a rooftop collector for sunbelt rooftops

Technical Progress White Paper and Economic Analysis as of July 2011

Executive Summary:
The Hot-Hybrid Solar Collector's goal is to exploit the heat content of sunlight alongside the photovoltaic potential at high temperatures. Unlike flat panel hybrids that generate large quantities of warm water, the Hot Hybrid is a concentrating model intended provide sufficient thermal energy density to drive processes such as adsorption or absorption cooling/chilling in the summer. High temperatures are also advantageous for space and water heating applications. The goal is to cool the PV array (improving its performance) while avoiding the requirement of an expensive low-temperature heating load like a swimming pool or a ground-loop heat dump. By combining high temperature heat collection, cool photovoltaic cell operation, solar driven cooling/chilling, solar heat and comprehensive building shade into one low cost collector array we can drive an industry leading ROI.

Further, where electricity and or natural gas prices are high (parts of the US sunbelt for example) and anywhere time of use contracts are available, this system will be even more competitive on a straightforward dollar basis. Add to this the applicability of rebates and incentives for both electricity and renewable hot water/space heating and the value proposition is even stronger.

The following is a report on both system design progress and the analysis of the economic prospects and market environment for the product at the current calculated efficiency rates of 12% electric, 33% process heat and 93% broadband shade.

Just in time for Intersolar.... Round Two Tuning Analysis
The collector has 5 directions into which energy has been tracked. Two are the traditional PV and Thermal paths. The remaining are warming the device, returning energy to space and passing through to the building below. This design, as described below, amounts to a shade structure powerful enough to impact cooling loads on buildings in the sunbelt.

Round Two Optical Tuning Results
After the preliminary optimizing and with no anti-reflection coatings, the device is already close to the performance of superimposed flat plate hybrids (See reading room for more on alternative products for comparison) but at higher output temperatures and a lower estimated cost of goods.

Study of energy paths within concentrating device:
expressed as fractions of direct light, segmented by bands useful and not useful to the photovoltaic element:
 Standard 1 sun power 1000 W/m2 Into PV Into Heat Circuit
Warming the
Rejected back
to Space
Directed Down to
Light that is Visible & UV (.36 to 1 micron) 0.64 640 50% 26% 11% 12% 2%
Long Wave IR (1 to 2.6 microns) 0.24 240 7% 72% 11% 9% 2%
outside definitions above 0.12

(results of analysis by Breault Research Organization reordered here to describe net results rather than the device's optical elements)
Some of the assumptions:
Direct radiation in the .5 degree cone angle of Sun
Rays traced in middle of cone and middle of the two spectral ranges

Wattage implications for the fractional analysis above (plus concentration calculations):
1) The UV and visible light .64 * 1kWm2  = 640 Wm2 incident on the device. Cell gets 640 Wm2*.50 = 320W at 51kWm2 (51 Suns minus IR)
2) The IR at .24 * 1kWm2  = 240 Wm2 incident on the device. Heat circuit gets 240Wm2*.72 = 172.8 Wm2 + UV and visible "spill" and bypass of .26 *.64 * 1kWm2  = 166 Wm2.  Heat circut = 172.8 + 166.4 = 339W.
3) Of special note is the scant 2% of the incident energy getting through to the building beneath. This places the system "off the chart" when it comes to cool roof calculators at LBL's cool roof website. A very good thing in the sunbelt - our projected best use case.
4) Unclear at this time is the performance of standard mono or poly crystaline silicon under light so composed (shifted blue and magnified) see below for the available tactics for responding to this uncertainty.

Conclusions from Round Two Optical Analysis:
Out of deference to future development partners we do not disclose here the method of operation (shared freely under non-disclosure agreements.) However, here are some of the avenues left for performance improvement and system evaluation that do not require advances by other parties (as for instance improvements in PV cell efficiencies) and should provide the reader with a sense of the project trajectory:

1) The heat circuit has gains from the indirect light path and simple measures are still on the table to increase the heat circuit's yield from direct light.
2) The tuning of the primary has been limited to single rather than faceted surfaces. The astigmatism inherent in the system has not yet been corrected (as an exercise in value engineering.) 50% above was arrived at quickly with only gross tuning.
3) The target PV area has been limited to 10 mm square and the concentration is at 51X, this can easily be stepped down (or even up) within the same device plan.
4) The heat circuit is far less dependent upon tracking and so can still work when the system is in PV turn-down mode
5) There is physical room for collimators and diffusers to improve the (as yet unanalzed) evenness of illumination at the PV.

Round Two Economic Analysis
The tumbledown of flat panel prices continues as production quantities grow and the manufacturers wring more efficiencies out of the production process. As if that were not enough, natural gas continues to deliver energy at very low costs. Fortunately the hybrid's virtues continue to generate notable value along the following fronts:

1) ROI
Even without further refinement the ROI for a system that collects 100 watts of electricity per square meter of collector and 340 watts of thermal energy at 200 degrees F should be around 1.5 times as fast. When the system is specified in outsized prescriptions (to exploit the shade effect) the ROI can benefit from shifting the cooling load (clarified below). Hedging against two different power sources (both of which you are going to use) with one purchase is also appealing.

2) COG
Cost of goods advantages in the micro concentrator design remain:
    a) mirrors are still cheaper than PV crystals
    b) the small PV targets have the potential to come from novel and very inexpensive sources
3) BOS & Installation Costs
Small subunits and tight, flexible configuration lowers cost of installation and balance of system elements:
    a) The sturdy and modular nature of the collector reduces the crew size required to safely install
    b) As a Hot Hybrid - this system can participate in incentives from both sides of the utility supply: R.E. hot water and electrical incentives
    c) The relatively lower peak wattage (and longer production day) can mean lower inverter costs
    d) Micro inverters integrated into the header reduce installation effort and high voltage DC wiring
    e) Tarpless turn-down means high voltage DC switches and spark arrest are not required

4) Distribution
Distribution channels are fostered through follow-on sales and service opportunities:
    a) High temperature output supports efficient thermal storage (exploiting the energy density of water) so sales of improved hot water storage naturally follow (or can be included in) a system sale.
    b) Large systems can inexpensively support replacement air heating and/or cold air return tempering or be used to boost heat pump performance.
    c) Large (30 square meters +) systems can drive ab/adsorptive cooling/chilling (thus the sales of same) or act in tandem with conventional AC*
    d) Chiller circuits through a building can support refrigeration
    e) maintenance, cleaning and surveillance services

*When applied to life safety loads like c) Air Conditioning and d) refrigeration can directly impact both Time Of Use and total use of electricity and thus significantly boost ROI.

5) Consumer advantages in addition to the strict energy for money axis:
    a) Only collector on offer that can integrate heat, electricity and light harvest into the same collector*
    b) Potentially colored to meet aesthetic needs with minimal consequences to the thermal collection
    c) Modules grow in 180mm wide increments to fit the available frame
    d) Thermal port can be placed anywhere along header to harvest heat most efficiently
    e) Micro modularity in the concentrators minimizes the consequences of point failures in the PV array

* All while maintaining the same look across the collector.

6) Safety
Advantages in the safety arena contribute to lower cost and better adoption:
    a) Tarpless shutdown - as a concentrator, if you stop tracking you stop the electricity
    b) Thermal can stop collecting within moments OR be left collecting without the PV producing
    c) Low wind-loading
    d) Can be installed in the "off" position and turned "on" when completely ready
7) Sales
Hybrids can enjoy a "like but better" positioning against conventional Thermal (the traditional value leader) and PV (the current mind share leader)
    a) Easy to buy: one stop shopping. Hybrids compete powerfully with thermal-only or PV-only offers with a comprehensive all-in-one solution at an attractive price. One trip to the calculator, one trip to the bank, one trip to the roof by one installation crew: one happy customer - a customer now done with their energy shopping for the next 20 years.
    b) Compelling % efficiency figures - PV products now have to explain why 20% is supposed to be impressive when they can get 50% for less.
    c) Hybrids sound good from the very start - capitalize on brand equity/mindshare/esteem occupied by hybrid cars
    d) Delivers the kind of power people use: electricity, heat & light - makes intuitive sense to customers
    e) Follow-on sales and service are a credible offer - people understand that moving parts need supervision and maintainance
    f) Hot water circuit can be pre-plumbed for add-ons later

Action Item for the Reader
If you have read this far you either see the economic promise of the Hot Hybrid or have detected some folly and are looking for a way to contact the developer to learn more or to share what you know. Either way I, Paul Bostwick, lead product developer of the Hot Hybrid, welcome your interest and contact at 510-872-8935 or email or skype: paulbostwickoakland.

This is copyrighted material and updates to it can be found at Hybrid Second Round Analysis.html