fuel required by the ICE (kWh) |

f_{ice} | ICE amortization factor (y^{−1}) |

F_{ice,c} | Fuel required by the centralized ICE (kWh) |

F_{mgt} | Fuel required by the MGT (kWh) |

f_{mgt} | MGT amortization factor (y^{−1}) |

f_{net} | DHCN amortization factor (y^{−1}) |

f_{pvp} | PV panels amortization factor (y^{−1}) |

f_{stp} | ST panels amortization factor (y^{−1}) |

f_{ts} | TS amortization factor (y^{−1}) |

h | Generic hour |

H_{abs} | Heat required by the ABS (kWh) |

H_{boi} | Heat produced by the BOI (kWh) |

H_{boi,c} | Heat produced by the central BOI (kWh) |

H_{boi,lim,c} | Centralized BOI operation limits (kW) |

H_{dem} | User thermal demand (kWh) |

H_{hp} | Heat produced by the HP (kWh) |

H_{ice} | Heat produced by the ICE (kWh) |

H_{ice,c} | Heat produced by the centralized ICE (kWh) |

H_{mgt} | Heat produced by the MGT (kWh) |

H_{net} | Thermal energy transferred through the pipeline (kWh) |

H_{net,c} | Thermal energy transferred through the pipeline of the central DHN (kWh) |

H_{net,lim} | Size limits of the pipelines (kWh) |

H_{stp} | Solar panel thermal production |

H_{stp,c} | Centralized solar field thermal production |

H_{ts} | Thermal energy storage input (kWh) |

H_{ts,c} | Thermal energy storage input (kWh) |

j | Generic component |

k | Generic site/user |

Kf_{ice} | ICE Performance curve linearization coefficient |

Kf_{ice,c} | Centralized ICE Performance curve linearization coefficient |

Kh_{ice} | ICE Performance curve linearization coefficient |

Kh_{ice,c} | Central ICE performance curve linearization coefficient |

K_{hp} | HP Performance curve linearization coefficient |

K_{los,ts} | Percentage thermal loss coefficient |

K_{pv} | Unitary PV production |

K_{stp} | Unitary solar thermal production |

l_{p} | Length of the pipeline (m) |

m | Generic month |

O_{boi,c} | Central BOI operation (binary) |

O_{hp,c} | HP cold operation (binary) |

O_{hp,h} | HP heat operation (binary) |

O_{ice} | ICE operation (binary) |

O_{ice,c} | Centralized ICE operation (binary) |

p_{t} | Pipeline thermal loss per unit length (km^{−1}) |

p_{t,c} | Pipeline thermal loss per unit length of the central DHN pipeline (km^{−1}) |

${\dot{Q}}_{p}$ | Heat transferred by a DHCN pipeline (kWh) |

Q_{ts} | Thermal energy stored in a thermal storage (kWh) |

s | Generic week |

S_{boi} | BOI size (kW) |

S_{boi,c} | Central BOI size (kW) |

S_{boi,lim,c} | Central BOI size limits (kW) |

S_{cc} | CC size (kW) |

S_{C,net} | Size of the cooling pipeline (kW) |

S_{cs} | Cooling storage size (kWh) |

S_{H,net} | Size of the thermal pipeline (kW) |

S_{H,net,c} | Size of the central DHN pipeline (kW) |

S_{hp,lim} | HP operation limits (kW) |

S_{ice,c} | Centralized ICE size |

S_{ice,lim,c} | Centralized ICE size limits (kW) |

S_{pvp} | Size of the PV panels equipment |

S_{stp} | Size of the solar equipment |

S_{stp,c} | Size of the central solar field |

S_{ts} | Thermal storage size (kWh) |

S_{ts,c} | Central thermal storage size (kWh) |

u, v | Generic unit |

v_{p} | Velocity of the medium inside the pipeline (m/s) |

V_{ts} | Thermal storage volume (m^{3}) |

wgt | Time interval weight |

X_{abs} | ABS existence (binary) |

X_{boi,c} | Central BOI existence (binary) |

X_{cp} | Existence of the cooling pipeline (binary) |

X_{hp} | HP existence (binary) |

X_{ice} | ICE existence (binary) |

X_{ice,c} | Centralized ICE existence (binary) |

X_{mgt} | MGT existence (binary) |

X_{net} | Existence of a network pipeline (binary) |

X_{net,c} | Existence of the central DHN (binary) |

X_{tp} | Existence of the thermal pipeline (binary) |

## Acronims

ABS | Absorption chiller |

BOI | Boiler |

CC | Compression chiller |

CHP | Combined cooling heat and power |

COP | Coefficient of performance |

CS | Cooling storage |

DCN | District cooling network |

DG | Distributed generation |

DHCN | District heating and cooling network |

DHN | District heating network |

HP | Heat pump |

ICE | Internal combustion engine |

MGT | Micro gas turbine |

MILP | Mixed integer linear programming |

PV panels | Photovoltaic panels |

ST field | Solar thermal field |

ST panels | Solar thermal panels |

TS | Thermal storage |

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**Figure 1.**Superstructure of the distributed generation solution (integrated with central solar system).

**Figure 2.**Plan of the possible path of the DHCN. 1—town hall; 2—theatre; 3—library; 4—primary school; 5—retirement home; 6—archive; 7—hospital; 8—school; 9—swimming pool; C—central unit site.

**Figure 4.**Hospital hourly energy demand patterns for a typical working day in winter (

**top**) and summer (

**bottom**).

**Figure 8.**Pareto front of the optimal distributed generation solutions, including the points representing isolated solutions (with and without TS) and the conventional solution.

**Figure 9.**Optimal DHN of the 60% environmental optimum—distributed generation solution. In red the DHN pipes have been reported together with their size (kW). The tables report the size of the components installed.

**Figure 10.**Superstructure of the distributed generation solution integrated with central solar unit.

**Figure 11.**Pareto front of the optimal distributed generation solutions integrated with central solar system.

**Figure 12.**Optimal operation of the storage in a typical week-economic optimization of the distributed generation solutions integrated with the central solar system.

**Figure 13.**Yearly optimal operation of the storage−70% environmental optimization of the distributed generation solution integrated with the central solar system.

**Figure 14.**Optimal DHN of the 70% environmental optimum—distributed generation solution integrated with the central solar system. In red the DHN pipes have been reported together with their size (kW). The tables report the size of the components installed.

**Figure 16.**Optimal DHCN of the 70% environmental optimum—complete distributed generation solution. In red the DHN pipes and in blue the DCN pipes have been reported together with their size (kW). The tables report the size of the components installed.

ELECTRIC | HEATING | COOLING | ||||
---|---|---|---|---|---|---|

USERS | Year Dem. | Peak Power | Year Dem. | Peak Power | Year Dem. | Peak Power |

(MWh) | (kWe) | (MWh) | (kWt) | (MWh) | (kWc) | |

Town Hall | 346,640 | 189 | 692,720 | 410 | 148,712 | 150 |

Theatre | 852,208 | 270 | 908,648 | 655 | 457,688 | 458 |

Library | 492,240 | 110 | 587,608 | 296 | 112,364 | 115 |

Primary School | 73,808 | 54 | 979,468 | 591 | 0 | 0 |

Retirement Home | 489,048 | 101 | 739,956 | 246 | 207,568 | 138 |

Archive | 82,516 | 36 | 429,604 | 238 | 78,652 | 91 |

Hospital | 3,284,416 | 628 | 7,884,141 | 1847 | 1,445,612 | 2087 |

Secondary School | 303,668 | 148 | 2,301,980 | 2084 | 0 | 0 |

Swimming Pool | 1,043,572 | 315 | 2,794,580 | 1425 | 297,416 | 435 |

Total | 6,968,116 | 1717 | 17,318,705 | 7017 | 2,748,012 | 3048 |

User Peak Power sum | 1851 | 7792 | 3474 |

Equipment | Unit 1 | Unit 2 | Unit 3 | Unit 4 | Unit 5 | Unit 6 | Unit 7 | Unit 8 | Unit 9 |
---|---|---|---|---|---|---|---|---|---|

MGT | 65 | 100 | 30 | 30 | 30 | 30 | 200 | 65 | 100 |

ICE | 70 | 140 | 50 | 50 | 50 | 50 | 200 | 70 | 140 |

ABS | 70 | 105 | 35 | 35 | 35 | 35 | 105 | 70 | 105 |

HP | 70 | 105 | 35 | 35 | 35 | 35 | 105 | 70 | 105 |

User | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|---|

Electric Peak (kW) | 189 | 270 | 110 | 54 | 101 | 36 | 628 | 148 | 315 |

Thermal Peak (kW) | 410 | 655 | 296 | 591 | 246 | 238 | 1847 | 2084 | 1425 |

Cooling Peak (kW) | 150 | 458 | 115 | 0 | 138 | 91 | 2087 | 0 | 435 |

Boiler (kW) | 294 | 479 | 217 | 418 | 205 | 179 | 1623 | 1673 | 1153 |

Comp. Chiller (kW) | 150 | 458 | 115 | 0 | 138 | 91 | 2087 | 0 | 435 |

Thermal storage (kW) | 544 | 375 | 312 | 766 | 173 | 298 | 690 | 2251 | 1564 |

User | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | Total |
---|---|---|---|---|---|---|---|---|---|---|

Natural gas (k€/y) | 44 | 58 | 37 | 62 | 47 | 27 | 498 | 146 | 177 | 1096 |

Electricity cost (k€/y) | 67 | 171 | 90 | 13 | 95 | 18 | 640 | 52 | 194 | 1340 |

Operating cost (k€/y) | 111 | 228 | 127 | 75 | 142 | 46 | 1138 | 198 | 371 | 2437 |

Maintenance cost (k€/y) | 1 | 2 | 1 | 1 | 1 | 1 | 11 | 2 | 3 | 23 |

Total investment cost (k€/y) | 58 | 141 | 43 | 33 | 47 | 35 | 597 | 127 | 188 | 1267 |

Annual investment cost (k€/y) | 7 | 18 | 6 | 4 | 6 | 4 | 77 | 16 | 24 | 163 |

Total annual cost (k€/y) | 120 | 248 | 134 | 80 | 149 | 51 | 1226 | 216 | 399 | 2622 |

Electricity emission (t/y) | 141 | 358 | 189 | 26 | 199 | 39 | 1341 | 108 | 407 | 2807 |

Natural gas emission (t/y) | 148 | 194 | 125 | 209 | 158 | 92 | 1677 | 492 | 596 | 3691 |

Total annual emission (t/y) | 289 | 551 | 314 | 236 | 356 | 130 | 3018 | 600 | 1003 | 6497 |

User | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | Total |
---|---|---|---|---|---|---|---|---|---|---|

Bought Electricity (MWh) | 396 | 1005 | 530 | 74 | 558 | 109 | 3766 | 304 | 1143 | 7884 |

Electricity user demand (MWh) | 347 | 852 | 492 | 74 | 489 | 83 | 3284 | 304 | 1044 | 6968 |

Electricity required by CC (MWh) | 50 | 153 | 37 | 0 | 69 | 26 | 482 | 0 | 99 | 916 |

Heat produced by BOI (MWh) | 696 | 911 | 590 | 984 | 741 | 432 | 7888 | 2312 | 2802 | 17,357 |

Thermal user demand (MWh) | 693 | 909 | 588 | 979 | 740 | 430 | 7884 | 2302 | 2795 | 17,319 |

Wasted heat (MWh) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

Cooling energy by CC (MWh) | 149 | 458 | 112 | 0 | 208 | 79 | 1446 | 0 | 297 | 2748 |

Cooling energy user demand (MWh) | 149 | 458 | 112 | 0 | 208 | 79 | 1446 | 0 | 297 | 2748 |

Wasted cooling energy (MWh) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

Economic Optimization | Environmental Optimization | ||||
---|---|---|---|---|---|

Conventional Solution | Isolated Solution without TS | Isolated Solution | Isolated Solution without TS | Isolated Solution | |

ICE (kW) | 0 | 2820 | 2840 | 4920 | 4920 |

MGT (kW) | 0 | 0 | 0 | 3900 | 3900 |

BOI (kW) | 6241 | 2145 | 984 | 609 | 431 |

ABS (kW) | 0 | 840 | 735 | 3570 | 3570 |

HP (kW) | 0 | 1750 | 980 | 3570 | 3570 |

CC (kW) | 3474 | 1274 | 1763 | 1948 | 2008 |

PV panels (kW) | 0 | 225 | 225 | 45 | 0 |

ST panels (m^{2}) | 0 | 0 | 0 | 1438 | 1800 |

TS (kWh) | 6973 | 0 | 15,016 | 0 | 36,000 |

CS (kW) | 0 | 0 | 0 | 0 | 36,000 |

Economic Optimization | Environmental Optimization | ||||
---|---|---|---|---|---|

Conventional Solution | Isolated Solution without TS | Isolated Solution | Isolated Solution without TS | Isolated Solution | |

CHP natural gas cost (k€/y) | 0 | 1458 | 1561 | 624 | 600 |

BOI natural gas cost (k€/y) | 1096 | 67 | 50 | 13 | 2 |

Buoght electricity cost (k€/y) | 1340 | 29 | 28 | 1216 | 1257 |

Sold electricity income (k€/y) | 0 | 365 | 490 | 138 | 140 |

Photovolatic incentive (k€/y) | 0 | 68 | 68 | 16 | 0 |

Operating cost (k€/y) | 2437 | 1121 | 1081 | 1699 | 1720 |

Maintenance cost (k€/y) | 23 | 120 | 128 | 53 | 52 |

Total investment cost (k€/y) | 1267 | 4288 | 4021 | 12,138 | 12,518 |

Annual investment cost (k€/y) | 163 | 421 | 395 | 1175 | 1206 |

Total annual cost (k€/y) | 2622 | 1661 | 1604 | 2927 | 2977 |

Reduction wrt conv. solution | 36.7% | 38.8% | −11.6% | −13.5% | |

Electricity emissions (t/y) | 2807 | 61 | 59 | 2545 | 2633 |

Sold electricity emissions (t/y) | 0 | 1363 | 1806 | 508 | 499 |

Natural gas emissions (t/y) | 3691 | 6769 | 7173 | 2844 | 2701 |

Total annual emissions (t/y) | 6497 | 5467 | 5427 | 4882 | 4836 |

Reduction wrt conv. solution | 15.9% | 16.5% | 24.9% | 25.6% |

Economic Optimization | Environmental Optimization | ||||
---|---|---|---|---|---|

Conventional Solution | Isolated Solution without TS | Isolated Solution | Isolated Solution without TS | Isolated Solution | |

ICE electricity | 0 | 11,563 | 12,455 | 4599 | 4591 |

MGT electricity | 0 | 0 | 0 | 331 | 175 |

PV panels electricity | 0 | 239 | 239 | 48 | 0 |

Bought electricity | 7884 | 173 | 166 | 7150 | 7395 |

Electric user demand | 6968 | 6968 | 6968 | 6968 | 6968 |

CC electricity | 916 | 137 | 191 | 398 | 394 |

HP electricity | 0 | 1042 | 628 | 3337 | 3399 |

Sold electricity | 0 | 3828 | 5073 | 1426 | 1,4012 |

ICE thermal energy | 0 | 16,906 | 18,133 | 6603 | 6628 |

MGT thermal energy | 0 | 0 | 0 | 566 | 299 |

BOI thermal energy | 17,357 | 1064 | 787 | 205 | 36 |

HP thermal energy | 0 | 2103 | 946 | 9279 | 9419 |

ST panels thermal energy | 0 | 0 | 0 | 1108 | 1387 |

Thermal user demand | 17,319 | 17,319 | 17,319 | 17,319 | 17,319 |

ABS thermal energy | 0 | 1733 | 1604 | 290 | 131 |

Wasted thermal energy | 0 | 1022 | 399 | 152 | 92 |

CC cooling energy | 2748 | 410 | 574 | 1194 | 1182 |

ABS cooling energy | 0 | 1127 | 1055 | 163 | 79 |

HP cooling energy | 0 | 1213 | 1121 | 1392 | 1488 |

Cooling energy demand | 2748 | 2748 | 2748 | 2748 | 2748 |

Wasted cooling energy | 0 | 2 | 1 | 0 | 0 |

Environmental Opt. | 90% Env. Opt. | 60% Env. Opt. | 30% Env. Opt. | Economic Opt. | |
---|---|---|---|---|---|

DHN pipes (n°) | 18 | 13 | 9 | 7 | 7 |

ICE (kW) | 1920 | 2190 | 2290 | 2590 | 28,403,900 |

MGT (kW) | 3900 | 0 | 0 | 0 | 0 |

BOI (kW) | 502 | 0 | 0 | 0 | 0 |

ABS (kW) | 3570 | 0 | 0 | 595 | 770 |

HP (kW) | 3570 | 2590 | 2380 | 1715 | 1050 |

CC (kW) | 1593 | 1682 | 1759 | 1620 | 1656 |

PV panels (kW_{p}) | 0 | 0 | 134 | 225 | 225 |

ST panels (m^{2}) | 1800 | 1800 | 734 | 0 | 0 |

TS (kWh) | 36,000 | 6316 | 8553 | 12,337 | 15,017 |

CS (kWh) | 36,000 | 0 | 0 | 0 | 0 |

**Table 10.**Total economic and environmental results of the optimizations—distributed generation solutions.

Environmental Optimization | 90% Env. Optimization. | 60% Env. Optimization. | 30% Env. Optimization. | Economic Optimization. | |
---|---|---|---|---|---|

CHP natural gas cost (k€/y) | 389 | 643 | 994 | 1296 | 1614 |

BOI natural gas cost (k€/y) | 0 | 0 | 0 | 0 | 0 |

Buoght electricity cost (k€/y) | 1603 | 1030 | 446 | 130 | 21 |

Sold electricity income (k€/y) | 76 | 82 | 126 | 264 | 539 |

Photovolatic incentive (k€/y) | 0 | 0 | 30 | 58 | 67 |

Operating cost (k€/y) | 1917 | 1591 | 1284 | 1105 | 1028 |

Maintenance cost (k€/y) | 37 | 57 | 84 | 107 | 132 |

Annual investment cost (k€/y) | 1519 | 410 | 378 | 381 | 397 |

Total investment cost (k€/y) | 17,422 | 4403 | 3968 | 4050 | 4178 |

Total annual cost (k€/y) | 3472 | 2058 | 1746 | 1593 | 1558 |

Reduction wrt conv. solution | −32.44% | 21.52% | 33.42% | 39.23% | 40.59% |

Electricity emissions (t/y) | 3358 | 2157 | 934 | 273 | 44 |

Sold electricity emissions (t/y) | 269 | 292 | 456 | 972 | 1981 |

Natural gas emissions (t/y) | 1710 | 2885 | 4461 | 5818 | 7244 |

Total annual emissions (t/y) | 4699 | 4750 | 4940 | 5120 | 5307 |

Reduction wrt conv. solution | 27.68% | 26.89% | 23.97% | 21.20% | 18.33% |

Environmental Optimization | 90% Env. Optimization | 60% Env. Optimization | 30% Env. Optimization | Economic Optimization | |
---|---|---|---|---|---|

ICE electricity | 3131 | 5190 | 8014 | 10,341 | 12,933 |

MGT electricity | 0 | 0 | 0 | 0 | 0 |

PV panels electricity | 0 | 0 | 141 | 239 | 239 |

Bought electricity | 9431 | 6060 | 2625 | 767 | 123 |

Electric user demand | 6968 | 6968 | 6968 | 6968 | 6968 |

CC electricity | 217 | 230 | 266 | 180 | 231 |

HP electricity | 4617 | 3232 | 2266 | 1560 | 532 |

Sold electricity | 760 | 820 | 1281 | 2729 | 5565 |

ICE thermal energy | 4619 | 7468 | 11,510 | 15,022 | 18,742 |

MGT thermal energy | 0 | 0 | 0 | 0 | 0 |

BOI thermal energy | 0 | 0 | 0 | 0 | 3 |

HP thermal energy | 11,581 | 8663 | 5406 | 3450 | 834 |

ST panels thermal energy | 1387 | 1387 | 566 | 0 | 0 |

Thermal user demand | 17,319 | 17,319 | 17,319 | 17,319 | 17,319 |

ABS thermal energy | 0 | 0 | 0 | 809 | 1704 |

Wasted thermal energy | 0 | 0 | 0 | 0 | 9 |

CC cooling energy | 652 | 689 | 797 | 540 | 692 |

ABS cooling energy | 0 | 0 | 0 | 523 | 1140 |

HP cooling energy | 2096 | 2059 | 1951 | 1686 | 917 |

Cooling user demand | 2748 | 2748 | 2748 | 2748 | 2748 |

Wasted cooling energy | 0 | 0 | 0 | 0 | 1 |

**Table 12.**Optimal configurations of the distributed generation solutions Integrated with central solar unit.

Environmental | 70% Env. | 30% Env. | Economic | |
---|---|---|---|---|

Opt. | Opt. | Opt. | Opt. | |

DHN pipes [n°] | 14 | 8 | 7 | 7 |

Central pipe size (kW) | 7500 | 6323 | 3579 | 1907 |

ICE (kW) | 4920 | 1840 | 2380 | 2500 |

MGT (kW) | 0 | 0 | 0 | 0 |

BOI (kW) | 3480 | 3408 | 2730 | 2023 |

ABS (kW) | 3570 | 1260 | 1155 | 1085 |

HP (kW) | 3570 | 1120 | 1225 | 1155 |

CC (kW) | 1584 | 1056 | 1053 | 1233 |

PV panels (kWp) | 225 | 225 | 225 | 225 |

ST panels (m^{2}) | 0 | 0 | 0 | 0 |

TS (kWh) | 0 | 0 | 2315 | 5134 |

CS (kWh) | 0 | 0 | 0 | 0 |

Central ICE (kW) | 0 | 0 | 0 | 0 |

Central BOI (kW) | 0 | 0 | 0 | 0 |

ST field (m^{2}) | 27,736 | 23,585 | 19,013 | 8035 |

Central TS (kWh) | 400,000 | 173,935 | 41,855 | 19,025 |

**Table 13.**Total economic and environmental results of the optimizations—distributed generation solutions integrated with central solar unit.

Environmental | 70% Env. | 30% Env. | Economic | |
---|---|---|---|---|

Opt. | Opt. | Opt. | Opt. | |

CHP natural gas cost (k€/y) | 86 | 741 | 1059 | 1339 |

BOI natural gas cost (k€/y) | 1 | 10 | 9 | 33 |

Buoght electricity cost (k€/y) | 1482 | 451 | 221 | 32 |

Sold electricity income (k€/y) | 30 | 125 | 234 | 373 |

Photovolatic cost (k€/y) | 75 | 53 | 55 | 66 |

Operating cost (k€/y) | 1464 | 1025 | 1000 | 965 |

Maintenance cost (k€/y) | 10 | 63 | 88 | 113 |

Total investment cost [k€] | 22,314 | 8248 | 6368 | 5359 |

Annual investment cost (k€/y) | 1760 | 705 | 569 | 453 |

Total annual cost (k€/y) | 3233 | 1792 | 1657 | 1531 |

Reduction wrt conv. solution | −22.32% | 31.64% | 36.89% | 41.61% |

Electricity emissions (t/y) | 3104 | 945 | 463 | 67 |

Sold electricity emissions (t/y) | 190 | 461 | 856 | 1385 |

Natural gas emissions (t/y) | 388 | 3362 | 4748 | 6268 |

Total annual emissions (t/y) | 3301 | 3846 | 4392 | 4950 |

Reduction wrt conv. solution | 49.20% | 40.80% | 32.41% | 23.81% |

**Table 14.**Total optimal annual energy magnitudes (MWh)—distributed generation solutions integrated with the central solar system.

Environmental | 70% Env. | 30% Env. | Economic | |
---|---|---|---|---|

Opt. | Opt. | Opt. | Opt. | |

ICE electricity | 693 | 5957 | 8482 | 10,956 |

MGT electricity | 0 | 0 | 0 | 0 |

PV panels electricity | 239 | 239 | 239 | 239 |

Bought electricity | 8718 | 2656 | 1302 | 188 |

Electric user demand | 6968 | 6968 | 6968 | 6968 |

CC electricity | 209 | 137 | 121 | 162 |

HP electricity | 1938 | 453 | 529 | 363 |

Sold electricity | 534 | 1294 | 2404 | 3889 |

ICE thermal energy | 1024 | 8547 | 12,284 | 15,979 |

MGT thermal energy | 0 | 0 | 0 | 0 |

BOI thermal energy | 11 | 161 | 146 | 529 |

HP thermal energy | 3991 | 675 | 866 | 497 |

ST panels thermal energy | 20,931 | 17,880 | 14,651 | 6191 |

Thermal user demand | 17,319 | 17,319 | 17,319 | 17,319 |

ABS thermal energy | 0 | 2316 | 2336 | 2341 |

Wasted thermal energy | 7850 | 6918 | 8018 | 2571 |

CC cooling energy | 626 | 410 | 364 | 487 |

ABS cooling energy | 0 | 1521 | 1522 | 1549 |

HP cooling energy | 2122 | 820 | 872 | 717 |

Cooling user demand | 2748 | 2748 | 2748 | 2748 |

Wasted cooling energy | 0 | 3 | 10 | 4 |

Environmental | 70% Env. | 30% Env. | Economic | |
---|---|---|---|---|

Optimization | Opt. | Opt. | Optimization | |

DHN pipes (n°) | 14 | 8 | 7 | 7 |

DCN pipes (n°) | 7 | 4 | 3 | 3 |

Central pipe size (kW) | 7500 | 4980 | 4118 | 1922 |

ICE (kW) | 4920 | 1840 | 2270 | 2380 |

MGT (kW) | 0 | 0 | 0 | 0 |

BOI (kW) | 12 | 1954 | 1406 | 1252 |

ABS (kW) | 3570 | 1435 | 1190 | 1120 |

HP (kW) | 3570 | 1890 | 1680 | 1680 |

CC (kW) | 778 | 250 | 174 | 306 |

PV panels (kWp) | 225 | 225 | 225 | 225 |

ST panels (m^{2}) | 0 | 0 | 0 | 0 |

TS (kWh) | 0 | 0 | 2176 | 4939 |

CS (kWh) | 0 | 0 | 0 | 0 |

Central ICE | 0 | 0 | 0 | 0 |

Central BOI | 0 | 0 | 0 | 0 |

ST field (m^{2}) | 22,736 | 21,764 | 17,664 | 8710 |

Central TS (kWh) | 400,000 | 169,926 | 30,980 | 20,366 |

**Table 16.**Total economic and environmental results of the optimizations—complete distributed generation solution.

Environmental Optimization | 70% Env. Opt. | 30% Env. Opt. | Economic Optimization | |
---|---|---|---|---|

CHP natural gas cost (k€/y) | 202 | 757 | 1026 | 1242 |

BOI natural gas cost (k€/y) | 0 | 7 | 4 | 33 |

Buoght electricity cost (k€/y) | 1474 | 486 | 218 | 38 |

Sold electricity income (k€/y) | 126 | 153 | 266 | 340 |

Photovolatic incentive (k€/y) | 75 | 53 | 56 | 66 |

Operating cost (k€/y) | 1475 | 1045 | 926 | 908 |

Maintenance cost (k€/y) | 10 | 60 | 88 | 107 |

Total investment cost (k€/y) | 24,806 | 8114 | 6909 | 5219 |

Annual investment cost (k€/y) | 1611 | 680 | 592 | 466 |

Total annual cost (k€/y) | 3095 | 1785 | 1606 | 1481 |

Reduction wrt conv. solution | −18.05% | 31.93% | 38.75% | 43.50% |

Electricity emissions (t/y) | 3087 | 1018 | 457 | 80 |

Sold electricity emissions (t/y) | 197 | 453 | 864 | 1157 |

Natural gas emissions (t/y) | 403 | 3262 | 4769 | 5974 |

Total annual emissions (t/y) | 3292 | 3827 | 4362 | 4897 |

Reduction wrt conv. solution | 49.33% | 41.10% | 32.87% | 24.63% |

Environmental | 70% Env. | 30% Env. | Economic | |
---|---|---|---|---|

Optimization | Opt. | Opt. | Optimization | |

ICE electricity | 726 | 5807 | 8510 | 10,412 |

MGT electricity | 0 | 0 | 0 | 0 |

PV panels electricity | 239 | 239 | 239 | 239 |

Bought electricity | 8671 | 2858 | 1283 | 226 |

Electric user demand | 6968 | 6968 | 6968 | 6968 |

CC electricity | 78 | 54 | 34 | 51 |

HP electricity | 2035 | 610 | 604 | 607 |

Sold electricity | 501 | 1257 | 2411 | 3234 |

ICE thermal energy | 1057 | 8313 | 12,241 | 15,187 |

MGT thermal energy | 0 | 0 | 0 | 0 |

BOI thermal energy | 0 | 112 | 63 | 527 |

HP thermal energy | 3974 | 820 | 916 | 1054 |

ST panels thermal energy | 17,520 | 16,771 | 13,612 | 6711 |

Thermal user demand | 17,319 | 17,319 | 17,319 | 17,319 |

ABS thermal energy | 0 | 2077 | 2431 | 2560 |

Wasted thermal energy | 4436 | 5930 | 6753 | 3439 |

CC cooling energy | 234 | 162 | 101 | 153 |

ABS cooling energy | 0 | 1377 | 1594 | 1676 |

HP cooling energy | 2537 | 1227 | 1074 | 933 |

Cooling user demand | 2748 | 2748 | 2748 | 2748 |

Wasted cooling energy | 0 | 4 | 8 | 5 |

**Table 18.**Summary of the different compromise solutions obtained for the different configurations considered.

Conventional Solution | Isolated Solution | Distributed Generation Solution | Distributed Generation Solution with Central Unit | Complete Distributed Solution | |
---|---|---|---|---|---|

DHN pipes (n°) | - | - | 9 | 8 | 8 |

DCN pipes (n°) | - | - | - | - | 4 |

Central pipe size (kW) | - | - | - | 6323 | 4980 |

ICE (kW) | - | 1840 | 2290 | 1840 | 1840 |

MGT (kW) | - | 0 | 0 | 0 | 0 |

BOI (kW) | 5241 | 984 | 0 | 3408 | 1954 |

ABS (kW) | - | 735 | 0 | 1620 | 1435 |

HP (kW) | - | 980 | 2380 | 1120 | 1890 |

CC (kW) | 3474 | 1763 | 1759 | 1056 | 250 |

PV panels (kWp) | - | 225 | 134 | 225 | 225 |

ST panels (m^{2}) | - | 0 | 734 | 0 | 0 |

TS (kWh) | 6973 | 15,016 | 8553 | 0 | 0 |

CS (kWh) | 0 | 0 | 0 | 0 | 0 |

Central ICE | - | - | - | 0 | 0 |

Central BOI | - | - | - | 0 | 0 |

ST field (m^{2}) | - | - | - | 23,585 | 21,764 |

Central TS (kWh) | - | - | - | 173,935 | 169,926 |

Operating cost (k€/y) | 2473 | 1080 | 1284 | 1025 | 1045 |

Total investment cost (k€/y) | 1267 | 4020 | 3968 | 8248 | 8114 |

Total annual cost (k€/y) | 2622 | 1604 | 1746 | 1792 | 1785 |

Reduction wrt conv. solution | - | 38.8% | 33.4% | 31.7% | 31.9% |

Total annual emissions (t/y) | 6497 | 5427 | 4940 | 3846 | 3827 |

Reduction wrt conv. solution | - | 16.2% | 24.0% | 40.8% | 41.1% |

© 2019 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**

Casisi, M.; Buoro, D.; Pinamonti, P.; Reini, M.
A Comparison of Different District Integration for a Distributed Generation System for Heating and Cooling in an Urban Area. *Appl. Sci.* **2019**, *9*, 3521.
https://doi.org/10.3390/app9173521

**AMA Style**

Casisi M, Buoro D, Pinamonti P, Reini M.
A Comparison of Different District Integration for a Distributed Generation System for Heating and Cooling in an Urban Area. *Applied Sciences*. 2019; 9(17):3521.
https://doi.org/10.3390/app9173521

**Chicago/Turabian Style**

Casisi, Melchiorre, Dario Buoro, Piero Pinamonti, and Mauro Reini.
2019. "A Comparison of Different District Integration for a Distributed Generation System for Heating and Cooling in an Urban Area" *Applied Sciences* 9, no. 17: 3521.
https://doi.org/10.3390/app9173521