Integrated design and operation

BITES systems can be categorized into passive and active systems. Active systems embody charge/discharge systems to actively charge and/or discharge thermal energy. Hydronic radiant floor with significant storage mass is a common example of an active closed-loop BITES system. For passive BITES systems, control is normally external to the system, such as control of the transmitted solar radiation (through motorized shading) and night time building pre-cooling. For active BITES systems, operation has to be taken into account during the design stages.

Key challenges for proper operation of active BITES systems include the following:

(1) Due to the relatively strong thermal coupling between the BITES system and the rest of the room, the operative temperature (sense of occupants) will be significantly influenced by the exposed BITES surface temperature and the possible advective thermal output of the BITES (e.g. the supply air from the BITES in Fig. 1.8). The BITES temperature has to be controlled in the thermal comfort range well;

(2) High thermal inertia of BITES systems. This means slow response and significant amount of thermal energy is needed to regulate the BITES temperature. High power intensity and precise schedule are needed for fast regulation without high overshoot. The dynamic response of indoor objects (e.g. furniture and wallboards) also need to be considered;

(3) Improving building energy performance through use of active BITES systems. o Pre-conditioning of BITES and indoor mass to utilize off-peak energy and

reduce peak power demand.

o Efficient utilization of renewable energy. This requires storing energy as much as allowed with respect to energy availability (e.g. night time for relatively cooler air) and other constraints (e.g. room temperature). o Allowing room temperature to float with exterior weather conditions to

utilize the thermal zone as thermal collector and thermal storage (e.g. passive solar heating or night time free cooling). For example, room temperature is allowed during sunny daytime to rise up to the upper comfort limit to capture useful passive solar heat gain.

The first two factors are the main challenges for control because the active BITES systems have to provide good thermal comfort as their first priority; while optimizing the energy performance of the active BITES in a whole building context.

Using passive measures with possible fan assistance, excess thermal energy from a thermal zone can be transferred to its BITES, and released back to the zone when needed. In this process, the temperatures of the active BITES system and its thermal zone swing within allowable thermal comfort limits. The time period for the swing should suit that of the

thermal load – ideally the zone temperature reaches its peak at the time or after when the thermal load switches from cooling to heating. The design of the active BITES system should provide a suitable dynamic response that matches the time period required. For example in passive solar design, an active BITES system should be able to absorb significant portion of the transmitted solar heat gain and release it back to the room after a suitable time to avoid space overheating. Thermal coupling between the BITES system and the thermal zone is also critical in these storage and release processes. It can be enhanced with open- loop design (e.g. local re-circulation of room air through BITES systems (Fig. 1.8-b)). Pre-conditioning takes advantage of available ambient renewable and/or off-peak purchased energy. These two kinds of energy can be used to pre-heat or pre-cool the BITES and its zone to reduce purchased energy consumption. Proper active BITES design will enable sufficient charge/discharge capacity to store desirable energy within a limited time. Fig. 1.11 demonstrates the passive measures and pre-conditioning of an active BITES system and its thermal zone during a shoulder season period – it is sunny and cool during daytime but heating may be needed during night time. Through optimizing passive measures with possible fan assistance, the BITES system keeps the room cool during the daytime by absorbing the excess heat from the room, but warms the room during the night time by releasing previously absorbed heat. However, passive measures with possible fan assistance alone are not sufficient, and some auxiliary heat injection to the BITES system is required during the night time in order to keep the room temperature within comfort range. With pre-conditioning operation (action “6”), heat is injected to the BITES system and

consequently to the room (action “3”) during the daytime. Consequently, the temperatures of the room and the BITES systems are raised (action “1” and “2”). Thus, the thermal

energy originally needed for the night time is reduced (action “5”). Heat injection for actions “3” and “6” can be accomplished with solar thermal collectors or heat pump.

Ideal operations should make use of active BITES systems to reduce space heating/cooling through storage and buffering, and to store useful thermal energy during favorable periods (e.g. ambient renewable energy) and release it in the following unfavorable periods. Also, peak power demand reduction should also be addressed. These objectives require energy- conserving operation strategies and predictive control. The acceptable range of thermal comfort offers flexibility but also imposes limits on operating strategies.

The design and operation of an active BITES system are interrelated, but the operation leads the design. This is mainly because the BITES has become part of a service system that supplies primary space conditioning. The operations aim to maintain room air temperature within comfort zone in a low cost manner, by managing the thermal energy input with respect to source choice (e.g. ambient renewable energy vs. purchased energy) and scheduling (e.g. on-peak purchased energy vs. off-peak). To accomplish the desired

operations, a suitable design is needed. On the other hand, operations have to adapt to the allowable design and attain potential benefits. In some cases, designs may be limited by other building parameters. For example, the maximum amount of thermal mass may be limited by the allowable structural load.

1. Room temperature increases within comfort range due to action “6”; 2. BITES becomes warmer due to action “6”;

3. More heat gain into the zone;

4. Auxiliary thermal energy is stored ahead for predicted heating load; 5. Use of auxiliary energy is avoided due to action “4”;

6. BITES pre-heating operation.

Fig. 1.11: Schematic of passive measures and pre-conditioning concepts ( is the heat gain/loss of the room; _ is the thermal output from the BITES to the room; is the

thermal energy injection rate to the BITES)