In 2018, when George Berbari, CEO, DC PRO Engineering, decided to build his house, he sat down and defined a set of broad objectives. One of them was to minimise energy and water use to the lowest level by using the latest commercially feasible technologies. Another was to offset the energy used by the building by generating power onsite through dual-purpose solar PV structures, such as carport and gazebo, to optimise the use of space and to preserve the aesthetics.
A third objective was cost control – of having a simple payback period of fewer than five years for incremental investment through energy and water savings. A fourth objective was to create a knowledge base available to the public that would produce the best-in-class green design. The result is what Berbari describes as his near-zero villa, in the Dubai Hills development. The house has a total gross floor area of 1,510.7 square metres, plus a 70-square-metre swimming pool; a total conditioned area of 991.6 square metres; and a connected electrical load of 78.94 kW (52.3 W/m2 gross area; 79 W/m2 AC area). The power consumption and solar PV generation amount to 75,000 kWh/ year, or per AC area of 75 kWh/m2 /year. The villa has an installed AC capacity of 17 tons (59.8kW) and an actual peak of 12.4 tons (43.6 kW), or per AC area of 80 m2 / ton (43.9 W/m2).
Among salient features is a clean roof concept that has a solar PV gazebo, high-reflectance roof materials, and peripheral roof planters with water-saving, low-height plants that are irrigated by recycled grey water. Another salient feature is the orientation of the building. Due to the land orientation and Dubai Hills master plan, the villa’s main entrance faces north-east, whereas the rear side faces south-west.
The glazing on the south-west is reduced to a minimum, and the first-floor balcony and ground floor are overhung to provide shading. All walls and hardscape have high-reflectance index. Speaking of glazing, the villa’s design optimises the envelope.
Typically, glazing in Dubai transmits over 12 times the heat compared to insulated walls. The villa uses standard double-glazing glass but has a low glazing-to-wall ratio (less than 20%). It uses neutral glass with light transmission over 70% to maximise daylight. It also has in place insulated rolling shutters to close during unoccupied hours, which reduces up to 80% the losses through glazing during unoccupied hours. Yet another salient feature of the villa is the use of insulated walls and roof. The use of 300mm autoclave aerated blocks wall followed by 30mm-thick rockwool fire-rated insulation produces a U Value of 0.3 W/m.K, compared to 0.57 W/m.K – the current Dubai regulation.
Further, the villa has combo roof and balconies insulation and waterproofing using 70mm-thick sprayed polyurethane, which is covered by a 50mm-thick screed. All these features support the air conditioning system of the villa, the highlight of which is a geothermal heat rejection system, which Berbari is particularly happy to point out. “Dubai is blessed with a high-water table with a relatively low water temperature of 32 degrees C, and has developed low-cost, eight-inch manual well-drilling techniques, which are resulting in water make up for the cooling tower of 2m3 /day,” he says. In addition, the heat rejection of the chillers and the geothermal water are used simultaneously to heat the swimming pool in winter by more than 700 kW of heat daily using less than 30 kWh of electricity for the pump and saving more than 150 kWh a day, in the event a typical air-cooled heat pump is utilised.
The villa’s geothermal cooling system is an open-loop system for water chiller heat rejection. The open-loop system comprises two wells – one for intake, with a submerged pump at the bottom, and another well to return the water to the same aquifer. It requires to reach the groundwater to have the highest heat rejection capacity. “In the Gulf, the depth can vary from as shallow as 10 metres to as deep as 20 metres,” Berbari says. The six-inch borehole can have up to 15 tons capacity, and larger boreholes of 10 inches or 12 inches have been used with up to 400 tons of cooling capacity. Berbari says the performance of the geothermal system is based on data – geothermal water temperature versus depth – collected over a year. “The data has indicated temperature consistency all year round,” he says.
The villa has a cooling tower, which is used as a backup, in the event of maintenance of the well or the submersible pump, as well as to provide thermal relief, in the event of raised underground water temperature due to prolonged operation. The air conditioning system also includes radiant cooling, which operates with higher chilled water temperature (14 degrees C), reducing compressor lift and power, resulting in power savings of 2kW.
The hydronic radiant systems in the villa are air-and-water systems that separate the task of ventilation and thermal space conditioning by using primary air distribution to fulfill the ventilation requirements and by using secondary water distribution to thermally cool the space – sensible heat. The radiant cooling systems reduce the amount of air transported through the villa significantly, particularly in bedrooms, where there is no air circulation at all; ventilation is provided by treated outside air systems as well as recirculated air to help in dehumidification in central areas of each flat of the villa. The radiant cooling system provides half the cooling requirement of the villa, using chilled water (from 14 degrees C to 19 degrees C) as the transport medium.
The villa also uses ceiling, wall and floor radiant cooling, which provides cold cave[1]like comfort, better temperature control and zoning. Traditional AC ductworks dictate the false ceiling height – typically 50-70 cm – while radiant cooling height is limited to 15cm. Further, radiant cooling eliminates 100-200 watts/ton of fan motor energy, which is needed in the case of a typical AC system. Other features of the air conditioning system are fresh air and exhaust air heat recovery, and dehumidification. The villa has an ultra-high-efficiency heat wheel (>80%) with dedicated chilled water loop for dehumidification, resulting in power savings of 2kW. The air conditioning system provides free swimming pool heating and cooling. The villa uses the chiller condenser heat rejection or geothermal energy for heating during winter, and free evaporative cooling from the cooling tower during summer.
It has the option of chilled water swimming pool cooling, which to date, was needed only for four days during the summer, where it utilised less than 28 ton-hours (100 kWh) a day. Further, it uses hot water heat pump and heat trace for piping network to optimise domestic hot water generation energy. Inside the house, the kitchen has a gas oven, as opposed to an electrical cooker. The oven provides 80% primary energy efficiency, as compared to the 35% for the cooker. The villa’s air conditioning system also includes home automation and PLC industrial controls, inclusive of sensors, electric, BTU and water sub-metering to control and monitor power and water usage.
To realise the near-zero-energy (low-carbon) dream and achieve minimal electricity bills, the villa exports solar PV power to offset most of the imported grid power. As mentioned earlier, the optimised villa results in minimal power consumption – estimated at 82,000 kWh – and a situation of offsetting by PV power generation.
The villa contains 142 solar PV dual-glass panels – 77 for the roof solar gazebo, 15 for the garden solar gazebo, 28 for the solar carport and 22 on the flat top roof. Each PV panel generates 275 watts, or 54,000 kWh/year, which is sufficient to cover 70% of the home electricity consumption. Other innovations in the villa include the use of a packaged, pre-insulated modulating control system for all FCUs and radiant cooling distributors, and the elimination of P-trap drainage and replacing it with an innovative M-Valve. The villa posed architectural challenges, given the objective of incorporating elements that would contribute to the near-zero-energy goal. The first challenge was designing a modern villa with the least glass opening possible – a glass-to-wall ratio of around 18.2%.
The second challenge was integrating the solar panels in the design while keeping the elegant look of the villa intact. The third challenge had to do with materials selection; considering the heat in the summer, and the rain in the winter, it was essential to arrive at a mix involving ceramic tile cladding, paint and low-E glass. In the end, Berbari says, it was worth it, because the villa surpassed the requirements of all green certifications. For instance, he says, LEED Platinum requires >80 points and >2,000 kWh of renewable energy produced per year.
The villa, he says, is achieving 85 points and is generating 37,500 kWh of renewable energy per year. Likewise, it meets Dubai’s Sa’fat Platinum conditions, which require 10% of renewable energy, whereas the villa achieves 100% renewable energy. Further, it more than meets Estidama 5 pearls requirement of exceeding 1,000 kWh/year of renewable energy. Perhaps, just perhaps, it makes sense to have a villa atop a plantroom.
LIVING GRACE
Here, George Berbari presents the one-year data compiled from two summers at his near-zero-energy villa…
OVING into a high-tech villa is not just bliss but also amazing grace, combined with hard work of adjusting or fixing errors or unforeseen circumstances. The one-year data for the 1,000-square-metre air conditioned space and 500 square metres of outdoor terraced areas and balconies and 70 square metres of the outdoor pool is now compiled. It is worth noting that the entire 1,000 square metres of indoor space was kept between 21 degrees C and 22.5 degrees C all year round; and the humidity was kept in the 60- 65% range.
The villa had hot water available all year round and reached the farthest tap within 10 seconds. The motorised, insulated rolling shutters, on coming down, provided outdoor noise damping and full light blackout; when combined with indoor noise-free radiant cooling, they provided us with the ultimate sleeping experience. The living areas felt like cold caves with the ceiling, walls, and floor chilled, while all central areas were provided with treated fresh air and additional dehumidification fan-coil units to assist in humidity control. The entire house felt pleasant with no or very little variation between rooms and spaces. In addition, the swimming pool was heated almost from the beginning of November 2021 till the end of April 2022 through a combination of geothermal heat (70%) and heat rejected from the water-cooled chillers.
It was cooled for most summer months by free evaporative cooling through a cooling tower; we had to cool it with chilled water – taking less than 50 ton-hours (175 kWh) per day only for four days. It is worth noting that swimming pools in the UAE are dominated by heating energy. We had to heat it by 700 kWh on the coldest days, and the swimming pool temperature dropped to around 27 degrees C on the coldest windy day.
The greywater recycling and rainwater harvesting, artificial turf and geothermal cooling combined to offer over 55% water savings. The average water consumption was 2.7m3 /day, whereas the swimming pool makeup averaged 650 litres/day, or 9.3mm losses per day (24.1%), and the irrigation and WC makeup consumed 1.36m3 /day, out of which 0.3m3 /day was recycled water and the balance 1.06m3 / day (39.3%) was Municipal potable water. The cooling tower makeup averaged 310 litres/day (11.4%), and the domestic water was barely 680 litres/day for an average of five occupants, out of which the hot water constituted 152 litres/day. The 280m2 artificial turf, if replaced with natural turf grass, would have added 2.8m3 /day on its own.
The hot water heat pumps worked amazingly well, delivering year-round 55 degrees C hot water with one-hour night temperature boosting by an electric heater for legionella control. The entire heat pump’s annual production was 1,184 kWh thermal, and the electric consumption of 313.4 kWh per year. For every kW of energy, the heat pump delivered 3.78 kW of thermal energy and 2.78 kW of cooling energy to cool and dehumidify the MEP room. Hence, for every 1 kW of electricity, the villar received 6.56 KW of thermal energy, making it one of the most efficient applications of heat pumps in the world. To put it in context, typically, these units are installed or ducted outdoors in Europe, delivering barely 2 kW of heat for each kW of power; and the cooling energy is wasted. In addition to the 313.4 kWh/year heat pump consumption, the single pipe heat tracing that maintains the hot water in the pipe consumed an estimated 1,500 kWh/ year (0.171 kW) of electricity.
Although that sounds high, when compared to the heat pump itself, an alternative circulating pump would have consumed 0.25 kW per hour, or 2,200 kWh, and the thermal losses from the return pipe would have added another 1,000 kWh of heat loss. The average winter temperature lift was 21 degrees C, whereas the average summer temperature lift was 15 degrees C. Intentionally, I have kept the interesting data to the end. The air conditioning system operated 40% as sensible radiant cooling, with an independent chilled water system operating at a supply of 14 degrees C.
The balance 60% was done through dehumidification and cooling system, with another independent chilled water system operating at a supply of six degrees C. The radiant cooling was expected to work more; however, the stubborn high humidity in Dubai forced the six degrees C system to operate extensively – more than originally anticipated. That said, the peak load was recorded on July 4, just before my family and I left the house on July 5 for a summer vacation, where the peak cooling combined load was merely 11.65 tons, and the entire day was 212 ton-hours, or 18.2 equivalent full load hours (FLH). This made the villa the most efficient unit of housing in a tropically hot and humid climate, where the performance exceeded the 80m2 /ton (44 W/m2 ) mark. When we chilled the swimming pool, the peak load reached 17 tons.
The annual cooling energy was slightly above 40,000 ton[1]hours/year or 3,435 FLH/year. The above are unprecedented numbers; however, the efficiency was slightly worse than the selection – the anticipated and annual combined chiller plants’ efficiency was under 0.90 kW/ton, whereas the actual for total cooling energy was 1.186 kW/ton, with the radiant cooling system (14 degrees C) averaging 1.158 kW/ton, and the chilled water system (six degrees C) averaging 1.205 kW/ton, taking into account upper adjustment of electric meter inaccuracy to reflect the actual worst-case scenario. The fresh air fans consumed 2,232 kWh/year or 0.092 kW/ton, and the operating six FCUs consumed 3,112 kWh/ year, or 0.128 kW/ton, which meant the total chilled water system consumption was 1.425 kW/ton, inclusive of the air side, and the radiant cooling system proved to be one of the world’s most comfortable ACs, and one of the most efficient, offering 16.7% energy saving compared to a chilled water system operating at similar conditions.
The variance in the chiller plants’ performance was mainly due to three reasons:
• The chiller manufacturer’s deviation from selected data by more than 20%, as these small, water-cooled chillers are rare and mostly tailormade and their performance is not certified by a credible authority
• Higher than expected pumping energy, and operation at a higher temperature of geothermal wells in summer, and due to heating requirement of the swimming pool in winter.
• All centralised AC systems, such as District Cooling, chilled water plants and VRF systems, suffer from parasitic partial load There is a potential saving with relation to the chiller plant – of 7,000 kWh per year with better chiller design and operation optimisation. The entire villa had one of the lowest power consumption profiles of an all[1]electric residential villa in tropically hot and humid climate – slightly lower than 75,000 kWh per year, or 75/kWh/m2/year, based on an air-conditioned space of 1,000 square metres and excluding the impact of 500 square metres of outdoor space and a 70-square-metre swimming pool; and from that, 33,000 kWh of solar-produced PV power needs to be deducted, resulting in a net imported consumption from DEWA of 42,000 kWh/year.
A typical villa in Dubai averages 150kWh/m2 /year, or 150,000 kWh/ year of power consumption for indoor equipment and another 35,000 kWh for swimming pool circulation pump, swimming pool heat pump and outdoor lighting, totalling 185,000 kWh/year and costing around AED 85,000/year, in addition to 6m3/day of water, with natural turf costing an estimated AED 25,000/year, or a total of AED 110,000/year for electricity and water. The near-zero energy villa delivered a low cost of utility (electricity and water) of AED 25,740/year, or an annual saving of AED 84,260/year. And in terms of net electricity used, it saved 143,000 kW/ year (77.3%); and once the balance solar PV is installed, it will save 159,325 kWh/year (86.1%), when compared to typical villas.
The current 25.85 kW horizontally installed solar PV produced around 33,000 kWh (1,277 FLH/year), with one-time monthly cleaning, as compared to the 35,600 kWh simulated (actual produced power is seven per cent less than projected) and an expected loss of around five per cent for flat horizontal panels as compared to the south-facing 23 degrees inclined installation. The 1,400-1,500 FLH/year projected solar output often talked about is overstated and is difficult to achieve in the field.
The generated solar PV power in the villa constituted around 45% of the total power consumption. I am planning the addition of 12.79 kW solar PV for installation before year end, which would add around 16,325 kWh of solar PV-produced power, taking the villa’s production to around 49,325 kWh/year or 65.8% of the power consumed. The additional solar PV area was authorised by exceptional approval, allowing the use of 50% of the roof area for an elevated solar gazebo in the interest of scientific research.
