Generation of high resolution electricity demand and PV supply: Loughborough Model.
Richardson (et al, 2010) have developed an open source, freely downloadable, validated electricity demand model in the form of an Microsoft Excel worksheet. The model stochastically generates synthetic demand data at one minute time steps, and aims to represent the variability of the electrical demands in individual dwellings. It allows researchers to build up electrical demand profiles using individual appliances as the building block.
The model also incorporates the relationship between the activities of building occupants and energy use, which is important in the study of electrical demand and demand side management. Using a Markov-Chain technique the model generates stochastic occupancy data, simulating active occupancy patterns based on UK Time of Use Survey (2000) (Richardson et al, 2008).
A domestic lighting component is also incorporated into the electricity demand profile model. A key factor determining use of electric lighting in a building is human perception of natural light levels, and active occupancy. Outdoor irradiance profiles are used within the model to generate lighting demand profiles. These profiles come from the CREST irradiance database, providing high resolution irradiance data recorded on site at Loughborough University (Richardson et al, 2009).
For our own modeling purposes, we populated our dwelling with 4 occupants, a fixed appliance list and a fixed set of compact fluorescent lighting units. High resolution irradiance data for Dundee could have been substituted into the model for more accurate profiles for our geographic area of study. However, this data was not available without incurring a cost.
Based on Novas 300, a mechanical ventilation with heat recovery unit with the highest Passivhaus certified MVHR rates, 4 occupants would require an air change of 120m³ per hour @ 0.24 Wh per m³. This is equivalent to a constant demand of 0.48 W mins. So each minute, the MVHR acts as an electrical load of 0.48 W mins. Assuming the unit runs continuously (for simplicity), it consumes 252.3 kWh per year.
The heat pump electrical demand input is provided by the customized heat pump supply model.
For the generation of the photovoltaic supply model, we again rely upon the work of Richardson (et al, 2011). A PV generation model is integrated into the fully validated high resolution electricity demand model. Again, irradiance inputs play an important role in both demand and PV supply profile generation. High resolution clear sky surface irradiance and clearness index are used to simulate the rapid variations in PV output caused by changing weather and passing cloud cover. This model is capable of generating synthetic irradiance data for any given location. For the purpose of our profiles, we selected a Dundee location, Latitude 56.46, Longitude -2.97. The PV model takes into account the orientation of the panel using the tilt angle (40°) and azimuth of the array (180°), and the system efficiency (15%), and area of array (initially 1 m²).
The program was then run for each day for a 1 year period to generate the electrical demand and the PV supply profiles. A one minute time step creates 525600 data points over 1 year. However, this high resolution is important in order to see the high frequency variation of loads, and to avoid underestimating the proportions of electricity import and export which can occur when data is averaged over periods longer than 1 minute.
The annual electrical demand provide by the model was 4555 kWh, so the supply required from PV to meet net zero carbon requirement is also 4555 kWh. It would take an area of 74.5m² of PV panels to achieve this. This area takes into account 10% losses incurred in the PV to Grid transformer.
Each of the models developed by Richardson (et al) has limitations which are clearly expressed in the literature available on each model component. Example software is available at http://www-staff.lboro.ac.uk/~eliwr/
Richardson, I., Thompson, M., Infield, D., Clifford, C. 2010. Domestic electricity use: A high-resolution energy demand model. Energy and Buildings. Vol. 42, Issue 10, pp 1878-1887.
Richardson, I., Thomson, M., Infield, D., Delahunty, A. 2009. Domestic lighting: A high resolution energy demand model. Energy and Buildings. Vol 41, Issue 7, pp 781-789.
Richardson, I., Thomson, M., Infield, D. 2008. A high resolution domestic building occupancy model for energy demand simulations. Energy and Buildings. Vol 40, Issue 8, pp1560-1566.
Richardson, I., Thomson, M. 2011. Integrated simulation of photovoltaic micro-generation and domestic electricity demand: a one minute resolution open source model. Microgen II: 2nd International Conference on Microgeneration and Related Technologies. Glasgow. 4th – 6th April, 2011.
http://www.energystar.gov/ia/products/downloads/CFL_PRG.pdf
US Department of Energy - Energy Star Qualified Light Bulbs - 2009 Partner Resource Guide