HEAT RECOVERY | THERMAL STORAGE Storage-tank capacity (kWh) Cooling load (kW) Chiller output (kW) 12000 2500 10000 2000 8000 1500 6000 1000 4000 500 2000 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of day 24 hours Thermal storage tank capacity and building load Water-side free cooling may be appropriate in very cold climates at normal chilled water temperatures (6C) and in temperate climates where there is a use for chilled water at a supply temperature of 10C or higher. Manufacturers of free cooling air cooled chillers advise that some free cooling is available once the ambient temperature falls to 3K below return chilled water temperature. Challenges and benefits in refurb and new-builds For efficient, high-density energy storage, large temperature differentials are required; at least 10K is desirable. Most air source heat pumps and air or water-cooled chillers cannot generate large enough temperature ranges for storage systems unless cascade arrangements are used. There is currently a very small selection of suitable products on the market that can produce efficiently the required water temperatures for storage, so refrigeration technology must adapt to support these conditions if thermal storage is to be a common solution for buildings in the future. Phase-change materials can offer denser energy storage, but are limited and, historically, operationally unreliable. Further, very large volumes and usable space for the storage tanks are required. The spatial requirements can be challenging; usually, these tank installations require very tall tanks and huge volumes of water. An obvious benefit of integrating thermal storage into a buildings design is that primary heating and cooling plant capacity can be reduced, thereby also reducing plant area and electrical infrastructure. Buildings can change their peak electrical demands to be more diversified, and make use of off-peak tariffs. Reducing plant areas can enable more flexible rooftop designs. Take, for example, the recently approved refurbishment of Millennium Bridge House in London. Norman Disney & Youngs (NDYs) thermalstorage design negates the need for heating or cooling plant to be roof-mounted, so allowing more than 2,500m2 of roof space to be used by the public, retail and office building tenants. The existing buildings roof offered no rooftop amenities, and all usable space was dedicated to HVAC plant. This Breeam excellent scheme developed with Piercy&Company Architects for Angelo Gordon and Beltane Asset Management uses large chilled and low-temperature hot-water storage tanks, located in the existing basement, with watercooled chillers to transfer and recover heat. Using chilled and low-temperature hot-water tanks in this way allows for full heat recovery, so no primary heating plant is required. The dry air coolers, located on the level 2 faade, reject heat from the water cooled plant in the basement when the heating storage systems are full. We have, therefore, avoided excessive pipework routes to roof level, reducing pumping energy and riser space while keeping the rooftop free of HVAC plant. Thermal-storage systems offer engineers an opportunity to challenge how new and existing buildings are designed. There could be a more sustainable future in which primary heating plant isnt needed in office buildings as we push towards net-zero carbon design. CJ JOSHUA MARTOO is a senior project engineer at Norman Disney & Young 50 September 2020 www.cibsejournal.com CIBSE September 2020 pp48-50 Heat recovery thermal storage.indd 50 21/08/2020 15:16