To meet the target of a 100% reduction in the UK’s carbon emissions by 2050 [1
], it is necessary to reduce the carbon emissions associated with the residential sector, which accounts for some 17% of the UK’s total energy consumption in 2017 [2
]. The UK has set a target of nearly-zero-energy new housing in compliance with the EU directive [3
], by the end of 2020, which means ‘a building that has a very high energy performance’, where the ‘nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby’. The regulated energy in the UK is for heating, cooling, lighting and ventilation. Under the forthcoming regulation changes, renewable energy will also potentially contribute to the unregulated electrical appliance loads. Even so, in most cases there will still be a net energy import from the grid over the whole year [4
A zero-energy building (ZEB) can be defined as a net zero emission building in terms of carbon dioxide emissions, where the carbon emissions generated from grid-based fossil fuel energy use are balanced by the renewable energy generation on the building itself [6
]. Case studies in a number of countries have shown the potential for zero-energy housing [7
]. They are usually grid connected, and the zero-energy performance is based on the annual net zero input and output from the grid. There are also examples of off-grid zero-energy autonomous housing [9
], although this would be more challenging. Total autonomy is not necessary unless the building is off-grid: the combination of renewable energy supply and energy storage to provide total autonomy will generally be too expensive and impractical in relation to the size of systems needed. In most situations the grid is available, so it can be used. In future the grid itself will become increasingly decarbonised with a higher proportion of large-scale renewable energy generation connected to it, for example, from wind farms and large-scale solar [10
An energy-positive building can be defined, as a building where the total energy generated over the year is significantly greater than the energy needed to operate the building, including heating, ventilation, lighting and appliances [11
]. Energy-positive buildings are generally based on a Passivhaus approach to reduce demand [12
], with renewable energy generation a major feature [14
]. An early example is the Frieburg Solar Community [15
], which demonstrated that a whole estate could achieve energy-positive performance, although some individual houses could not due to design variations. Figure 1
illustrates the energy-positive performance in relation to energy demand and renewable supply.
A whole-house system approach integrates across passive design, efficient heating and ventilation, and the use of renewables [17
]. The use of heat pumps is generally applicable for energy-positive housing, where a lower-temperature heating system is more appropriate [19
]. Research has identified the importance of the balance between renewable energy and heat pump performance and opportunities for heat recovery in colder climates [20
]. Simulation has been used to show the potential of energy-positive performance for groups of houses using heat pump technology [22
There is currently a shift in European housing, from gas to electric heating, with the smart integration of solar PV (photovoltaic), heat pump technology and energy storage [23
]. The UK Committee on Climate Change report, UK housing: Fit for the future [24
] has recommended that from 2025 at the latest, no new homes should be connected to the gas grid. They should instead be heated through low carbon sources, have ultra-high levels of energy efficiency, alongside appropriate ventilation and, where possible, be timber-framed. It also recognises that addressing the ‘performance gap’ in new homes could save between £70 and £260 in energy bills per household per year.
Energy storage may be used to maximise the self-consumption of renewable energy used in the building [25
], including heating domestic hot water [27
]. Energy storage may be based on electricity, for example battery storage, or on thermal energy, for example energy stored in hot water, or in the construction materials of the building. Energy storage does not directly save energy, but it does reduce the amount of energy imported from the grid, which may contribute to reducing peak loads, and therefore ‘destressing’ the grid. From a householder’s point of view, in the UK, importing electricity from the grid incurs a higher cost compared to any payment received from exporting renewable energy to the grid. So from a householder’s point of view, the most economical solution is to be as near energy-autonomous as possible [28
Zero energy buildings can benefit from a modular approach [29
]. This may be a volumetric or panel-based system. Such offsite manufacturing has the potential to improve quality, reduce any performance gap, minimise waste and speed up the construction process. A modular construction approach is encouraged by the UK government to help deal with improving the efficiency of the construction industry [30
The transition from a nearly-zero-energy building to an energy-positive one should be marginal in terms of cost, as the basic requirements of reduced energy demand and renewable energy supply are common to both. An energy-positive building has a number of potential benefits. It complies with (and exceeds) the nearly-zero-energy target [3
]. It has cost benefits to the householder in relation to net zero annual energy bills, and the potential to be cost-positive by selling excess energy to the grid. Also, it helps to destress the grid and reduce peak loads.
Many studies relating to zero energy and energy-positive housing have been based on computer modelling. However, there is a need to test performance in use by monitoring of energy use and system performance, in order to identify any performance gap between design and operation [31
]. A number of studies have shown differences between simulation and as-built performance energy use [16
], with a performance gap between 13% and 250%. The gap is often related to technical performance issues, but the way the building is used also can significantly affect energy performance [36
]. It is therefore important to monitor innovative designs to understand how they perform in practice in comparison with theoretical predictions. Now that the European Directive is prescribing nearly-zero-energy buildings, there is a need to gather information on exemplary energy-positive buildings as a future potential transition [37
This paper describes how a combination of energy modelling and monitoring has been used to evaluate the smart operation for a low carbon energy region (SOLCER) house design, identifying any performance gap, and to help understand how an energy-positive performance can be realised in practice.
The paper is presented in four parts:
The main design elements of the house are described.
The energy simulation modelling is described, together with plugins for modelling specific elements of the design, including the transpired solar collector (TSC) and mechanical ventilation heat recovery (MVHR) system.
The energy modelling results are compared to measured data over a continuous period for the house used for office activities.
The energy model is then used to simulate the whole-house energy performance for typical household occupancy patterns.
The detailed stages of the methodology with outcomes are presented in Table 1
2. SOLCER House Design
The SOLCER house is a three-bedroom detached house of 100 m2
floor area, intended for the social housing market (Figure 2
). Although the house is detached, it would normally form part of a semi-detached or terraced development. The design of the SOLCER house used a number of technologies and design approaches that were developed through the Low Carbon Research Institute (LCRI) Low Carbon Buildings Programme [38
]. These have been optimised using a systems approach, through two stages of integration. Firstly, the electrical and thermal technologies were integrated, with solar PV and battery storage powering a heat pump, combining with a transpired solar (thermal air) collector (TSC) and mechanical ventilation heat recovery system (MVHR), to provide space heating and domestic hot water (DHW). The solar PV and battery storage also provide power for lighting and electrical appliances. Secondly, the energy systems were integrated into the building design, with the PV panels as the south-facing roof, and the TSC as the external south-facing, first-floor external wall finish.
The architectural design and construction of the house was carried out by Cardiff University. The house was designed based on available technology and using local supply chains. A major design criterion was affordability, and the estimated cost of replication at the time of construction was £1200/m2 of usable floor area, which was considered comparable to the then-current social housing costs (2015).
2.1. Building Design for Reduced Energy Demand
The energy demand of the house was reduced to a near ‘Passivhaus’ level [39
]. However, the design did not follow the Passivhaus standard rigorously, in order to allow the use of technologies and local suppliers that may not be compliant with Passivhaus accreditation requirements. The house was designed to have high levels of thermal insulation and a low air leakage. The house used a structural insulated panel system (SIPS) method of construction with 172 mm of climate EPS insulation contained between two layers of oriented strand board (OSB). The windows were double-glazed timber frame, with an aluminium external finish. The thermal insulation levels for the main design elements are summarised in Table 2
. The south-facing roof comprises of a large (34 m2
area) solar PV panel, which is fully integrated into the design of the building, with the southerly roof space, naturally lit through the transparent areas around the PV cells. This approach provides a different aesthetic, and potentially reduces costs, compared to a solar PV system ‘bolted-on’ to a standard roof. The north-facing roof is constructed from an SIPS panel with a standing seam metal cladding external finish.
The south-elevation first-floor of the house incorporates a transpired solar collector (TSC) with an area of 14 m2
, which is used to collect solar thermal energy, preheating the incoming ventilation air to the building to supplement space heating. External air enters the TSC through a grid of small (1 mm2
) holes in the metal cladding. These systems were previously investigated as part of the LCRI Low Carbon Buildings Programme [38
The building is modular in its design, based on a 0.6 m by 0.6 m dimension grid. The whole building took sixteen weeks to construct, taking place during the mid-winter period. The house is located at the Cenin ‘energy cluster’ industry park in South Wales, where there is a range of renewable energy generation systems, including solar PV, wind and anaerobic digestion. The house is used as a test facility and is currently fully occupied as an office and meeting space.
2.2. The Design of the Energy Systems
presents a schematic of the SOLCER house’s electrical and thermal system. The energy supply is all electric, providing space and domestic hot water heating and powering electrical appliances. The 34 m2
solar PV array has a capacity of 4.3 kWp. This is combined with a 6.9 kWh lithium-ion-phosphate Victron battery, which is located within the roof space. The battery and PV array are connected via a DC-coupled system, which is connected to an inverter to provide AC power to the house. The backup grid supply connects into the AC circuit. The PV and battery storage system provide power to the ring-main, LED lights and the heat pump and mechanical ventilation system. Electricity is drawn from the grid when there is insufficient power available from the PV and battery system. The aim is to maximise the use of the renewable energy within the house, and only export to the grid when all the house energy needs are fulfilled.
The thermal system comprises a transpired solar collector (TSC), mechanical ventilation with heat recovery (MVHR) integrated with an exhaust air heat pump and a thermal water store. This system provides space heating through the ventilation, with fresh air supplied mechanically to the main living spaces and extracted from the kitchen, bathroom and shower room. The building has a low heat demand and so can be heated through the ventilation system. For space heating, external air enters the TSC and is preheated from incident solar radiation, when available. The air then passes through the heat exchanger of the MVHR and then, if necessary, is topped up with heat by the heat pump. Exhaust air leaves through the MVHR, exchanging heat with the incoming supply air. It then passes to the evaporator of the heat pump, which heats both supply air and the thermal water store. The heat pump collects heat from the exhaust air, which is at internal air temperature (minus the heat exchanged through the MVHR), thus maintaining a relatively high coefficient of performance (COP) over the heating season. The exhaust air therefore can be considered to contribute to the space and DHW heating through the operation of the heat pump. When space heating is not required during warmer weather, the TSC and heat recovery stages are bypassed, and the MVHR acts solely as a mechanical ventilation supply air system. The heat pump is powered either from the solar PV coupled to battery system, or from the grid when there is no direct PV solar supply and the battery is exhausted. The MVHR, heat pump and thermal store are all contained within the single Genvex Combi 185LS EC unit. The thermal system details are presented in Table 3
The house has been continuously monitored on a 5 min interval, using the equipment using the sensors detailed in Table 4
. The monitored data have been used to make comparisons with simulations at component and whole house scale.
3. Building Energy Modelling: System Component Testing
Energy modelling was used both in the design of the building and in the evaluation of its performance. During the design stage, the solar PV and battery storage were initially sized using the buildings as power stations (BAPS) tool, which was developed during the early design stages to predict the balance between energy demand reduction, renewable energy supply and battery storage [41
]. However, for performance evaluation, a more detailed building energy model was needed to simulate the whole house performance, including all the elements of the heating and ventilation system. The analysis of the energy design was carried out using the building energy model, HTB2 [42
], with external plugins to simulate specific components of the energy system. This model is able to simulate the annual hourly thermal performance of the building, using local weather data, building construction details and occupancy profiles. HTB2 has been developed at Cardiff University over a period of nearly 40 years, and has undergone extensive testing and validation, including the IEA Annex 1 [43
], IEA task 12 [44
] and the IEA BESTEST [45
The thermal and energy system components of the SOLCER house have been added to the model, either through submodels within HTB2 (for the MVHR and TSC), or through separate ‘plugin’ submodels operated at the post-processing stage (for the heat pump, battery, thermal storage), as illustrated in Figure 4
. This has resulted in a ‘bespoke’ version of HTB2, specifically set up for modelling the SOLCER house. The modelling framework for the thermal and electrical system is summarised below.
3.1. Transpired Solar Collector (TSC)
The TSC is modelled as four vertical spaces within the HTB2, one above the other, as shown in Figure 5
a. External air enters each space and air passes upwards through the higher spaces until it is extracted from the top space and fed into the MVHR submodel. The total amount of input air is divided equally across the four spaces of the TSC. The TSC submodel simulations have been compared to measured data from the SOLCER House for the air temperature rise between the inlet (external air temperature) and the outflow of the TSC for the heating season (Figure 5
b). There is reasonable agreement between the simulated hourly values and the average measured data, indicating that the TSC submodel is providing a realistic simulation. It has been suggested [46
] that the relatively warm external air boundary layer of the TSC provides a useful contribution to the overall energy performance, in addition to the transfer of heat from the solar-heated metal surface in the TSC cavity. This might account for the small under-prediction (generally around 1 °C) compared with the measured results. The relative high exposure of the site to wind may also reduce the external boundary heat-gain. The TSC can deliver in excess of 20 °C rise in air temperature to the incoming air (for an incident solar radiation level of 600 W/m2
3.2. Mechanical Ventilation Heat Recovery (MVHR)
The MVHR has been modelled within the HTB2, using two spaces, one a path for the supply air and the other a path for the exhaust air. The spaces share a common metal ‘wall’ with a surface area equal to the sum of the heat exchange plate areas of the MVHR unit. A plate-surface heat transfer coefficient of 72 Wm−2
was selected as an appropriate value for airflow through a narrow cavity [47
]. HTB2 then simulates the hourly heat exchange between the exhaust air and the supply air. The results of the MVHR simulation are compared to measured data in Figure 6
. The temperature rise through the MVHR of incoming air is plotted against the difference between the inlet and outlet air temperatures. The agreement between measured and predicted results is good with a maximum difference of around 1 °C at the extremes of operation.
The performance of the TSC linked to the MVHR is presented in Figure 7
, as an hourly distribution of temperature rise, comparing predicted with measured data. The results imply that the level of fit between measured and predicted data is such that the model can be reliably used to assess the overall performance of the house with respect to the combined elements of the TSC and MVHR. The measured inlet and outlet flow rates were different from the manufacturers values (Table 3
). For the year 2015/6, the average flow rates were 41 L/s for the inlet and 31 L/s for the outlet, implying an over -pressure mode of operation. The measured rates were used in the simulation for the office situation in Section 4
3.3. Exhaust Air Heat Pump
The heat pump supplies heat as required to the incoming air for space heating, and for heating the thermal water store. The manufacturers stated COP value of the heat pump is 3.21 (Table 3
). The electricity needed to operate the heat pump is predicted on an hourly time-base for space heating, calculated by HTB2, and for hourly DHW demand, based on occupancy use. The DHW temperature is calculated based on its storage capacity, the use pattern and cylinder losses. When the predicted temperature of the water in the thermal store falls below 50 °C, the heat pump operates to raise the water temperature to a maximum 52 °C.
The Genvex Combi unit has an evaporator coil and two condensing coils. One condensing coil is used for heating DHW, and the other for space heating. The unit’s controls are set to prioritise either heat for DHW or for heating the supply air for space heating. It cannot do both simultaneously. The default setting prioritises DHW. However, the user can change the priority to space heating if required. Once the DHW tank is up to temperature, the unit will automatically switch to space heating, if it is required. If there is no demand for heat, the heat pump will switch off, but heat recovery through the MVHR will continue.
3.4. Thermal DHW Store
The thermal DHW store is activated between a temperature of 52 °C and 65 °C. The heat pump raises the DHW temperature to 52 °C, when it requires heat input. When excess solar PV electricity is available, after the demand from the heat pump, small-power-demand battery storage is met; it will be used to heat the DHW. It will raise the DHW temperature up to a maximum of 65 °C. The temperature required to eliminate Legionella bacteria is 60 °C, and if it is not met by PV electricity once a week, an electric immersion heater will operate. The DHW storage tank heat loss is calculated considering its thermal insulation.
3.5. Solar PV and Battery Storage, Inverter and Grid Electricity Supply
The solar PV array is simulated within the HTB2 from the predicted hourly solar radiation incident on the PV surface. A module efficiency of 15% was applied to the simulated incident solar radiation. The cell efficiency in standard test conditions (STC) is 19.6%, but this is reduced due to the spaces between cells in the module panel and the panel frame. The battery is allowed to discharge down to 20% of its capacity; discharging below this is detrimental to the lifetime of the battery. Figure 8
compares the measured and predicted PV energy generation for the second year of monitoring. The average measured value of 15.6% compares well with the manufacturers value of 15%.
A systems approach, integrating reduced energy demand, renewable energy supply and energy storage has been shown to have the potential to deliver an energy-positive house performance (Figure 16
). The integration of thermal and electrical technologies with architectural design features can potentially reduce costs and provide an improved aesthetic, in comparison to a more traditional ‘bolt-on’ technology approach which generally incurs a higher overall cost. The affordable construction costs make the house applicable for the housing market, especially for social housing, where low energy costs have greatest impact on householders.
The research has illustrated how a combination of energy modelling and detailed monitoring has led to a better understanding of how an energy-positive house performs and the relative contribution of its individual components combined within a whole systems approach. The analysis identified a performance gap, which was mainly attributed to infiltration between the heated space and roof space, disturbed thermal insulation in the ceiling and mechanical ventilation inlet and extract flows varying from their design specification. The performance gap identified would detract from the overall energy-positive performance, and potentially increase the annual heating costs by an estimated £140. This emphasises the need to ensure that a building is constructed to a good standard and that checks are made to identify any potential problems. This is especially the case with energy-positive performance where the energy demand is low and any performance gap can have a relatively higher impact on overall performance and the ability of systems to cope, in relation to their capacity to heat. A thermographic survey can identify major causes of poor thermal design and workmanship.
The study has indicated that, without a performance gap and with an efficient pattern of occupant use, the total electricity demand was predicted to be 4191 kWh/year (around 42 kWh/m2/year) with an annual grid import of 1112 kWh compared to the exported value of 1458 kWh. The building is predicted to import about 26% of its energy needs from the grid, but over the year its energy export to import ratio is 1.3:1. The annual space heating demand is 1464 kWh (14.64 kWh/m2), which is less than the 15 kWh/m2 Passivhaus target. The results indicate that the space heating and DHW heating accounts for some 44% (1852 kWh/year) of the total electricity demand, with the remaining 66% (2339 kWh/year) used for other electrical demand (i.e., lights and appliances). The DHW heat demand is higher than that for space heating. The energy-positive performance can be further improved using water-efficient equipment, producing an annual energy export to import ratio rise to 1.4:1.
The performance of the MVHR and TSC contributes an equivalent of some 75% to the space heating, while the MVHR on its own contributes some 72% to space heating. The TSC is therefore of marginal benefit. However, it is potentially a relatively low-cost item and also provides the external element to the construction. A future modification to the house would be to use the TSC for all-year-round DHW heating, and possibly as a source for inter-seasonal chemical heat storage.
The results show the potential for energy-positive performance with existing technology and for an affordable cost, provided the performance gaps can be addressed. The SOLCER house has provided a detailed understanding of energy-positive performance, which is being followed up by the Welsh Government’s Innovative Housing Programme on a number of new social housing schemes [52