JX Crystals

Adapting TPV for Use in a Standard Home Heating Furnace

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Courtesy of JX Crystals

Abstract, A novel TPV configuration will be presented that can fit into a standard home furnace cabinet. This system incorporates an externally faceted glass cylinder with a dichroic filter deposited on its outer surface and a GaSb IR cell array bonded to the outer surface on top of the filter. This cylindrical array is then surrounded with an envelope containing a low boiling point liquid for evaporative cooling. The liquid is in direct contact with the backside of the cell array. An air-cooled condenser is then mounted above the photovoltaic converter array. Evaporative cooling potentially allows a heat removal rate of 20 W cm2. Additional novel features of this design are described. The goal is to design a cost-effective retrofit forced-air warm air furnace that can work either as a self-powered furnace or as a Combined Heat and Power appliance. In order to achieve low cost, the design point for the GaSb cell electric power density is 2.5 W/cnr.

Keywords: Photovoltaic, Thermophotovoltaics. TPV, GaSb. IRPV cell. PACS: 84.60Jt

Furnace Company Perspective
In a companion paper [1], two hydrocarbon-fired TPV generator configurations using GaSb IR sensitive PV cells were described. The Midnight Sun TPV stove is a simple combined heat and power unit providing heat and electricity for small off-grid homes. As summarized in table 1 (Case 1), its primary advantage is simplicity and its primary disadvantage is low electrical conversion efficiency.

A second cylindrical TPV generator has been designed for the US Army as a quiet battery charger. As also summarized in table 1 (Case 2), the primary advantage of this unit is high electrical conversion efficiency but its primary disadvantage is complexity. This unit uses exhaust gas recuperation but its complexity and efficiency both come from its efficient spectral control. It incorporates an anti-reflection coated tungsten foil emitter for improved TPV spectral efficiency but this then requires the complexity of a vacuum emitter thermos.

These efficient cylindrical TPV generators are designed around a family of commercially available SiC radiant tube burners. We have previously noted [2] that one of these efficient cylindrical TPV generators could be adapted as a Combined Heat and Power (CHP) furnace-generator for use in a home. Its size would be a 40,000 BTU/h (12 kW) furnace with a 1.2 kW TPV electric generator.

However, after discussions with furnace companies, it is now clear that they have a different perspective. The US heating market demands furnaces with higher heating capacity, e.g. 85,000 BTU/h or 28 kW. They argue that this higher heating capacity is required to heat a home on the coldest day and that customers want to heat a cold home rapidly when returning home. Further, the bulk of US furnace sales are for replacement of old furnaces that are naturally found in older, less well insulated homes. There also appear to be competitive reasons for favoring higher 'numerical' furnace ratines.

The US furnace market also favors relatively low power generation for two interesting reasons. First, there is a significant demand for self-powered furnaces that will operate during power outages, particularly those due to ice-storms. These self-powered furnaces require only 600W of generator output and within this segment there is essentially no demand for excess generation. Second, the demand for CHP furnaces - that is furnaces that generate more electricity than they use and return the excess to the grid - favors a furnace generator configuration that can be attached to existing house wiring as this simplifies and minimizes the cost of retrofit installations. Existing US electrical codes require a dedicated 15A, 120V circuit for furnaces and a retrofit furnace connected to such a circuit can deliver no more than 1.4 kW to the grid. Rewiring existing homes would be necessary to accept retrofit CHP furnaces with greater generating capacity, and this would make the sale of such furnaces difficult and the rewiring costs would adversely impact the economics.

There are other requirements as well. Ideally, the TPV generator should fit entirely within existing furnace dimensions, function properly under on-off operation in forced-air furnaces, operate quietly, and of course be low cost.

Our initial TPV generator design for the US furnace market has 1.2 kW electric output with 28 kW fuel input, and 4.3% conversion efficiency. To achieve the desired size, cost and operating characteristics, it operates at very high power density, reducing overall size and the quantity of IR cells.

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