System aspects

The energy demands of each house, and the neighbourhood as a whole, are met through the consideration and combined use of a pool of energy supply alternatives, including:

(i) DER, (ii) local energy sharing through microgrid and pipeline infrastructure, and (iii) central energy services. This determines the boundaries of the system, see Figure 3.1.

Note that not all units need to be installed but an optimal selection is made.

73

House 1

House 2 House n

tech

tech tech

Neighbourhood

IN OUT

RES

E import

NG import

E export

Emissions

Figure 3.1: Boundaries of the energy system of a neighbourhood with n^h houses, in-cluding technologies (tech) and energy integration (grey dashed arrows). E=electricity,

NG=natural gas, RES=renewable energy resource (e.g. sun, wind).

3.1.1 A hybrid small-scale poly-generation approach

Residential energy demands require small-scale, mini and micro units < 30 kW (see Table 1.1 and [242]). A generic pool of technologies is selected for consideration in the optimisation process. The chosen technologies are established, commercially available and able to exploit locally available resources. As new technologies become available, they can be added to the database. The considered technologies are presented below.

Appendix B summarises their technology operational behaviour.

Distributed generation (DG) units encompass small-scale electrical or thermal genera-tion units that can interact with energy sharing infrastructure and the central energy system. The considered intermittent units – based on renewable energy resources – are wall-mounted small-scale wind turbines (wind) and rooftop photovoltaic (PV) units (sun). The considered dispatchable DG unit is a small-scale combined heat and power unit (CHP). CHP units can come in various forms based on operational procedures and fuel [105, 131]. A natural gas fuelled CHP unit is selected due to its appropriate op-erational parameters and micro-range capacity [105]. By coupling a thermally driven refrigeration unit to the CHP unit, waste heat can also be used for cooling purposes [105].

Absorption chillers – requiring both waste heat and limited electricity for refrigeration – are the most established, suitable small-scale thermally driven cooling technology [105].

Several co-generation operational modes can be considered; electricity demand follow-ing or heat (indirect coolfollow-ing) demand followfollow-ing [105, 129, 131]. Since thermal demands mostly exceed electricity demands in residential applications, electricity-following modes

would require auxiliary condensing boilers for supplementary space heat generation [129].

Heat-following modes, in contrast, require additional electricity supply alternatives in order to meet varying electricity loads [129]. To fully use the installed CHP unit in com-bination with renewable electrical DG units, heat-following operation is implemented.

Additionally, conventional thermal generation units are considered that are only able to supply energy to their accommodating house, i.e. natural gas fired gas heaters or condensing boilers for space heating, electricity fuelled air-conditioning units for space cooling and a potential connection with the conventional distribution network (see Sec-tion 1.1.2). Furthermore, both electrical as well as hot and cold thermal storage are considered. Design choices determine whether storage can interact with external infras-tructure or can only be employed in the accommodating house. Figure 3.2 details the energy supply and interaction options of each individual house.

Eload

(b) Heating and cooling supply Figure 3.2: Black-box diagram of the considered energy supply alternatives for each house in the neighbourhood. Note that the CHP unit forms the link be-tween the electrical and thermal supply systems. AC=absorption chiller, airco=air-conditioning unit, B=boiler, CHP=combined heat and power unit, Cload=space cooling load, Cpipe=cold pipeline network, CST=cold storage, dump=dump load, Eload=electricity load, EST=electrical storage, G=gas heater, Hload=space heating load, Hpipe=hot pipeline network, HST=hot storage, MG=microgrid electricity shar-ing, NG=natural gas supply, PV=photovoltaic unit, WT=small-scale wind turbine.

black lines=electricity, double lines=heat, dashed lines=cooling, diamonds=DG units, circles=conventional thermal technologies.

3.1.2 An energy integrated approach

Energy efficiency improvements and cost savings can be achieved through energy in-tegrating a neighbourhood in terms of energy services and sharing [105]. Microgrid

infrastructure can be installed, allowing for sharing of locally generated electricity be-tween neighbourhood houses facilitated through a central control unit. Residential tri-generation, furthermore, allows for fully integrated thermal supply through optimised thermal pipeline networks with water as working fluid. A schematic of the energy in-tegrated approach is presented in Figure 3.3. Although an energy inin-tegrated model is adopted, the main focus is on the electrical system.

CCHP

Eload

Cload Hload EDG

EST

Grid

H/CT

H/CST

MG

Hpipe

Cpipe

Figure 3.3: Energy integrated system design with energy sectors electricity (black-/Eload), space heating (dark grey/Hload) and space cooling (light grey/Cload).

Energy integration through microgrid (MG) for electricity and optimised heat-ing (Hpipe) or coolheat-ing (Cpipe) pipeline networks. CCHP=combined cooling heat-ing power, EDG=electricity distributed generation units, EST=electrical storage,

H/CT=conventional heating or cooling technologies, H/CST=hot or cold storage

3.1.3 A superstructure black-box approach

DES design can be analysed through a systems thinking approach [80]. This facilitates flexible model building with building block components and interactions. Each com-ponent is represented as a black-box characterised by a set of operational and design parameters, which transform component power/energy inputs to component power/en-ergy outputs [34]. The interactions and relations of implemented component building blocks are hence analysed on a superstructure level with loss-dependent component in-teractions rather than detailed thermodynamic or electrical analyses [34]. Black-box system building blocks can be aggregated into a superstructure model, which allows for less variables and degrees of freedom for complex energy systems [34]. Superstructure

models allow for high-level design of complex systems with large numbers of components and interactions to aid decision-making. Figure 3.4 presents an example superstructure black-box approach of a section of the considered DES system, namely a single house with an installed CHP unit as main component. The optimisation approach (see Sec-tion 3.2.1) selects the implemented components and interacSec-tions for each neighbourhood house based on a pool of potential components and interactions, eliminating or adopting features of each neighbourhood house system (illustrated in Figure 3.2).

CHP η_el, η_th, HER Component I

Input Output

Natural

gas electricity

Input

Eload

heat Hload

Hpipe loss

AC COP/ECR

Component II Output

cooling heat

electricity

HST

loss heat

Figure 3.4: Superstructure black-box design of a section of the considered DES system with a CHP unit as main component. η_el=electrical efficiency, η_th=thermal efficiency, AC=absorption chiller, COP=coefficient of performance, ECR=electricity to cooling ratio, Eload=electricity load, HER=heat to electricity ratio, Hload=space heating load,

Hpipe=heating pipeline, HST=heat storage unit.