15. Power Distribution
DaedalusaL4 will obviously require electrical power for all of its primary functions; communications, industry, research and, of course, life support. Therefore we must not only consider the question of the generation of power but also the storage, conversion and distribution of it. Steps must also be taken to minimize the risk of electrical hazards to both people and equipment. The main method of power generation on the station is through the use of photovoltaic cells (see 2.2 Power Generation Systems).
The basic power distribution system is very simple. Power is generated in the PV cells and then distributed as primary power via transmission lines. Before power is delivered to residential users its voltage is reduced via a ‘step down’ transformer to the secondary power voltage. This is then further distributed and routed by distribution boxes so as to provide power for a great many individual users. Electricity is distributed at high voltages to minimise the current flow and thus the amount of electrical energy converted to heat in the wires, and thus lessen wastage and the risk of fire (ref. 84). In general, primary power is used by heavy industrial users or when power is distributed over long distances. Secondary power is for domestic use and distribution over local networks.
Power Distribution Command & Control Unit (PDCCU)
This is the linchpin of the entire power distribution network. It connects the batteries and the PV cells to the power network. It controls the charge and discharge of the batteries when required and also the operation of the power storage disk (see below). Secondary power to critical systems is backed up directly from the PDCCU to safeguard against cases when the standard transmission lines to these systems become inoperable.
Photovoltaic Cells Control Unit
This links the power distribution network to the generating equipment. The power task of this system is to regulate the output voltage of the cells. Power from the cells is received through the rectenna, regulated and the output is maintained within primary power voltage parameters. All systems relying on primary power must be able to cope with voltages within this variable range. Further regulation of power will take place in the PDCCU. The reasons for this necessity are many but most notable are the performance characteristic of PV cells and the fact that energy may be lost when transported over long distances.
Power Storage Systems
Power can be stored using a flywheel storage system. This involves using energy to spin a disk of large mass, (possibly of carbon fiber or other ultrastrong material), resting on magnetic bearings, to a high velocity in a vacuum. When the power is needed the kinetic energy of the disc is recovered and used to turn a dynamo. This is a very efficient method of storing power, and has reached first commercial applications stage on Earth. It provides numerous advantages over batteries, such a more energy stored per unit mass, longer usable life, etc. The ready availability of high vacuum in space should help to further improve efficiency.
Banks of batteries can provide a further back-up power storage system in situations where the solar arrays are not functioning. The charging and discharging of these batteries is controlled by the PDCCU. The actual storage devices are Nickel Hydrogen (NiH2) batteries. Given complete failure of all other power generation facilities the batteries could provide power for a limited period with frugal use of energy resources. The batteries are connected to a control unit that monitors their status and transmits this data to the PDCCU. Also included in this bundle are charge/discharge units for each of the batteries.
Primary Power Distribution System
Transmits power to heavy industries, research areas and any other systems which may require higher voltages than can be provided by the secondary power distribution network.
Secondary Power Distribution System
The first step in this system is the conversion from primary power to secondary power voltage. This conversion occurs in many ‘step down’ transformers located throughout the station. Each transformer has one primary power input and one secondary power output. Primary power is variable but is typically 16 kilovolts while secondary power is specified at 124 volts (ref. 68). If any other voltage is required the user must, within safe limits, convert it for themselves.
The electrical power distribution could, if not properly maintained, be hazardous to the personnel on board the settlement and also damaging to equipment. As such the provision of adequate safety features was seen as a priority, some of these features are outlined below.
· All components in the system are maintained at common potential and all structures and components are electrically tied to a common point (the metal infrastructure of the station). These facts combined minimise electrical shock hazards to personnel and equipment.
· A further safety feature is that in response to over-current conditions, the system is designed such that each downstream circuit protection device is set to a lower current rating and responds more quickly than the protection device directly upstream (ref.68). This feature ensures that faults in the system do not propagate towards the power source.
· The potential difference between the station and space could be significant and would result in micro-arcing, potentially causing damage to the settlement’s outer hull. To minimise this potential difference Plasma Contactor Units will be used. These devices, first developed for the ISS, generate plasma from xenon gas or, in the case of DL4, another element sourced from the Moon or asteroids, and emit a stream of electrons into space. This electron emission effectively grounds the station to the space environment minimising potential difference as well as related hazards to personnel and equipment.
In the event of a serious or total failure of the power generation system, power will have to be allocated in a more parsimonious manner than usual. As such, a set of emergency allocation protocols has been drawn up to outline the hierarchical distribution of limited energy resources. In the event of a reduction in available electrical power that poses a danger to the continued habitability of the station, the following steps will be taken to ensure continued safe operation.
1. All scientific research and industrial operations will be ceased until the situation is resolved.
2. All energy intensive recreational activities will not be permitted.
3. The unnecessary use of internal transport systems will be discouraged except in extreme cases
4. The use of electrical devices in residential dwellings may be prohibited. In a lesser crisis minimum power can be made available for essential services.
5. In all cases the first priority must be the provision of adequate life support for all personnel. It may be deemed necessary to seal off unoccupied areas of the station for the conservation of life support resources. In the most extreme cases contingency plans will be put into operation to evacuate part of the station’s habitable volume.
6. After the continued provision of life support the next priority should be to remedy whatever malfunction has caused the power available to be reduced. The isolation and repair of faults is the next step to returning the settlement to normal operating procedures and resources should be directed accordingly.