Techno-economic analysis of a Cold District Heating system using sea water energy,
based on a study case.
Maxime Bollinger
Master of Science Thesis TRITA-ITM-EX 2018:463
Techno-economic analysis of a Cold District Heating system using sea water energy, based
on a study case.
Maxime Bollinger
Approved Examiner
Björn Laumert
Supervisor
Rafael Guedez
Commissioner
ENGIE Cofely, France
Contact person
Cédric Maisonneuve
Abstract
When it comes to energy efficiency for heating and domestic hot water demand, district heating systems are known to be energy efficient due to centralization of the production. More recently, district cooling are in development to meet a growing demand. If district heating and cooling systems using centralized plant are strongly implemented as the dominant design, the purpose of this study is to investigate a variant solution using decentralized production: A Cold District Heating system consists of a network carrying tempered water to customers’ substations, to be used as a heat sink or source by reversible heat pumps for heating and cooling purpose. In this study, a CDH system is designed based on the study case and using literature review insights.
Then, a model able to simulate a CDH system is developed so that its behavior can be analyzed.
Finally, a cost-estimation methodology is used to perform a techno-economic analysis of the system. Insights from this study concerning key factors for a Cold District Heating system profitability enhancement will be highlighted.
Sammanfattning
När det gäller energieffektivitet för uppvärmning och efterfrågan på varmtvatten är fjärrvärmesystem kända för sin energieffektivitet på grund av centraliseringen av produktionen.
Mer nyligen utvecklas stadskylning för att möta växande efterfrågan. Om fjärrvärme- och kylsystem med centraliserad installation används i stor utsträckning som en dominerande konstruktion är syftet med denna studie att studera en variant med decentraliserad generation: Ett fjärrvärmesystem ska användas som kylfläns eller källa med reversibla värmepumpar för uppvärmning och kylning. I denna studie är ett CDH-system utformat utifrån fallstudien och med hjälp av litteraturrecensioner. Därefter utvecklas en modell som kan simulera ett CDH-system så att dess beteende kan analyseras. Slutligen används en kostnadsberäkningsmetod för att utföra en teknisk-ekonomisk analys av systemet. Lärdomarna från denna studie om de viktigaste faktorerna för att förbättra värmeanläggningens verkningsgrad kommer att markeras.
Figure 1: Market shares for heat supply to residential and service sector buildings in Sweden between 1960 and 2014 with respect to heat delivered from various heat sources. (Werner S.,
2017) ... 9
Figure 2: Layout of a district heating system using centralized production (ScandAsia, 2016) ... 11
Figure 3: Evolution of the TICPE for HFO in France ... 12
Figure 4: Examples of substrates which can be anaerobically digested to generate biogas (Abbasi T., 2011) ... 13
Figure 5: Simple layout of a geothermal energy system (27) ... 14
Figure 6: Schematic diagram of a typical geothermal BHE (Andrew D., 2016) ... 15
Figure 7: Schematic of a multi-BHE configuration (Andrew D., 2016) ... 15
Figure 8: General schematic of an open loop (on the left) and a closed-loop (on the right) surface water heat exchange system (Andrew D., 2016) ... 16
Figure 9: Cornell University free-cooling diagram ... 17
Figure 10: Example of ASHP ... 18
Figure 11: Comparison of typical chillers efficiency using different prime driver (Tredinnick S., 2016) ... 19
Figure 12: Schemes of a branched network (left) and a meshed network (right) ... 20
Figure 13: District heating piping network picture ... 21
Figure 14:Layout of the pipe for calculation basis ... 22
Figure 15: A typical customer substation ... 23
Figure 16: A CDH system scheme ... 25
Figure 17: General scheme of an adiabatic cooler ... 26
Figure 18: Example of a substation scheme for a CDH system ... 27
Figure 19:Layout of a multi-purpose heat pump system (Pellegrini M., 2016) ... 27
Figure 20: Location of Sète in the south of France ... 29
Figure 21: Yearly distribution of the buildings ... 30
Figure 22: Final map for the study case ... 31
Figure 23: Space heating, DHW and cooling demand for the district from 2020 to 2037 ... 32
Figure 24:Location of the three points of temperature measurements ... 33
Figure 25: Layout basis of the CDH system ... 38
Figure 26: Electric power absorbed by circulating pumps as a function of the flow rate of sea water ... 40
Figure 27: Sea water temperature throughout the year at the point of pumping “Bassin Orsetti” 41 Figure 28: Electricity consumption of the adiabatic cooler unit as a function of air temperature 43 Figure 29: Customer’s substation diagram ... 44
Figure 30: Efficiency of the heat pump as a function of the cold ring temperature and the outlet temperature ... 45
Figure 31: Cold ring flow rate and hot water demand represented for the 22th of January ... 49
Figure 32: Difference of temperature between the supply temperature and return temperature .. 50
Figure 33 Flow rate as a response to cooling demand throughout the 22th of July ... 51
Figure 34:End-users demand and flow rate in the cold ring throughout the 13th of April ... 51
Figure 35:Difference of temperature between supply and return and end-users demand throughout the 13th of April ... 52
Figure 36: Heating and cooling demand resulting on a total demand upstream of the cold ring, throughout the 13th of April ... 52
Figure 37: Balancing Factor ... 53
Table 1:Consumption and peak power per floor area unit for housing and tertiary buildings ... 32
Table 2:Peak power and consumption of the district in 2037 ... 32
Table 3: French subsidies for district heating network ... 34
Table 4: Cost per linear meter for trenches opening and closing and for pipes purchase ... 38
Table 5: Average sea water temperature and daily fluctuations at the point of pumping ... 41
Table 6: Water temperature needed on user-side ... 45
Table 7: Summary of the pricing model used to calculate the LCoHC ... 47
Table 8: Financial hypothesis used to calculate the NPV ... 48
Table 9: Results for scenario A1 ... 54
Table 10: Substation scheme for scenario A2 ... 55
Table 11: Results for scenario A2 compared to scenario A1 ... 55
Table 12: Power production distribution for the scenario B1 ... 57
Table 13: Adiabatic coolers production analysis for scenario B ... 57
Table 14: Financial results for scenario B1 ... 58
Table 15: Power distribution for scenario B2 ... 59
Table 16: Financial results for the scenario B2 ... 59
Table 17: Power distribution for scenario B3 ... 59
Table 18: Financial results for scenario B3 ... 60
Table 19: Production type & year of commissioning for scenarios C1 & C2 ... 60
Table 20: Adiabatic coolers production analyses for scenario C ... 61
Table 21: Financial results for scenario C1 & C2 ... 61
Table 22: Production type & year of commissioning for scenarios D1 & D2 ... 61
Table 23: Financial results for scenarios D1 & D2 ... 62
Table 24: Summary of financial results for every solutions ... 62
Table 25: Optimal solution configuration (C2) ... 63
Table 26: Results to the regard of criteria for the optimal solution. ... 64
Equation 1: Power absorbed by circulating pump ... 19
Equation 2: Heat losses ... 21
Equation 3: Thermal resistances ... 22
Equation 4: Return temperature optimization ... 24
Equation 5: LCoHC ... 35
Equation 6: NPV ... 36
Equation 7: Production ... 39
Equation 8: Consumption ... 39
Equation 9: Power harvested from the sea ... 40
Equation 10: Heat pump ... 44
Equation 11: Heat extracted from the cold ring ... 45
Equation 12: Heat rejected to the cold ring ... 45
Equation 13: Energy balance at a substation’s boundary ... 45
Equation 14: Price model ... 46
Equation 15: Annual minimum flow rate ... 50
NOMENCLATURE
Abbreviations
DH District Heating
DC District Cooling
DHC District Heating & Cooling CDHC Cold District Heating & Cooling
IWH Incineration Waste Heat
BHE Borehole Heat Exchanger
SWHP Sea Water Heat Pump
DVSP Distributed Variable Speed Pump
DHW Domestic Hot Water
LCoHC Levelized Cost of Heat/Cold
NPV Net Present Value
COP Coefficient Of Performance
Table of contents
ABSTRACT 1
SAMMANFATTNING 1
NOMENCLATURE 4
TABLE OF CONTENTS 5
1 INTRODUCTION 7
1.1 Background 7
1.2 Objectives 7
1.3 Methodology 7
2 THERORETICAL FRAMEWORK 8
2.1 District Heating and Cooling History 8
2.1.1 District Heating 8
2.1.2 District Cooling 9
2.1.3 Cold District Heating 9
2.2 District Heating and Cooling 10
2.2.1 Introduction 10
2.2.2 Centralised heat and cold production 11
2.2.3 Distribution piping 19
2.3 Cold District Heating 24
2.3.1 Concept introduction 24
2.3.2 Temperature of the loop adjustment 25
2.3.3 The cold ring 26
2.3.4 Substations 27
3 CASE STUDY 29
3.1 Current context 29
3.2 Load assessment 31
3.3 Sea water energy 32
3.3.1 Point of pumping 32
3.3.2 Regulation 33
3.4 Subsidies 33
3.4.1 Distribution network 34
3.4.2 Sea water energy production 34
4 METHODOLOGY AND MODEL DESCRIPTION 34
4.1 Method 34
4.1.1 Simulation software 34
4.1.2 Criteria 35
4.2 Model 36
4.2.1 In-house tools 36
4.2.2 The “CDH” tool 37
5 RESULTS 48
5.1 Cold ring behavior 48
5.1.1 Power needed 48
5.1.2 Flow rate and temperature 49
5.2 Techno-economic optimization 53
5.2.1 Scenario A 53
5.2.2 Scenario B 56
5.2.3 Scenario C 60
5.2.4 Scenario D 61
5.2.5 Optimal solution 62
5.3 Limit of the study and suggestion for further work 65
6 CONCLUSION 66
BIBLIOGRAPHY 67
1 Introduction
1.1 Background
Choices made in the past created a path dependence on fossil fuels in the composition of worldwide energy mix. Nowadays, there is a pressure that pushes the transformation of the energy system and therefore goals are set to help the energy transformation being a success. In order to achieve those goals, governments use subsidies to induce sustainable development and to replace un-efficient and polluting systems.
When it comes to heating efficiency, district heating systems are widely used in many countries because centralization of heat production provides greater efficiency and less serious environmental impact than localized boilers. On the same level, district cooling systems are more and more used as a response to a recent increase of cooling demand. Hence, district heating and cooling systems have been proven to be sustainable, efficient systems and economically viable.
In the context of energy efficiency, buildings are submitted to regulations leading to an increase of the use of low-temperature heating systems. This represents an opportunity to cold district heating system which is a variant solution combining centralized and decentralized production to provide both heating and cooling to consumers.
This Master Thesis has been written in the context of an internship performed at ENGIE Cofely in France and it comes from a specific need of ENGIE Cofely. ENGIE Cofely is a French leader company of energy services and in particular, ENGIE Cofely is used to develop and run district heating and cooling systems. Hence, ENGIE Cofely already possesses its own tools to size and estimate the cost of a classical district heating and cooling systems. Due to recent customers need, ENGIE Cofely is missing a tool able to perform techno-economic optimization on cold district heating systems and this Master Thesis work aims to fulfill that need. Using the developed tool, the final goal is to perform a techno-economic analysis on a real study case and for which ENGIE Cofely will make a commercial proposal.
1.2 Objectives
The final goal of this thesis is to assess the profitability of a Cold District Heating (CDH) system based on a specific study case. It should be mentioned that CDH systems are used only singularly and so very few literature is available concerning this technology. Hence, there is firstly a need to understand differences brought by a CDH system. Once enough knowledge has been acquired on CDH systems, intermediate objectives are targeted in order to reach the final goal of the thesis:
- Context assessment, including project specification, load profile, regulations, subsidies, - Cold district heating system design and sizing,
- Development of the system operation modelling, - Optimal behavior finding,
- Most profitable system determination.
Another purpose of this thesis is to show to project developers the potential of cold district heating systems as a sustainable and profitable solution to provide both heating and cooling to a district level.
1.3 Methodology
In order to reach the objectives, a methodology has to be followed. The first step is to understand how a classical district heating and cooling system work and what differences bring a cold district
heating system. Hence a literature review will be carried out to gain knowledge concerning those systems. Secondly, a cold district heating system will be designed using the insights from the literature review, setting a basis for the further work. Once the design of the CDH system has been set, physical interactions between every components of the system can be deducted in the purpose of modelling the system behavior to a specific end-users’ demand. Then the optimal behavior of the system is deducted from results analysis of the model. Finally, the better techno-economic solution is deducted using criteria defined in consistence with objectives of affordability, reliability and energy efficiency.
2 Theroretical Framework
2.1 District Heating and Cooling History
2.1.1 District Heating
Historically, the first district heating system appeared in the city of Chaudes-Aigues (France) back in 1332. Indeed, the city was lucky enough to benefits from 30 geothermal sources coming from the earth, with a water temperature between 45°C and 85°C, depending on the sources, and with a total flow rate of about 700 l/min. The system back then consisted of a piping system going inside 40 houses, allowing them to benefit from the first district heating system in the world (Woods, 2016).
In the late 19th century, the beginning of the what is called modern district heating appeared in the United-States, but first for another purposes : Industry needed electricity, but there was no electrical network back at the time and so hot steam was used to generate electricity. Some years later, the idea to use such a hot steam network for heating purposes became a reality in the US with the New-York district heating in 1882, as an example. Such district heating systems became, and is still nowadays, the dominant design for district heating in the US.
In Europe, district heating was also developed in late 19th, but it took another direct : After developing some projects using the American steam design, engineers discovered that using hot water instead of steam would present lots of advantages. In general, DH systems had to be optimize and engineers were facing issues to make DH systems efficient enough to be profitable, such as return temperature control, insulated pipe development, operational costs diminution. Another major issues is that every existing buildings were built with their own heating systems, and so DH systems had to adapt to every building heating system requirements in terms of temperature and pressure at delivery point : This would result in additional costs compared to what is required if temperature and pressure requirements were standardized.
The first relevant example of DH system in Europe is The DH of Copenhagen which was built in 1903 (State Of Green): Excess heat from a waste incinerator was used to supply heat to the Frederiksberg neighborhood. In Europe, difference clear between the development of district heating in the Nordic and eastern countries compared to the others. In particular, the Swedish, Finish, Danish governments spent a substantial amount of money for research and development of district heating. In Sweden, the first district heating system appeared in 1948 in the city of Karlstad and started the expansion of district heating systems in Sweden, and nowadays every major cities in Sweden have district heating, with a total current list of about 500 systems (Werner, 2017).
Figure 1: Market shares for heat supply to residential and service sector buildings in Sweden between 1960 and 2014 with respect to heat delivered from various heat sources. (Werner S., 2017)
2.1.2 District Cooling
As for district cooling, the idea of distributing cold though pipes came just after the beginning of DH systems, and so early attempts of DC systems appeared in 1880 in the US. But it is only in 1930 that the first commercial DC system was built in the US (District Cooling Guide, 2013). The first Swedish experience of district cooling was introduced in 1992 in Västeras, and nowadays almost 40 urban areas are equipped with district cooling (Werner, 2017). District cooling is mainly of used to the service sector buildings and industrial processes in Sweden, and is the result of the ban of CFC in existing chillers. As for France, district cooling is more developed than in Sweden as a result of a warmer climate, with the example of the Parisian cooling network which was built in 1991 and was delivering 400 MW to 500 customers. The Parisian cooling network is still in expansion and make a better use of its cooling power using the biggest ice storage system in Europe (240 MWh).
2.1.3 Cold District Heating 2.1.3.1 Principle
Cold District Heating and Cooling (CDH) allows the possibility to provide both heating and cooling using only one network : If there is both heating and cooling demand on customer’s side, and ff the classical DH and DC systems are used, two different network, one for the heat and one for the cold have to be built. Hence, four pipes has to be put in to the ground, which is of a high impact on the investment cost of the project. The principle of this new solution consists of water going through the network at a temperature between 10 and 35 °C and heat pumps installed in every buildings provide either heating or cooling according to the customers demand. It can be seen as an alternative solution for classical DHC solutions. CDH presents many advantages but it is not a widely developed solution, some example are given in the following sections.
2.1.3.2 Switzerland
The first example of CDH system can be found in the village of Oberwald (200 inhabitants) in Switzerland, and was introduced in 1991. The system get energy from a geothermal sources which has a drained water outflow of 90 m3/h at a temperature of 16 °C, exchanging energy with the
tempered water loop network (Pellegrini M., 2018). Local heat pumps are used to provide thermal energy to buildings with a total installed capacity of 960 kW thermal power.
2.1.3.3 Norway
The university of Bergen is equipped with a CDH using a centralized heat pump, exchanging energy with the sea water and delivering heat at intermediate temperature to an uninsulated heat distribution network. Local heat pumps use the tempered water as a heat source or a heat sink, to provide either heating or cooling to buildings (Stene J., 1995). This system was introduced in 1995 and back then the choice toward this solution was driven by many arguments such as an increased energy efficiency, reduced investment costs due to using only one uninsulated pipeline system, a high flexibility with regard to future expansion of the system.
2.1.3.4 Netherlands
Another example of CDH using the sea water as a heat source or heat sinks can be found in the city of The Hague (Netherlands). The production of thermal energy upstream consist of a seawater centrally supply unit with a heat exchanger (Goodier, 2013). The seawater heat pump is used during winter when the temperature of the sea water is too low because it is not possible for the local heat pumps to provide domestic hot water from a heat source at 5°C up to 65°C. This system provides heating and cooling to about 750 houses.
2.1.3.5 France
One of the biggest example of CDH system can be found in Marseille (south of France) which is an achievement from the French company EDF. Sea water is pumped at 5 meters deep in the sea and go through heat exchangers, to keep the temperature of the water loop constant. Even if the upstream energy production uses sea water energy, this example differs from the Norwegian and the Dutch system because there is no sea water heat pump: The temperature of the sea water is hot enough to maintain a high enough water loop temperature during winter. The thermal power installed is of 21 MW and it delivers thermal energy to buildings with a total surface of 500.000 m² (L’usine nouvelle, 2017).
2.2 District Heating and Cooling
2.2.1 Introduction
As explained in the previous section, historically district heating and cooling were using centralized production of energy, and this layout became rapidly the dominant design for DHC :
Figure 2: Layout of a district heating system using centralized production (ScandAsia, 2016)
Firstly, water is heated in the primary circuit in the heating plant, this water then goes to an heat exchanger and delivers calories to the cold water coming back from the customers. Then, the hot water goes to the customers through pipes and delivers heat to the customers, the heat being exchanged via an heat exchanger in customers’ substation. Requirements for temperature and pressure may vary depending on the customers substations: Historically, ancient buildings need hot water around 100 °C, while new customers’ substation need water between 50 and 65°C.
Generally, the inlet temperature at customer substations has to be around 7°C for cooling purpose.
It is essential that those requirements are known before designing a DHC network.
Various designs exist for each components of DHC system and will be presented in the next sections, starting with heat and cold production upstream of the network and finishing with the network itself.
2.2.2 Centralised heat and cold production
In this section the aim is to establish a summary of every solutions for producing heat and cold energy upstream of the network, along with their respective advantages and specificities, covering ancient fossil solutions to more recent renewables and sustainable solutions.
2.2.2.1 Heat production 2.2.2.1.1 Heavy Fuel Oil
Heavy Fuel Oil (HFO) was historically used in the aviation and maritime industries, it was also very commonly used for heat production purposes. Fuel boilers are highly efficient and the technology is well-known, with automatic driving and remote controls (Sipilä K., 2016). HFO heat production benefits also from a great flexibility and can easily adapts itself to a changing demand.
However, burning HFO produces greenhouse gas in the combustion process and secondly it contributes to reduce fossil fuel reserves. Even though the investment cost for HFO boilers and surroundings equipment is low compared to other solutions, HFO is becoming more and more expensive throughout the years, due to a tremendous growth of the taxes. The environmental part of the TICPE is proportional to the quantity of carbon dioxide rejected to the atmosphere during the combustion process, with a goal of 100 € per ton of CO2 rejected, in 2030 (French government, 2008). Hence, the TICPE treats each and every fossil fuel in accordance with its environmental
impact. In France, the corresponding taxes is called the “TICPE” and it is applied to fossil fuels.
The evolution of the TICPE per mass unit for HFO is presented in the following graph:
Figure 3: Evolution of the TICPE for HFO in France
To summarise, using HFO for heat production in a DH system is not in accordance with nowadays policy, HFO will be too expensive very quickly due to the strong taxes.
2.2.2.1.2 Natural gas & Biogas
Natural gas is also a fossil combustible and present many similar advantages as the HFO: Gas boilers and burners are also highly efficient and present great flexibility toward demand. It is a multi-purpose source of primary energy, so natural gas is being used for various application such as heating, industrial processes, electricity production or transportation (Pustisek A., 2017).
Natural gas is mostly constituted of methane (CH4), and it comes from the decomposition of plant and animal matter over millions of years. Another advantage of natural gas is that the distribution network is widely developed in Europe, and so using natural gas as a combustible allows to avoid transportation of primary energy. However, natural gas combustion also produces CO2 and so it is submitted to environmental taxes, such as HFO, proportionally to the quantity of CO2 rejected in the atmosphere. But natural gas remains a relevant solution for heat production, especially when considering the possibility to use biogas instead of natural gas in the coming decades, when natural gas will be too expensive due to taxes: Biogas is not submitted to the carbon tax TICPE in France (French government, 2008). Biogas is obtained by decomposition of organic matter such as natural gas, but biogas is the result of anaerobic decomposition (without oxygen) which differs from natural gas (Abbasi T., 2011). The following scheme shows examples of organic matter used to produce biogas:
0 5 10 15 20 25 30
2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023
TICPE (c€/kg)
Figure 4: Examples of substrates which can be anaerobically digested to generate biogas (Abbasi T., 2011)
Biogas is produced in “biogas digesters” or “biogas plant” which are composed of a mixing tank where organic matter is mixed, a digester where the mixture ferments to produce biogas through bacterial action and a storage dome to collect the biogas.
On one hand, using gas boilers for heat production appears to be an interesting alternative since it is associated to a cheap investment cost, great flexibility of power production, easy to handle and biogas offers the alternative to use a “sustainable” combustible from a tax point of view. On the other hand, with biogas being the logic alternative to natural gas consumers, it can be expected that biogas prices will rise in accordance to natural gas prices, even though biogas is not submitted to carbon taxes.
2.2.2.2 Heat and cold production 2.2.2.2.1 Geothermal energy
Geothermal energy is thermal energy stored inside the Earth, it comes from the formation of the planet and from radioactive decay of materials. The core-mantle of the Earth reaches a temperature of 4000°C and so heat is transferred from the center of the Earth toward its surface, allowing the possibility to drill through the ground and use this geothermal energy for heating and cooling purposes. Geothermal energy is used since decades and as previously said in the history section, the first worldwide DH system was using geothermal energy coming out from the ground at multiple source. Geothermal energy presents the advantage to be renewable and environmental friendly.
2.2.2.2.1.1 Deep Geothermal
Deep Geothermal energy consists of drilling two wells (a ‘doublet’) through the ground, one well for getting the hot water and the other well to reinject it into the ground. At the surface, a geothermal plant contains circulating pumps which inject water and extract it back from the
“reservoir” which is a volume around the end of the two dwells, used as a water warmer. A simple layout of a geothermal energy system is presented on the following layout:
Figure 5: Simple layout of a geothermal energy system
The depth of the two wells depend on the temperature and power required. Since it is an open circuit, a proper pressure management is required to minimize water losses in the surrounding rocks, at the end of the wells (Jefferson W., 2006). The main disadvantage of deep geothermal energy is that drilling is very expensive compared to the available power. On the other hand, deep geothermal systems have a small land area footprint and emit almost zero greenhouse gas emissions. Moreover, the technology used in such systems is simple and so they have low cost of maintenance. Another advantage of geothermal energy is its availability everywhere in the world, even if grounds are different depending on locations (which leads to different depths of drilling).
From an economical point of view, numerous projects of DH systems using deep geothermal energy as heat producer have viable levelized cost of energy, but they also show that the positive result of the project strongly depends to the drilling campaign: There is always uncertainty about the ground properties at the supposed depth, so deep geothermal projects always present a risk to not get the expected power. Hence, volcanically active locations are favored for geothermal project since high temperatures occur at low to medium depth of drilling.
But geothermal energy allows also to produce cold which is interesting for a DHC system: The same geothermal system is usable to heat a DH system during winter and to chill a DC system during summer. If this solution is chosen, then lower depth has to be aimed to make cold production feasible, using heating pumps system before the end-user. The possibility of producing cold energy is of a great advantage to geothermal solutions considering the recent growth of DC market, especially because the cost of operation of a geothermal plant is very low compared to the energy that is harnessed.
2.2.2.2.1.2 Borehole Heat Exchanger
A Borehole Heat Exchanger (BHE) refers to a closed-loop pipe assembly installed in a vertical
Figure 6: Schematic diagram of a typical geothermal BHE (Andrew D., 2016)
Typical scheme of BHE consist of a single U-tube grouted in a borehole, with typical diameters of about 20 cm. The borehole is usually backfilled with a bentonite-based grout to facilitate the sealing of aquifers and to enhance the contact area for heat transfer.
The thermal power that a BHE can harness depend on the thermal conductivity of the ground in which it is installed, and according to (BRGM, 2012) this value can vary between 20 and up to 55 W per linear meter of BHE installed. A quick calculation shows that one BHE of 150m can harness 6 kW (with the hypothesis of 40W/ linear meter of power), which is very low, that is why multi- BHE array system is used (depending on needed power):
Figure 7: Schematic of a multi-BHE configuration (Andrew D., 2016)
The complicating factor when building a multi-BHE array is the thermal interaction between the boreholes: The Earth temperature in the ground is constant if undisturbed but constant thermal
exchange leads to significant temperature variation around the BHE array. That is also why the power that can be harnessed by the BHE is so low, regulation were put in place in order to avoid irreversible transformation of the ground (BRGM, 2012). Even worst, if heating is the major thermal energy demand, there is a risk to freeze the ground in the BHE array. For this reason, BHE have to be very long compared to what they could be if freezing of the ground was not an issue. If the system is well balanced between heating and cooling demand, more power per linear meter can be harnessed by BHE. This could also be achieved with smart management of the ground resources. Another requirement of BHE array is to respect certain distance between every BHE in order to minimize thermal interaction between BHE: According to (BRGM, 2012) a minimal distance of 8 m has to be respected between each center of BHE.
Since the temperature of the ground is between 5 and 15°C, it is not hot enough during winter nor cold enough during summer to provide heating or cooling and so a heat pump system has to be used after the BHE array and before the end-user.
Hence multi-BHE system present the advantage to be renewable, have a very low cost of operation, and is able to produce both heating and chilling but this technology is facing major issues in terms of capital cost of investment for the BHE and for the surface needed (even if they are placed under the ground).
2.2.2.2.1.3 Surface Water Energy
Surface Water Energy consists of using surface water (from lakes, ocean, rivers) as a heat source or sink, it is not a recent idea but it gained popularity in the recent years in parallel with the growth of geothermal heat pumps (Andrew D., 2016). Two general configurations can be identified: Open- loop systems where water is directly pumped, and closed-loop systems which exchange energy inside the water, using heat exchangers. Schemes of open-loop and closed-loop surface water heat exchange system are presented below:
Figure 8: General schematic of an open loop (on the left) and a closed-loop (on the right) surface water heat exchange system (Andrew D., 2016)
It is possible to directly cool another water circuit with an open-loop system if the temperature of the water is lower than the temperature of the fluid circuit, this is called “free-cooling”. Usually, cooling systems in buildings are built to work with a 7°C inlet water temperature and so a cold enough water has to be pumped, usually water from deep-lake is used. An example of free-cooling is the Cornell University deep-lake water cooling system:
Figure 9: Cornell University free-cooling diagram
In the Cornell University free-cooling system, water is pumped at a depth of 80 m at the bottom of the lake, at a temperature of 4°C, and so water at 7°C is delivered at the university to be in accordance with the buildings cooling system.
However, heat and cold production is possible with a surface water heat exchange system combined with a heat pump. If this system is initially very simple, cost has to be added when considering following factors: Equipment have to be protected in accordance with the water properties (mineral properties, corrosivity), length of the pipe from the surface water to the heat exchanger. On the other hand, surface water energy present advantages such as very low environmental impact, low cost of operation, lower cost of investment than geothermal energy.
The largest sea water heat pump (SWHP) was installed at the Värtan Ropsten plant in the neighborhood of Stockholm, in 1980 (Friotherm AG). The sea water heat pump have a total capacity of 180 MW and produces water up to 80°C using the sea water as a heat source (sea water temperature of 0,5°C during winter).
When designing a SWHP system, an important parameter is the sea water temperature at the expected point of water intake: Depth and bathymetry of the intake point are two factors that influence water temperature profile. First, water temperature has an impact on the efficiency of the SWHP and secondly, other issues could be raised such as limitation of water temperature rejection due to regulations or freezing risk. Also, the intake water point should be placed such that it stays under water at all time, even with water movement events, in order to avoid air entry in the system.
It should also be placed 2.5m above the sea bottom to avoid bottom materials being sucked in the pipe (32).
Surface Water Energy seems to have a great potential for DHC purposes in the sense that it provides both heating and cooling with the same equipment, and it present a very low operational cost. However, it is conditioned by the presence of a significant enough source of water closed to the DHC network: According to (Zhen L., 2007), the pipeline length is the biggest part of the investment cost.
2.2.2.2.2 Cooling technologies
DC system is similar to DH system, it is composed of three major components: A chilled water production facility (centralized cooling plant), the distribution pipping (network) and customer substations. There are two categories of water chiller technologies: Mechanical vapor compression and thermochemical (Tredinnick S., 2016). Currently available chiller technologies include
centrifugal or rotary screw vapor compression machines and hot water or steam absorption thermochemical chillers. The difference between those two technologies is that mechanical vapor compression use a compressor the compress the working fluid, while thermochemical chillers use a heat source to compress the working fluid.
2.2.2.2.2.1 Thermochemical chillers
Thermochemical chiller is an interesting solution in the case that a free heat source is available during the cooling season. An example would be a DHC system using a solar heat plant for heating purposes combined with a thermochemical chiller. The most common and well known solution using thermochemical chiller is when a DHC system uses a waste heat source from the combustion process of a combined heat and power (CHP) system, allowing to not discard the heat during summer and to produce cooling energy without electricity.
2.2.2.2.2.1 Air Source Heat Pump
The classical solution for cooling purposes in recent buildings is to use an Air Source Heat Pump (ASHP) . ASHP uses the principle of vapor compression refrigeration using a refrigerant system involving a compressor and a condenser, an example of those system is presented below:
Figure 10: Example of ASHP
This type of mechanical vapor compression chiller system is well known and widely used.
Numerous mechanical vapor compression chiller technologies exist and they all have their specifications and advantage, allowing various utilization. Hence, lots of solutions for cooling are available when designing a DHC system and the best cooling solution will depend on various parameters such as local resources, temperature requirements, electricity prices, peak power of the demand.
A typical comparison of chiller efficiency is given in following table:
Figure 11: Comparison of typical chillers efficiency using different prime driver (Tredinnick S., 2016)
Electric centrifugal compressor machines have high efficiencies compared to thermochemical chillers (in the lower part of the table), justifying the use of thermochemical chillers only in situations where there is an almost free-heating source available.
2.2.3 Distribution piping
After that heat/cooling is generated in the plant, energy is transferred to the water in the pipe network using an heat exchanger located in the plant. Then, circulating pumps (also located in the plant) push the water in to the pipping network allowing to the hot/cold water to be distributed to end-users.
2.2.3.1 Circulating pumps
Distributed Variable Speed Pumps (DVSP) has been receiving increasing attention in recent years.
Indeed, in most DHC systems the end-user load is strongly dependent to outside temperatures and so using DVSP reduce considerably electricity consumption (Zeng J., 2016). The number and design of circulating pumps depend on the network pressure losses (H) and the circulating flow (Q). Usually, three similar pumps are installed with two pumps working and the resting pump is installed as a backup. Having multiple pumps permit also to avoid electrical consumptions due to inefficient pumping (36). If the pumps are working inefficiently, it means that the Total Dynamic Head (TDH) is higher than what it should be, and so pressure has to be lost in customers substation, leading higher cost of electricity.
The electrical power of a circulating pump is given by the following equation (36):
Equation 1: Power absorbed by circulating pump 𝑃 = 𝑄𝐻
3,74𝑅 With:
P (W): Absorbed power of the pump Q (m3/h): Flow rate
R: Efficiency of the pump 2.2.3.2 Pipeline network 2.2.3.2.1 General scheme
The pipeline network is usually designed as a branched network because it is the simple layout, leading to less pipe length. In a branched network every substation is fed by an unique way from the heat/cold generation to the substation. Hence, if an incident occurs somewhere on the network, lots of substations could be deprived from heat/cold (Narjot R., 1996). To remedy to this issue, meshed network can be built instead: Substations can be fed by multiple ways, and thus when an incident occurs only the touched segment is deprived of heat/cold. Schemes of those two possible network designs are presented below:
Figure 12: Schemes of a branched network (left) and a meshed network (right)
Usually, a branched network is the chosen design since it leads to lower cost of investment. Indeed, the pipe network account for a large share of the total investment cost of a DHC system so adding length of network greatly increase the capital cost of investment.
2.2.3.2.2 Pipe Structure 2.2.3.2.2.1 General structure
Usually, two pipes are used for every network, the “hot water” pipe and the “cold water pipe”, which means that a DHC system needs four pipes (two for heating and two for cooling). The pipes are built to accommodate thermal expansion and avoid outer corrosion (Werner S., 2017) and nowadays the most common method is to use prefabricated steel pipe with polyethylene casing and pre-insulated with polyurethane foam. This solution is the result of widely use of DH networks and presents advantages such as low distribution cost, low heat losses and high reliability. A picture of such a pipe structure is presented below:
Figure 13: District heating piping network picture
2.2.3.2.2.2 Thermal insulation
When distributing hot (or cold) water through the ground, heat is lost to the environment and that is why insulation is usually used on the pipes. Generally, heat losses are quite important for classical DH systems with values between 5-10 % of heat generated in the plant (Werner S., 2017).
In order to calculate heat losses in a DH network, the following formulas are used (Narjot R., 1996):
Equation 2: Heat losses 𝛷 = 2 × (𝑇𝑑+𝑇𝑟
2 − 𝑇𝑔) × 𝐾 with 𝐾 = 1
𝑅𝑠+𝑅𝑖+𝑅ℎ With :
Φ (W):Heat losses
𝑇𝑑 (°C): Departure temperature 𝑇𝑟 (°C): Return temperature 𝑇𝑔(°C): Ground temperature 𝐾 (𝑚.𝐾
𝑊 ) : Conductivity coefficient 𝑅𝑔 (𝑚.𝐾
𝑊) : Ground resistance 𝑅𝑖 (𝑚.𝐾
𝑊) : Pipe thermal resistance 𝑅ℎ (𝑚.𝐾
𝑊 ) : Exchange resistance between the two pipes
In order to calculate the conductivity coefficient K, the following equations have to be used:
Figure 14:Layout of the pipe for calculation basis
Equation 3: Thermal resistances 𝑅𝑔 = 1
2𝜋𝜆𝑠ln (4𝑍𝑐
𝐷𝑐) 𝑅𝑖 = 1
2𝜋𝜆𝑖ln (𝐷𝑝𝑢𝑟
𝐷𝑜 ) 𝑅ℎ = 1
2𝜋𝜆𝑠ln[1 + ( 2𝑍𝑐
𝐶+𝐷𝑐)2] with :
𝑍𝑐(𝑚) ∶ Distance from the center of the pipe to the ground 𝜆𝑠 (𝑊
𝑚𝐾): Thermal conductivity of the ground 𝜆𝑠 (𝑊
𝑚𝐾): Thermal conductivity of the insulator
The optimal thickness of the insulation is discussed in (Keçebaş A., 2011) according to various parameters such as type of heat production, annual weather (degree days) and diameter of the pipe, using a financial optimization process. Practically, pre-insulated pipes are used and so no insulation thickness optimization has to be done but it could be a relevant optimization to look at, especially for DH systems in very cold climates.
2.2.3.2.3 Pipe diameter
Diameter optimization of the pipes in a DHC system is primordial to its economic viability since the price per meter of pipe depend on its diameter. The two “design parameters” when it comes to DHC network design are pressure losses and speed acceptable on the network (Narjot R., 2016).
Usually, acceptable pressure loss in a hot water network should be around 1 mbar/linear meter and water speed between 1 to 3 m/s. Smaller diameter and high water speed lead to higher pressure losses, leading to higher electricity consumptions by the pumps.
Another parameter to consider is that the problem is different for main pipes or disadvantaged substations (far from the production) and for substations close to the production: Water arriving at substations close to the production will have high pressure due to low pressure drop on the way:
In this case a small diameter is advised to loss the excess of pressure in the pipe rather than at the customer substation, because it would lead to lower efficiency of the substation. Linear pressure drop is also dependent on the material of the pipe used and its roughness, it is deducted from the diagram for friction factor in pipes. For a certain roughness of the used material and Reynold number (characterizing the type of flow, dependent on the velocity), approximated equations can
A customer substation is the link between the DH network and the customer. There are two types of designs, the first one is direct DH system where the water from the DH system goes straight in to the customer building and heating system. The second one is indirect DH system where the water coming from the DH network and the water used in the heating system of the customer are physically separated and heat is exchanged through an heat exchanger. A typical customer substation (indirect DH system) is presented below:
Figure 15: A typical customer substation
It has to be mentioned that both system do not differ much in terms of efficiency on a higher level (Tippenergy, 2013). The customer substation required around 10 to 20 m² in the basement of the building, and it includes the heat exchanger, the arriving pipe from the DH system, departing pipes to the building system and some metering devices.
2.2.3.3.2 Temperature requirements 2.2.3.3.2.1 Hot service water
For DH system, if hot service water is needed by the customers (mostly the case for housing) then it sets the lowest required DH supply temperature (41): The tap water system heats up cold water at an incoming temperature of 5/15°C to an outgoing temperature of 60°C.
2.2.3.3.2.2 Space Heating
Over the years, the design temperatures of radiators system are going down. Nowadays, the trend is to go toward low temperature radiators system, so if ancient buildings temperature requirements are around 80/60 °C, recent buildings use a 60/40 °C system (Gummerus P., 2016). Future trends is to furnish buildings with low temperature heating system because it is more cost-effective and it is suited to newest buildings insulation, the required temperature will be between 35 and 55°C depending on the technology used. However, if hot service water is needed, it will sets the lowest temperature of supply from the DH system, as discussed in the previous section.
2.2.3.3.2.3 Space Cooling
The supply temperature of the cold water depend on the cooling devices of the customer but usually, buildings are outfitted with an HVAC system working with a 7/12 °C system.
2.2.3.3.Return temperature optimization
Return temperature optimization is one of the main concern of DH system because it strongly impact on the efficiency of the system. The reason is that the supplied heat power by the DH system is proportional to the difference of temperature between the delivery point and the return:
Equation 4: Return temperature optimization
𝑄 = 𝑚𝐶𝑝∆𝑇
∆𝑇 = 𝑇𝑠𝑢𝑝𝑝𝑙𝑦− 𝑇𝑟𝑒𝑡𝑢𝑟𝑛 With:
𝑄 (𝑘𝑊ℎ):Heat carried by the water 𝑚 (𝑘𝑔
ℎ) :Mass flow rate 𝐶𝑝(𝑘𝑊ℎ
𝑘𝑔.𝐾) :Specific heat capacity of water
𝑇𝑠𝑢𝑝𝑝𝑙𝑦(𝐾):Temperature of arriving water at the substation 𝑇𝑟𝑒𝑡𝑢𝑟𝑛(𝐾):Temperature of leaving water at the substation
Since the specific calorific capacity is a variable that cannot be changed, the two only levers for increasing or decreasing the heat power are the temperature difference at the substation and the mass flow rate through the substation. As the power delivered is proportional to the temperature difference, an abnormally high return temperature inevitably leads to a decrease of the power delivered (39). Furthermore, the temperature difference and mass flow rate settings are also critical to the efficiency of the system for the primary circuit as well as for the secondary circuit. Indeed, a high difference of temperature is needed in order to have an efficient heat exchange at the substation. Moreover, considering the equation above, increasing the temperature difference leads to a decrease of the mass flow rate which means lower energy need for pumping the water and energy savings. On the other hand, a low supply temperature is preferable in order to have a good heat production efficiency and the minimum heat losses (Cuadrado S., 2009). The return temperature then needs to be as low as possible in order to optimize the efficiency of the system.
This reasoning is also true for DC system, in general the higher the temperature difference between supply and return, the better the energy efficiency.
2.3 Cold District Heating
2.3.1 Concept introduction
CDH is not a common system and few literature are speaking about it, a definition was given by (Pellegrini M., 2018): A CDH is a system which distributes cold water in a temperature between 10-25°C to end-users substations, where the energy is used to produce both hot and cold water at different temperatures and for different purposes (space heating, cooling, hot service water production) via heat pumps.
Figure 16: A CDH system scheme
This scheme was elaborated by M. Pellegrini et al. and it includes the three major components of a CDH: The end-users’ substation, the network which is called the “cold ring” and the heat/cold production upstream. The heat/cold production is composed of the water extraction and discharge (represented on the bottom of the scheme) and the cold ring pre-heater. This scheme supposes that the water loop exchange energy with water coming from another source. This definition is restrictive in the sense that various heating/chilling solutions can be used to adjust the temperature of the loop upstream, such as heating/chilling solutions presented in 2.2.2.2.2.
In a CDH system, a substation is constituted of more than one heat pump (or one multi-purpose heat pump) and is producing heating or/and cooling according to customers’ demand. The heat pumps are exchanging energy with the cold ring via heat exchangers similar to those used in classical DHC substations.
On the Figure, three cases are presented for the substations working conditions: The first substation presents a higher temperature of return than supply, so the substation is rejecting energy to the cold ring, which means that in total the cooling demand is higher than the heating demand in this substation. The second substation is heating, and the third substation represents the case where the cooling power balance the heat power inside of the substation.
The third case is particularly beneficial for a CDH efficiency because no energy has to be put upstream of the cold ring to adjust its temperature. Cooling and heating demands inside the substation balanced themselves and in total only electricity is consumed by heat pumps to produce heat and cooling. This phenomenon also appears between the substations: On the previous example, substation 1 is heating the cold ring while substation 2 is heating the cold ring, hence both substations’ demands balance themselves partially (or completely).
2.3.2 Temperature of the loop adjustment 2.3.2.1 Cold water source
After every substations have rejected or extracted heat from the cold ring, the temperature of the water loop changed, and so the cold ring has to be either heated or chilled to bring back the temperature to its setpoint. M. Pellegrini et al. supposes that water source is used for this purpose as stated before, and it is the solution that many systems adopted. The reason is that the cold ring works with a temperature range of 10 to 25°C, which is temperatures commonly found in
numerous water sources. One of the problem could be that the water source is not warm enough in winter for heating purposes and so some systems use a centralized heat pump to heat up the water before exchanging energy with the cold ring.
2.3.2.2 Classical solution
However, since the temperature requirements for the cold ring in a CDH system is much less restrictive than for a classical DHC system with centralized production, every solutions presented in 2.2.2 for heating and cooling could also be used for a CDH system. Hence, CDH systems allow a very large panel of heating/chilling solutions to be used.
2.3.2.3 Adiabatic cooler
For cooling purposes in CDH systems, adiabatic cooler is a technology that has to be put forward because it is able to cool down water around 30 degrees with a low consumption process. A scheme of an adiabatic cooler is presented below:
Figure 17: General scheme of an adiabatic cooler
On the contrary to classical cooling technologies, an adiabatic cooler does not possess a condenser and a compressor, it works similarly to a fluid dry cooler but enhancing its capacity with adiabatic pre-cooling of the air intake (Lucas M., 2014). Dry air is carried out through the adiabatic cooler by the use of ventilators and first passes through the adiabatic pad which reduces the ambient dry bulb temperature by humidifying the air. Then, water is evaporated extracting the energy from the hot fluid, excess water leaves the adiabatic section through a gutter system to the sewer. This technology is a replacement for cooling towers and present many advantages. There is no water spray in the air flow so there is no legionella risk, which was the problem of cooling towers, the maintenance is quite easy, water consumption is low, and the temperature of cooling is lower than the ambient temperature (Jacir).
2.3.3 The cold ring
constitutes the major part of the investment of a DHC system, a CDH solution benefits greatly from this advantage.
2.3.4 Substations
End-users substation is more complex for CDH system than for classical DHC because the final production of heating/cooling is done inside the substation. Hence, the substation have to first exchange energy to the cold ring and secondly to produce energy. M. Pellegrini et al. propose a scheme for a CDH customer substation:
Figure 18: Example of a substation scheme for a CDH system
On the right bottom of the layout, energy is exchanged with the cold ring, then a multi-purpose heat pump produce heating or cooling, as well as domestic hot water (DHW), according to the end-user’s demand. Before going through the heating and cooling circuit of the end-user, water is stored in hot and cold storage tanks. The multi-purpose heat-pump is a concept introduced and explained by M. Pellegrini and al. and produces both heating and cooling, with only one interface with the cold ring, by the use of different valves and working conditions for every component. A layout for such a multi-purpose heat pump is presented:
Figure 19:Layout of a multi-purpose heat pump system (Pellegrini M., 2016)
Multi-purpose heat pumps can achieve electrical efficiencies up to 7, however a constant need for both cooling and heating demand is required, which is quite restrictive.
Another solution would be to install two classical heat pumps, one for DHW production (if needed) across the year, and the second one being a reversible heat pump that produces cold water during summer and hot water during winter.
However, those complex equipment that have to be installed in a CDH substation have a significant cost and require also space in the building. This last issue could present a problem for existing buildings which do not have dead space, leading to the use of CDH only for newly built district.
3 Case study
3.1 Current context
The case study used for this thesis is located in a city called Sète in the south of France. Sète is a small coastal city with a total of 92.000 inhabitants, but it keeps growing due to intensive tourism.
Figure 20: Location of Sète in the south of France
The specific context of the study case is that a new neighborhood will be built starting in 2020 and ending in 2037. This new neighborhood will be constituted of both housing and small businesses and the city of Sète expressed the will to implement a renewable system that provides both heating and cooling to the buildings. So, a cold district heating system will be looked at since it is able to provide thermal energy using lower primary energy compared to a reference system where classical boilers and chilling technologies are used.
As previously shown in the section 2.2.2, various ways of producing thermal energy upstream of the distribution network are available. However, Sète is a coastal city and so sea water energy will be used as the major thermal energy producing method for this study case. A scheme of the to be built district is given:
Figure 21: Yearly distribution of buildings
The previous map was the initial document and the some modifications have to be taken into account:
- Buildings will arrive with 3 years of delay, so the first building is expected for 2020, - Existing buildings are not taken into account because they already possess heating and
cooling equipment, and their heating system will not fit with low temperature produced by the CDH system,
- Buildings marked in orange are cancelled and so they are not taken into account,
- The three green buildings in the top of the map, expected for 2030 are not taken into account.
So the new scope of the study is presented below:
Figure 22: Final map for the study case
The final scheme can be divided in two parts: The south zone which is the first to come and for which exact surfaces and buildings type are known, and the north zone which is expected to come later (first building in 2023).
3.2 Load assessment
In order to assess the load for heating and cooling throughout the year, floor areas and type of the buildings are necessary input values:
- The total floor area for the south zone is 48.000 m² with a distribution of 72% of housing and 28% of tertiary buildings, including 31% of commercial buildings and 69% of offices, - The north zone is supposed to have a total floor area of 220.000 m² and to be constituted of 50% housing and 50% of tertiary buildings, including 50% of commercial buildings and 50% of offices,
- Only half of the housing will be equipped with space cooling.
According to the thermal regulation in France, new buildings have to respect low energy consumption and so the city of Sète announced following ratios for consumption and peak power for space heating, district hot water and cooling:
Housing Tertiary Space heating Consumption kWh/m²/year 18,50 16,00
Peak power W/m² 25,00 25,00
DHW Consumption kWh/m²/year 26,30 0,00
Peak power W/m² 10,00 0,00
Space cooling Consumption kWh/m²/year 40,00 40,00
Peak power W/m² 40,00 40,00
Table 1:Consumption and peak power per floor area unit for housing and tertiary buildings
Knowing the floor areas according to their type of building and the expected consumptions of those buildings, the final values for consumption and peak power are given in the following table:
Peak Power Consumption
kW MWh
Space heating 6706 4654
DHW 1450 3813
Space cooling 7831 7831
Table 2:Peak power and consumption of the district in 2037
The yearly consumption distribution is also plotted to have an idea of how demand will evolve throughout the years:
Figure 23: Space heating, DHW and cooling demand for the district from 2020 to 2037
This graph shows that demand will increase from 2020 to 2037 and so new investments will have to be made constantly from 2020 to 2037 in order to meet the energy demand. Staggering of the project is one of the most restrictive aspect and will be a major point of attention during this study.
The exact floor areas per building is not known and so it is assumed that every buildings have the same demand and power, leading to two different types of substations: 14 substations of the south type and 46 substations of the north type, for a total of 60 substations.
3.3 Sea water energy
3.3.1 Point of pumping
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
2018 2020 2022 2024 2026 2028 2030 2032 2034 2036 2038
Consumption (MWh)
Space heating DHW Space cooling
Figure 24:Location of the three points of temperature measurements
The study concluded that the most profitable location for pumping is the “Bassin Orsetti” point which is at a depth of 9 meters under water surface. Water temperatures was measured between 12 degrees in winter and up to 24 degrees in summer. The sea water temperature is high enough during winter to avoid the use of an ammoniac heat pump.
3.3.2 Regulation
The use of sea water energy is restricted by the French regulation, an environmental impact study has to be carried out in order to assess the influence of the project on the environment. Considering water temperatures of pumping and rejection the regulation states:
- The temperature of rejection should be under 30°C
- The temperature difference between collection and rejection should be less than 15°C - The temperature difference between rejection and receiving environment should be less
than 6°C
Since the receiving environment is the surface water, the temperature difference between collection and rejection should be around 5°C in order to keep a safe margin to the regard of the law.
3.4 Subsidies
In order to promote renewable and sustainable systems, France subsidizes district heating and cooling projects. Documents for subsidies requirements and calculations are available on the internet (ADEME, 2018).
3.4.1 Distribution network
A district heating network can be subsidized if the thermal energy produced by renewable energy is bigger than 50%. It is also required that an efficient network is used and in the case of a classical district heating network, the maximum temperature from departure is 60°C for a new DH system.
It imposes the buildings to use low-temperature space heating system which is more energy efficient. Also, highly insulated pipes have to be used in order to ensure low thermal losses.
The calculation of the expected subsidies for a district heating network depend on the diameter of the pipe and is directly proportional to its length:
Pipe diameter Subsidy mm €/linear meter
Up to 65 331
From 80 to 125 382 More than 150 522
Table 3: French subsidies for district heating network
Those subsidies are of a great help because the distribution network constitutes the greater part of the investment, as mentioned before. However, subsidies cannot be greater than 60% of the investment cost due to the network. District cooling networks are not subsidized yet in France, but it is not a problem since a CDH system uses only one network for both heating and cooling.
3.4.2 Sea water energy production
The production of renewable energy for district heating purposes is also subsidized by the government and for sea water heat pumps it is of 10 €/MWh of heating produced. There is no difference made between a classical DH system using centralized production and CDH, but the principle is the same since the difference is that production by the heat pumps is decentralized in the second option, so it is assumed that the subsidies for a CDH will be the same as for a classical DH system.
4 Methodology and model description
4.1 Method
In order to carry out a techno-economic analysis of the CDH system in the context of the study case explained in the previous section, the method used has to be described. First the simulation software will be presented, then criteria of optimization will be explained, and finally models used and developed during the thesis will be detailed.
4.1.1 Simulation software
In-house tools developed under excel and VBA are already operational for techno-economic analysis of classical DHC systems. Those models allow to try various solutions for every major components of a DHC system and to find the optimal solution for a system, according to specification:
