Design, modelling, and simulation of solar photovoltaic driven air-conditioning systems for herbaria collection in Botswana

The demand for cooling has grown exponentially over the past century to meet economic development, availability, comfort and social needs, accounting for over 10% of the global electricity consumption and growing. Rising global temperatures have also contributed to the growing need for cooling in buildings. Cooling, is a very energy-intensive process that can account for 20% to 75% of a building's energy, depending on the building use. Solar photovoltaic (PV) driven air-conditioning offers a great cost-effective and environmental alternative for adoption in both residential and non-residential buildings to offset grid electricity, particularly in countries with high irradiation, such as Botswana.

This thesis explores the potential of a grid-assisted solar PV air-conditioning system for Peter Smith Herbarium at the University of Botswana (PS-HUB) Okavango Research Institute (ORI) in Maun, Botswana. Tourism and economic development in the Okavango region have contributed to the environmental strain and the loss of botanical species, some unique to the area and the world. The herbarium plays a critical role in collecting and preserving botanical collections for reference and research with data dating over 100 years. The herbarium requires special thermal comfort settings in terms of temperature and humidity to preserve the botanical collections and thus requires huge amounts of energy for cooling.

At the start of this study, the thermal comfort demand could not be substantiated in the herbarium storage. In fact, with recurrent power interruptions, and an inadequately designed air-conditioning system housed in a prefabricated structure, PS-HUB experiences severe challenges in keeping its primary operation mandate. This bares the risk that valuable plant specimen with immense importance faces severe deterioration over in time, which would be a great loss.

Therefore, this study explores the design, model, simulate and evaluation of the potential of a solar PV variable air volume (VAV) air-conditioning system in an energy-efficient herbarium building in Botswana using the DesignBuilder software and PVsyst. The PS-HUB is a case study for the research. Due to the herbarium’s specific needs, it operates throughout the day and year, in attempts to maintain a constant herbarium temperature of 16°𝐶. The research involved design of the building envelope and dynamic annual cooling load calculation using a typical meteorological year (TMY), and a supply air temperature of 14°C to meet the specified temperature needs. The variable-air- volume HVAC system was designed with a system rating of 30 kW to meet these needs.

The results of the study showed that the HVAC system accounted for 68.9% of the building's total electricity at 293 509.60 kWh annually. To offset the grid electricity, a 175.1 kWp nominal power rated PV system requiring 416 modules to match the required power, covering an area of 928 m2, which spanned the roof and carport areas to meet the HVAC system annual needs. The PV system produced a total of 329 535.61 kWh annually to meet the herbarium HVAC electrical demand, of which the extra electricity produced was fed into the grid at a fixed feed-in tariff of 0.45BWP/kWh determined by the Botswana Power Corporation.

An economic assessment using PVsyst found that for an installation priced with average solar PV prices in Botswana totalled to be 1 014 841.00 BWP, with annual operating costs of 33 000 BWP/year. With self-project financing, the project is estimated to have recouped its initial investment within 8.8 years. The PV system has able to meet the HVAC electrical load.