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Control strategy in BREEZE.
BREEZE contains different installation- and control components representing generic models rather than specific implementations. The components can be combined freely to correspond to the principal operation of any HVAC system.
The control strategy is not described by profiles as in most common two-step (loads-systems) programs. Instead the control is based on a sequence of restrictions on the possible sources for heating, cooling or heat recovery. The restrictions are relaxed successively within each time step in the building model until a solution is found. The order in which the restrictions are to be relaxed may be varied, the capacity intervals can be open ended on one side, etc.
By combining loads and systems minimum energy strategies can be defined and found by the program. When limiting the installed capacities the building dynamics will be more active in the control process which has shown to give a surprisingly high potential to reduce peak power problems and energy use.
In common design and control strategy for HVAC systems today, space temperature is prescribed and heating and/or cooling is supplied in the required capacity to maintain this temperature. Instead, the temperature that can be maintained with equipment of prescribed capacity. Thus the design procedure is to determine the frequency of out-of-range temperature as a function of equipment capacity, forming the basis for final size specification.
This is coupled with a control policy in which conditioning capacity is supplied at low amplitude over a longer period of time, rather than at high capacity when space temperature gets out of bounds as in current practice. The philosophy is to work with the building thermal mass, trying to keep it within bounds with gentle nudges appropriate times, rather than attempting to overcome the large load that would develop when this mass is allowed to approach maximum set point bounds as in current practice.
Simulations show that nearly all zone loads at any particular instant comes from the solid surfaces. Thus if these surfaces are kept within comfort range, the air temperature will "take care of itself," (more or less), and loads will be small.
The potential benefits of successful application of these principles are immense. Perhaps of greatest immediate interest is reduction of demand for air conditioning. Many application of these principles in Sweden have shown that cooling capacities can be reduced to less than half of those required under current practice. This translates into kW demand reduction, shifting demand to off peak, and equipment first cost reduction. Significant energy savings are also realized because the cooling is delivered throughout the day and night, maximizing opportunities for economizing.
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