The present work analyses the potential of a combined solar thermal and heat pump (HP) system on new and existing multifamily buildings. It is based on a case study of a new housing complex located in Geneva (Switzerland), in which solar collectors are used either for direct solar heat production, via a heat exchanger, or as absorbers for the HP.
Monitoring over an entire year (2012) shows that the 68.1 kWh/m2 of heat demand is covered with a renewable heat fraction of 68%, with a 32% complementary electricity consumption. The relatively low annual SPFsys of 2.9 is inherent to a peculiar hydraulic configuration (single distribution loop for SH and DHW, with individual DHW storage), which does not allow for solar preheating of DHW, but only for DHW production above 55°C. Nevertheless, and due to the low heat demand, the system electricity consumption (HP and direct electric heating) represents an acceptable value of 23.0 kWh/m2.
As a complement, the developed numerical simulation model allows to understand the relatively low performance of the monitored case study, as well as to further analyse the potential of such systems, in particular for the case of building retrofit. The system components are modelled by way of energy balance considerations, and a specific effort is set on proper system integration and control strategy, taking into account the diverse possibilities and priorities of heat production, storage and demand.
Simulation results are validated with the monitored values, at component and system level. The model is shown to be sufficiently robust to reproduce the various energy flows, in terms of monthly profiles and yearly integrals.
In a first step, the validated model is used for a sensitivity analysis concerning the technical layout of the specific case study. We start by considering a standard system layout consisting of a centralized SH and DHW storage in the tech-nical room, with separate distribution for SH and DHW, which allows for proper solar preheating of DHW. As a conse-quence, the electricity consumption of the system drops to 15.7 kWh/m2 and the SPFsys reaches a value of 4.4. Second-ly, we analyse the possible added value of insulating the rear face of the unglazed solar collectors, which results in a higher efficiency in summer (increased solar gains), but lower efficiency in winter (reduced heat exchange with ambi-ent air, leading to lower evaporator temperature), so that the global SPFsys remains unchanged.
In a further step, we investigate the performance of the system for the case of diverse SH and DHW demands, as well as SH distribution temperature, in particular concerning its applicability in the case of building retrofit. For Geneva’s weather conditions, a sizing factor of 3 m2 solar collector per kW of HP capacity (HP sized for 100% coverage of heat demand) seems to be a good compromise between system size and system performance, resulting in a SPFsys between 3.1 and 4.1, depending on the SH distribution temperature. The associated electricity consumption strongly depends on the building heat demand: between 12 and 20 kWh/m2 for low energy demands (new building and best case retro-fit); around 30 kWh/m2 for less performant retrofit; around 45 kWh/m2 for a non-retrofitted building. Similarly, the specific collector area (m2 per m2 heated area) depends strongly on the building heat demand: around 0.10 for low energy demands (new building and best case retrofit); around 0.15 for less performant retrofit; around 0.20 for a non-retrofitted building. For multifamily buildings which do not benefit from a low SH and DHW demand (up to around 70 kWh/m2), implementation of such a system may hence be restricted not only by the associated electricity consump-tion, but also by the available roof area and the investment cost.
Finally, and to answer the question in the chapter’s title, a SPFsys of 5 could potentially be achieved, but only in newly constructed buildings with a high efficient envelope, a low SH distribution temperature, and with a collector area of at least 0.20 - 0.25 m2 per m2 heated area. Such may however be compromised by the available roof area, and the relat-ed investment may not be worthwhile given the rather small associatrelat-ed electricity saving (2 - 4 kWh per m2 heated area), as compared to the proposed sizing factor of 0.10 m2 / m2. This shows that a blind search for the highest SPF possible is misleading.
Chapter 4
Potential and constraints of available heat sources in relation to various building de-mands.
The content of this chapter will be submitted to a peer-reviewed journal (to be finalized after the Phd defence)
Abstract
This chapter covers a comparative analysis of the potentials and constraints of different heat sources (air, geothermal, deep lake, river, groundwater and solar), valorised by heat pumps (HPs) implemented in various types of multifamily (MF) buildings (new, retrofitted and non-retrofitted). After characterizing the various heat sources and building de-mands and presenting the numerical model, we study two distinct situations: i) combination of the HP systems and the MF building demands, disregarding possible constraints on available roof area (solar HP) or ground area (geo-thermal HP); ii) combination of HP systems and MF buildings, taking into account the aforementioned space con-straints, as well as possible photovoltaic (PV) production on the remaining roof area. For buildings with a combined space heating and domestic hot water heat demand up to 80 kWh/m2, which correspond to current best case build-ings, combined HP & PV systems should lead to an annual final purchased electricity inferior to 15 kWh/m2 (with a factor 2 between the resource extremes), with an associated daily peak load up to 150 Wh/m2/day. For such buildings, the decision of which resource to choose will most likely fall on other factors than the system’s energy performance (resource availability, legal restrictions, investment costs, social acceptability, system integration in existing buildings,
…). For buildings with a higher demand, the final purchased electricity can rise to 35 kWh/m2 and the daily peak load to 500 Wh/m2/day. Aside from the final purchased electricity, the annual electricity injected into the grid is in the order of 15 – 20 kWh/m2 for low-rise buildings, and half that much for high-rise buildings. As an exception, in solar HP systems, the reduced available roof area for PV leads to significantly lower values. If the entire MF building stock of the Canton of Geneva (19.3 million m2) was renovated to current best case buildings and would all use such HP sys-tems, the total final purchased electricity would remain below 300 GWh which represents 10% of the total Cantonal electricity demand. However, the associated daily peak load could rise up to 3 GWh/day, corresponding to 30% of the cantonal daily peak load. Lastly, SPF alone is not a sufficient indicator for characterization of the HP system perfor-mance, since it doesn’t reflect the absolute value of the electricity demand, which primarily depends on the building heat demand. Furthermore, both SPF and annual electricity demand are limited to annual balance considerations. As a complement, an indication of the peak electricity load gives valuable indications of the potential stress on the grid.
Potential and constraints of available heat sources in relation to various building demands.
Nomenclature
Abbreviations
DHW domestic hot water
HP heat pump
SH space heating
MF multifamily PV photovoltaic panels Latin letters
Ageo geothermal footprint per heated area, taking into account surface availability [m2/m2] Ageo.0 geothermal footprint per heated area, without considering surface availability [m2/m2] Asol specific solar collector area per heated area, taking into surface availability [m2/m2]
Asol.0 specific solar collector area per heated area, without considering surface availability [m2/m2] COP coefficient of performance [-]
Edir electricity, direct heating [kWh/m2]
Efinal electricity, purchased from the grid HP system consumption not covered by PV [kWh/m2]
Ehp electricity, HP [kWh/m2]
Einject electricity, production from the PV panels not self-consumed and injected into the grid [kWh/m2] Epv electricity, total production from the PV panels [kWh/m2]
Eself electricity, production from the PV panels self-consumed by the heat pump system [kWh/m2] Esys electricity, HP + direct heating [kWh/m2]
Gh.avg global solar irradiance (in horizontal plane), hourly average [W/m2] Gh.sum global solar irradiance (in horizontal plane), annual sum [kWh/m2]
Pmax,dem maximum hourly heat load, DHW + SH [W/m2]
Pmax,dhw maximum hourly heat load, DHW [W/m2]
Pmax,sh maximum hourly heat load, SH [W/m2]
Pnom,hp nominal capacity, HP (at 0°C evaporator input / 35 °C condenser output for the water/water HP,
2°C/35°C for the air water HP) [W/m2]
Pnom,sh.0°C nominal heat load, SH (at 0°C outdoor) [W/m2] Qdem heat demand, DHW + SH [kWh/m2]
Qdhw heat demand, DHW [kWh/m2]
Qdhw,is heat consumption for DHW demand (including storage and distribution heat losses) [kWh/m2] Qhp HP heat production [kWh/m2]
Qhp,dhw HP heat production, to DHW [kWh/m2]
Qhp,sh HP heat production, to SH [kWh/m2]
Qhp,st HP heat production, to storage [kWh/m2] Qres,hp resource heat , to HP [kWh/m2]
Qsh heat demand, SH [kWh/m2]
Qsol,dhw direct solar heat production, to DHW [kWh/m2]
Qsol,dir direct solar heat production, to SH + DHW + storage [kWh/m2] Qsol,hp solar collector heat production, to HP [kWh/m2]
Qsol,sh direct solar heat production, to SH [kWh/m2]
Qst,dhw storage heat discharge, to DHW [kWh/m2]
Qst,loss storage heat losses [kWh/m2]
Qst,sh storage heat discharge, to SH [kWh/m2]
Potential and constraints of available heat sources in relation to various building demands.
SPFfinal seasonal performance factor, combined HP and PV system
SPFhp seasonal performance factor, HP SPFsys seasonal performance factor, system Sthp upper DHW storage capacity [L/m2] Stsh SH storage capacity [L/m2]
Stsol lower DHW (solar HP system only) storage capacity [L/m2] Tavg temperature, hourly average [°C]
Tmax temperature, hourly maximum [°C]
Tmin temperature, hourly minimum [°C]
Tsh.0°C temperature of SH distribution, at 0° outdoor temperature [°C]
Tsh.15°C temperature of SH distribution, at 15°C outdoor temperature [°C]
Tsh.off temperature on/off set-point for SH [°C]
ΔGh.avg global solar irradiance (in horizontal plane), difference between hourly averages [W/m2] ΔGh.max global solar irradiance (in horizontal plane), difference between annual sums [kWh/m2]
ΔGh.ms global solar irradiance (in horizontal plane), mean square difference between hourly sorted values [W/m2]
ΔTavg temperature, hourly, difference between averages [°C]
ΔTmax temperature, hourly, difference between maximum [°C]
ΔTmin temperature, hourly, difference between minimum [°C]
ΔTms temperatures, mean square difference between hourly sorted values [°C]
Potential and constraints of available heat sources in relation to various building demands.
4.1 Introduction
In Geneva, the present CO2 emissions related to the energy sector represents 4.2 ton of emitted CO2 per capita, of which 2.2 emitted by the heating sector, 1.1 by the transport sector (not including the airport) and 0.8 by the electrici-ty sector (Quiquerez et al., 2016). Consequently the main CO2 emissions reduction potential lies in the heating sector, which represents about half of the final energy consumption in the city (Quiquerez, 2017). In 2014, the energy con-sumed by the heating sector in Geneva amounts to 5’444 GWh or 40.6 GJ/capita, mainly based on fossil fuels (Figure 4:1).
Figure 4:1 Geneva final energy consumption, in 2014.
Energy consumption for the heating sector surrounded by the red dashes (source: Quiquerez, 2017).
Even though MF buildings only constitute 27% of the Geneva building stock, they represent almost half of the heated floor area of the canton, namely 19.3 out of 40.9 million m2 (Khoury, 2014). About half of these MF buildings, which were built between 1946 and 1980, are nowadays in need of retrofit and possess a strong energy saving potential.
In parallel to reducing of the heat demand of the building stock, in particular by way of retrofit, reducing of the CO2
emissions can also be achieved by replacing fossil fuels by renewable energies.
In Geneva, the potential of renewable resources for the heating sector has been estimated at 5’500 GWh/year (Faessler, 2011), not including the heat potential of the outdoor air, which is difficult to quantify. The main potential resources are shallow geothermal energy (1’000 GWh) and the thermal energy of the lake (4’000 GWh). Although the potential of renewable energy resources covers the heating demand of the Canton, their integration in the energy system can be limited by: (i) their spatial availability, temporal dynamics and quality (e.g. temperature); (ii) their costs;
(iii) their social acceptance.
One of the possible options to valorise locally available renewable heat sources, independently of their quality (e.g.
temperature), is by using heat pumps. Heat pumps can be centralized (connected to district heating, i.e. at a regional scale) or decentralised (at building level, i.e. at a local scale/ individual). Nowadays, neither of these solutions occupy an important place in Geneva’s heating sector (1% of supplied heat demand, see Figure 4:1). Nevertheless, prospec-tive scenarios show that the development of heat pumps (to supply 15 to 25% of the heat demand of the Canton) in combination with other heat supply solutions and reduction of the heat demand, would reduce the Canton’s CO2
emissions to the goals aimed for the year 2035 (Quiquerez et al., 2016).
Potential and constraints of available heat sources in relation to various building demands.
In this regard, and as a complement to the numerical sensitivity analysis concerning solar assisted HP systems (Chapter 3), we will focus on a comparative analysis of the potentials and constraints concerning: 6 different heat sources (air, geothermal, deep lake, river, groundwater and solar), valorised by way of HPs implemented at a local scale, for 6 types of MF buildings (2 new, 3 retrofitted, 1 non retrofitted), based on monitored case studies situated in Geneva.
After a characterization of the various studied heat sources and building demands, we present 2 distinct system lay-outs: i) specific solar assisted HP system; ii) generic HP system for other resources. After recalling the main features of the previously described numerical model and describing the new elements (in particular concerning geothermal boreholes), we study two distinct situations:
In a first step, the combination of the 6 HP systems and the 6 MF building demands disregards possible con-straints on available roof area (for solar HPs) or ground area (for geothermal HPs). The simulation results are discussed at HP and system level, as a function of the various heat sources and building demands.
In a second step, the combination of HP systems and MF buildings takes into account the aforementioned space constraints, as well as the possible PV production on the remaining roof area. The simulation results are discussed at system level, in relation to the self-produced electricity and of the complementary electricity purchased from / injected into the grid.
Finally, the results are discussed in relation to the MF building stock and the total electricity consumption of the Can-ton, in terms of annual balance as well as daily peak loads.
Potential and constraints of available heat sources in relation to various building demands.