Glasgow Caledonian University, School of Engineering and the Built Environment1
Modern multi-stage ammonia heat pumps can now efficiently and reliably produce up to 90Â°C water from a large thermal mass such as a ground or water resource, of which good, high thermal conductivity examples are rivers, effluent discharge and large bodies of surface water.
Within the cityscape there are many disparate but exploitable water resources that could be used to heat buildings and help alleviate fuel poverty. As the direct burning of fossil fuels to generate heat is steadily being replaced by electric heating derived from low-carbon sources, there is a strong impetus (and Government incentives) to develop technologies to use "green" electricity to generate low-carbon heat cost-effectively.
However, in order to ensure long-term integrity of supply, the ramifications of large-scale energy extraction on a water body must be fully considered. There are various problems to be solved: How much energy could be safely extracted from a resource without significant downstream impacts, ecological harm or ever-diminishing yield? How do different asset types differ, and what will the impact of climate change be? Is it possible to model assets using readily available meteorological data, removing the need to physically measure assets over many years? It is possible to derive a model than can support forecasting of and backcasting for asset potential?
A Glasgow Caledonian University mature student investigated the potential of buried and surface water bodies, canals, and waste water discharge in Glasgow. Temperature loggers were located in key locations and local meteorological and historical flow rate data used. Although some resources were quickly discounted, others showed impressive potential.
The data gathered by the loggers was correlated against local meteorological data to determine if simple modelling of asset potential was possible using cheaply and readily available data. Some correlation was found, but of greatest significance was the finding that the peak yield from an asset is delayed by many hours, allowing the greatest amount of energy to be extracted at the time of greatest need - early evening. Surface water assets gathered a useful quantity of exploitable energy from the environment during the warm seasons, but more data gathering is needed to determine by how much that energy diminishes during winter. Buried water sources were found to be more closely coupled to groundwater temperature than air temperature, allowing more dependable energy extraction during winter time from a buried rather than a surface asset. Canals showed interesting potential due to the clay lined bottom inhibiting groundwater flow and creating an insulating layer, helping reduce the loss of energy to the ground. Very distinctive stratification was also recorded, with rapid de-stratification when the solar gain ceased. Finally, it was discovered that the amount of heat that could be recovered from waste water treatment discharge without any downstream degradation can be measured in the Megawatts. Local heat maps were derived for the buildings that could be serviced by district heating systems fed by this energy, and demonstrated that a wide community of housing, many of which are occupied by the fuel poor, and businesses could be heated by cheap low-carbon energy.
The student also studied relevant research spanning many decades to help determine impacts of exploitation and how asset potential could be modelled for use with historical and/or current meteorological data. Various models are available, ranging from simple but inaccurate regression models to highly computational multi-variable self-calibrating models that have been proven to be very accurate. The student recommends that the next phase of the study includes creating a tailored model for determining water asset potential. An accurate self-calibrating dynamic model would allow heat yields to be determined across Glasgow and beyond, which could then be mapped to local heat loads for servicing by a heat pump and district heating network. The student also recommends that to confirm the accuracy of any models created, detailed flow rate and thermal sensing is performed across many heating seasons.
In conclusion, the study in its early phases has already demonstrated the significant potential of city-wide water bodies to provide dependable forms of low-carbon heating energy. Further study and analysis is required to fully map the potential of the resources, but this initial study has illustrated the clear potential for these resources to play a useful part of our future energy mix. Mohseni, O & Stefan, H.G. (1998) A nonlinear regression model for weekly stream temperatures. Water Resources Research. Iss 10. 2685-2692.
Mohseni, O & Stefan, H.G. (1999) Stream temperature / air temperature relationship: a physical interpretation. Journal of Hydrology. Iss 3, 128-141.
Ficklin, L et al (2012) Development and application of a hydroclimatological stream temperature model within the Soil and Water Assessment Tool. Water Resources Research. Iss 1.
Barnhart, B et al (2014) Improved stream temperature simulations within SWAT using NSGA-II for automatic, multi-site calibration. Transactions. Volume 57, Iss. 2. 517-530.